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Keywords:

  • Arabidopsis (Arabidopsis thaliana);
  • branching;
  • CAROTENOID CLEAVAGE DIOXYGENASE;
  • kiwifruit (Actinidia chinensis);
  • leaf senescence;
  • perennial

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information
  • CAROTENOID CLEAVAGE DIOXYGENASE (CCD) genes have been demonstrated to play an integral role in the control of branch development in model plants, including Arabidopsis, pea (Pisum sativum), petunia (Petunia hybrida) and rice (Oryza sativa).
  • Actinidia chinensis is a woody perennial plant grown for commercial production of kiwifruit. CCD7 and CCD8 genes were isolated from A. chinensis and these genes are predominantly expressed in the roots of kiwifruit. AcCCD7 and AcCCD8 were able to complement the corresponding Arabidopsis mutants max3 and max4. The function of AcCCD8 in branch development was determined in transgenic kiwifruit plants containing an RNAi construct for AcCCD8.
  • Reduction in expression of AcCCD8 correlated with an increase in branch development and delayed leaf senescence.
  • The CCD pathway for control of branch development is conserved across a wide range of species, including kiwifruit, a woody perennial.

Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information

Branching is an integral factor in determining the architecture and productivity of a plant. The huge range of growth forms observed in plants is largely dependent on developmental decisions made at two major checkpoints: firstly, the formation of axillary meristems in the leaf axils; and, secondly, the outgrowth of these meristems. Axillary meristem outgrowth is controlled by multiple interacting environmental and genetic signals. Although the phenomenon of apical dominance is one point of control for meristem outgrowth (Sachs & Thimann, 1964; Cline, 1994), investigations of mutant plants from annual species exhibiting altered branching phenotypes have also revealed the presence of a root-derived branching signal. These mutants include the decreased apical dominance (dad) mutants of petunia (Petunia hybrida) (Napoli, 1996; Napoli & Ruehle, 1996), the ramosus (rms) mutants of pea (Pisum sativum) (Beveridge, 2000; Morris et al., 2001; Rameau et al., 2002), the more axillary growth (max) mutants of Arabidopsis (Arabidopsis thaliana) (Booker et al., 1999; Stirnberg et al., 2002; Sorefan et al., 2003) and the high-tillering dwarf (htd) and dwarf (d) mutants of rice (Oryza sativa) (Zou et al., 2005; Arite et al., 2007). Identification of the CAROTENOID CLEAVAGE DIOXYGENASE (CCD) genes CCD7 and CCD8 that are affected in a group of these branching mutants (Sorefan et al., 2003; Booker et al., 2004; Snowden et al., 2005; Johnson et al., 2006; Zou et al., 2006; Arite et al., 2007; Drummond et al., 2009) has demonstrated that there are conserved pathways acting in diverse plants involving a carotenoid-derived signal. DAD1, MAX4, RMS1 and D10 are orthologous genes encoding CCD8, while DAD3, MAX3, RMS5 and D17 are orthologous genes encoding CCD7. The CCD family includes proteins in animals and plants that act on carotenoid substrates. The first plant member to be identified was Vp14 from maize, which encodes a 9-cis-epoxycarotenoid dioxygenase (NCED) (Schwartz et al., 1997). A number of other CCD genes have been shown to have carotenoid cleavage activity (Schwartz et al., 2004; Auldridge et al., 2006; Huang et al., 2009).

Studies in pea and rice have revealed the involvement of strigolactones in the branch signal (Gomez-Roldan et al., 2008; Umehara et al., 2008). These investigations have shown that branching mutants lacking CCD7 or CCD8 have reduced concentrations of strigolactone and applications of synthetic strigolactones can restore the wild-type branching phenotype to the mutant. Strigolactones are a group of terpenoid lactones initially characterized as a plant-derived signal promoting germination of the parasitic plants Striga and Orobanche (Bouwmeester et al., 2007) and have also been shown to be involved in symbiotic interaction with arbuscular mycorrhizal fungi (Akiyama et al., 2005).

Although annual plants have been the focus for investigating the control of branch development, perennial plants, such as kiwifruit and fruit trees, have the potential for many additional points of regulation. The architectural framework of a perennial plant is dependent on growth in the previous year, any pruning of that growth, and the organogenesis of the new season’s meristems. Exogenous signals from the environment, including extremes in temperatures and day length, are integrated over months by the whole plant and influence the progress of axillary meristems through the stages of dormancy induction and release (Arora et al., 2003). To determine the role of the CCD pathway in the control of branch development in the more complex system of a perennial plant, we investigated branch development in kiwifruit (Actinidia chinensis).

Kiwifruit, a deciduous woody perennial, as grown in commercial orchards commonly consists of a single trunk with two horizontal branches (leaders) trained in opposite directions along a support. Shoots that have developed from the leaders are tied down in winter and the next season’s shoots arise from axillary buds formed during the previous season (Snowball, 1997; Walton et al., 1997). The architecture of the kiwifruit plant is important for obtaining good yields of fruit and allowing adequate light for flower initiation in the following season (Sale & Lyford, 1990). Currently this is managed through manual pruning. The development of shoots and the potential to produce flowers and fruit are influenced by low winter temperatures, which increase the proportion of buds that break dormancy (McPherson et al., 1994, 1995). In kiwifruit, both the shoot architecture and yield can also be influenced by the rootstock (Wang et al., 1994; Clearwater et al., 2004, 2006), suggesting that root-derived signals, such as strigolactones, may be important for controlling kiwifruit branching.

In this paper we show that kiwifruit contain CCD7 and CCD8 genes that are predominantly expressed in the roots. Furthermore, we report that reducing the expression of AcCCD8 alters branch development and delays leaf senescence. Therefore, we conclude that the control of branch development observed in model annuals is conserved in a woody perennial plant.

Materials and Methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information

Plant material

Plant tissue used in the analysis of AcCCD7 and AcCCD8 expression in wild type plants was harvested from Actinidia chinensis Planch. var. chinensis plants grown in orchards in KeriKeri and Bay of Plenty, New Zealand. Arabidopsis mutants max3-1 and max4-1 (a kind gift from O. Leyser) and wild-type Columbia were used in the complementation experiment. Arabidopsis plants were grown under glasshouse conditions with natural light supplemented when required to provide 16 h of light.

Isolation of AcCCD7 and AcCCD8

Degenerate primers (Table 1) were designed based on published Arabidopsis, petunia and rice CCD7 and CCD8 sequences and used for the isolation of the initial portion of the CCD7 gene from A. chinensis genomic DNA and of the CCD8 gene from Actinidia deliciosa genomic DNA (kindly donated by R. Atkinson). PCR amplification used Platinum Taq (Invitrogen) and the following conditions: 94°C for 2 min, followed by 35 cycles of 94°C for 30 s, 58°C for 30 s, 72°C for 2 min, followed by 72°C for 5 min. All further genomic DNA and RNA used in this study was from A. chinensis. Additional genomic sequence was isolated using successive rounds of gene walking (BD GenomeWalker; BD Biosciences, Franklin Lakes, NJ, USA).

Table 1.   Primers used for amplification of kiwifruit (Actinidia chinensis) genes
Purpose of primersTarget geneSequence 5′ to 3′
Degenerate primers for gene isolationCCD7ACNGTNCAYCCIYTIGAYGGNCAYGGNTA
NGTRTTIGCIACRTTYTTCATNACYTT
CCD8GGNMGNGGNATGGAYATGTG
GGATCCAGCAACCATGCAAGCC
cDNA isolationCCD7GTTCATTAAAGGCCAAAAATGC
CTCGAGAATCTAGGAGGGTGCCCA
CCD8GCACTCGCTAGTACTAGTACCTCTC
GTATAGAGTTCTAGTTCTTTGGGAYCCAG
qRTPCRCCD7CTATTGATGGGGTCAGTGGA
CCATCTTGCCACCTTTCAAC
CCD8CTAATGTCGCGACGGAATTG
CAACAAGTTCTCCCTCCCAC
UBCTTGCTTTAATGGCACATCCA
TCATTTCACCCCTTCTTTGG
ACTINCCAAGGCCAACAGAGAGAAG
GACGGAGGATAGCATGAGGA

The 3′-region of AcCCD7 was isolated by 3′-RACE (GeneRacer Kit; Invitrogen) using root mRNA. Full-length cDNA copies of CCD7 and CCD8 were then isolated from root mRNA using PCR with primers shown in Table 1.

Generation of transgenic A. chinensis plants

To generate a construct to reduce the expression of AcCCD8, a 134 bp fragment from the genomic sequence of the AcCCD8 gene was cloned as an inverted repeat into the vector pTKO2 (Snowden et al., 2005) to produce pTKO2SAcCCD8. The construct was introduced into Agrobacterium tumefaciens and used to generate 11 independent RNAi transgenic A. chinensis plants (R1–11) following the procedure described in Wang et al. (2007). Six control transgenic plants (C1–6) were generated using the control vector pART27-10 (Yao et al., 1996), which contains a uidA gene under the control of the 35S promoter from Cauliflower mosaic virus (CaMV). Plants were transferred into potting mix and cultivated in a glasshouse under natural daylight and temperature conditions for 15 months, incorporating an initial spring/summer growth soon after transfer from tissue culture, a winter dormancy and a second spring/summer growth. At the end of the first spring/summer growth, the main shoots of plants were pruned to 1.7 m to allow for glasshouse management.

Root tissue was collected from the transgenic plants at three time points after transfer of the plants to the glasshouse (4 , 8  and 13 months) by carefully removing the plants from the pots and harvesting root tissue from several regions around the root mass before repotting the plants. Root tissue was quickly washed free of potting mix, frozen in liquid nitrogen and stored at −80°C. At the first time point, roots from only five of the six control plants were collected.

Quantitative RT-PCR

Total RNA was extracted using the procedure of Chang et al. (1993) modified with the addition of a 1 min polytron of the tissue with the extraction buffer, filtration of the aqueous phase through autoclaved miracloth following the first chloroform extraction and a final concentration of 2 M LiCl in the first precipitation. The extracted total RNA was treated with DNase (using Turbo DNA-free; Ambion) and the quality and concentration of RNA determined using an Agilent 2100 Bioanalyser (Agilent Technologies, Palo Alto, CA, USA). cDNA was synthesized with Superscript III Reverse Transcriptase (Invitrogen) using poly(dT)23V primer and 1 μg RNA. Quantitative RT-PCR was performed with the primer sets shown in Table 1 to detect the level of expression of AcCCD7 and AcCCD8 and the control genes UBIQUITIN CONJUGATING ENZYME E2 (UBC, accession number FG518635) and ACTIN (accession number FG440519; Walton et al., 2009). At least three technical replicate RT-PCR reactions were performed per primer pair using an aliquot of cDNA (1/500th), Power SYBR Green PCR Master Mix (Applied Biosystems, Carlsbad, CA, USA) and 200 nM of each primer on an ABI Prism 7900 HT machine (Applied Biosystems). Data were analysed with the SDS 2.0 software (Applied Biosystems), and relative expression was calculated using the comparative cycle threshold method (Pfaffl, 2001) with normalization of data to the geometric average of the internal control genes (Vandesompele et al., 2002).

Chlorophyll analysis

Leaf tissue for the chlorophyll analysis was collected from three leaves on the main stem of each of the 17 transgenic plants. To collect tissue of similar age and experiencing similar environmental conditions, leaf discs (1 cm diameter) were harvested from the blade of leaves at three sequential nodes starting 20 cm from the base of each plant. Leaf tissue was frozen in liquid nitrogen and stored at −80°C. Ground leaf discs were resuspended in 80% acetone (saturated with Na2CO3) and incubated for 1 h in the dark. The solutions were centrifuged at 16 000 g for 10 min and the absorbance of the supernatant was determined at 663 and 646 nm. The total chlorophyll (Ca+b) content was calculated according to Lichtenthaler (1988) (Ca+b = 7.15A663 + 18.71A646) and the average Ca+b was determined for each plant.

Complementation of Arabidopsis mutants

To generate constructs for the overexpression of AcCCD7 and AcCCD8, the full-length cDNA clone of AcCCD7 was inserted into pSAK778S (Drummond et al., 2009) and the full-length cDNA clone of AcCCD8 was inserted into pHEX2 (Hellens et al., 2005) both under the control of the CaMV 35S promoter. The overexpression constructs were transformed into A. tumefaciens strain LBA4404 and used to transform the Arabidopsis mutants max3-1 (for AcCCD7) or max4-1 (for AcCCD8) using the floral dip method of Clough & Bent (1988). T2 seeds were selected on kanamycin in tissue culture and then transferred to the glasshouse under long days. Control plants containing pHEX2, an empty CaMV 35S vector (Hellens et al., 2005), were germinated on kanamycin, while the Col, max3 and max4 seeds were germinated without kanamycin but under the same tissue culture conditions. The number of rosette branches was recorded 10 wk after planting, when flowering was almost complete.

Statistical analysis

The principal components analysis (PCA) was fitted in R 2.90 using the FactoMiner 1.12 package, while the Mann–Whitney U-test (P = 0.05), Spearman’s correlations and nonhierarchical clustering were calculated in Genstat 10.1 (VSN International, Hemel Hempstead, UK). For the analysis of the transgenic Arabidopsis branch number, Tukey’s multiple comparison ANOVAs (= 0.05) were performed using Genstat 10.1.

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information

Isolation of kiwifruit CCD7 and CCD8

Searches of the available kiwifruit EST libraries (Crowhurst et al., 2008) failed to identify sequences with homology to published CCD7 and CCD8 genes. Therefore the kiwifruit CCD7 and CCD8 genes were isolated using a combination of degenerate primer PCR and gene walking. A large portion (5.6 kb) of genomic sequence for the AcCCD7 gene was isolated, lacking the 3′-end of the proposed coding region, which was subsequently isolated from root RNA using 3′-RACE. For the AcCCD8 gene, 6.4 kb of genomic sequence was isolated, including 134 bp of sequence upstream of the start codon and 1077 bp downstream of the stop codon. Full-length cDNA clones for AcCCD7 (including an open reading frame of 1814 bp, accession number GU206813) and for AcCCD8 (including an open reading frame of 1680 bp, accession number GU206812) were isolated from root RNA.

The predicted AcCCD7 and AcCCD8 amino acid sequences were used in a phylogenetic analysis of the CCD family (Fig. 1). This included the full-length CCD7 and CCD8 sequences from other plant species identified using multiple BLAST searches (Altschul et al., 1990), all of the Arabidopsis members of the NCED clade, and the first identified plant member of the CCD family from maize ZmVP14. The AcCCD7 gene falls into the clade that includes the Arabidopsis, pea, petunia and rice CCD7 genes identified through the max3, rms5, dad3 and htd1 branching mutants, as well as the predicted genes from Zea mays, Vitis vinifera and Medicago truncatula. Likewise, the AcCCD8 gene falls into the CCD8 clade with the Arabidopsis, pea, petunia and rice CCD8 genes identified through the max4, rms1, dad1 and d10 branching mutants as well as the predicted genes from Populus trichocarpa, M. truncatula, V. vinifera and the moss Physcomitrella patens.

image

Figure 1.  Unrooted phylogenetic tree of the CAROTENOID CLEAVAGE DIOXYGENASE (CCD) gene family. Only full-length members of the family are included. The predicted protein sequences were clustered using ClustalX (Thompson et al., 1997) and then further aligned manually with MacClade (Maddison & Maddison, 2000). Phylogenetic relationships were calculated using the maximum-likelihood principle, and bootstrap values with 1000 replicates were determined using the Geneious software package (Guindon & Gascuel, 2003; Drummond et al., 2007). Scale bar is the number of substitutions per site. Accession numbers for the sequences used are as follows: Actinidia chinensis AcCCD7 (GU206813), AcCCD8 (GU206812); Arabidopsis AtCCD1 (At3g63520), AtCCD4 (At4g19170), AtCCD7 (At2g44990), AtCCD8 (At4g32810), AtNCED2 (At4g18350), AtNCED3 (At3g14440), AtNCED5 (At1g30100), AtNCED6 (At3g24220), AtNCED9 (At1g78390); Chlamydomonas reinhardtii CrCCD (XP_001701620.1); Medicago truncatula MtCCD7 (AC152177.43), MtCCD8 (CR956392); Oryza sativa OsCCD7 (AL663000.4), OsCCD8a (AP003296.3), OsCCD8b (AP003376.3); Petunia hybrida PhCCD7 (FJ790878), PhCCD8 (AY743219); Physcomitrella patens PpCCD8 (XP_001754721.1); Pisum sativum PsCCD7 (ABD67496), PsCCD8 (AY557341.1); Populus trichocarpa PtCCD8a (EEE93066.1), PtCCD8b (EEF03362.1); Vitis vinifera VvCCD7 (CAO70581.1), VvCCD8 (CAO44021.1); Zea mays ZmCCD7 (AC211432), ZmVP14 (U95953.1).

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The kiwifruit CCD7 and CCD8 genes complement the Arabidopsis branching mutants

To investigate the function of the kiwifruit CCD7 and CCD8 genes, we tested for the ability of AcCCD7 and AcCCD8 to complement the corresponding Arabidopsis branching mutants, max3 or max4. The kiwifruit genes were introduced into Arabidopsis mutants under the control of the CaMV 35S promoter. Three of the five Arabidopsis AcCCD7 overexpression lines and four of the six AcCCD8 overexpression lines showed reversion to the low branch number of wild-type plants and pHEX2 control plants (Fig. 2, Tukey’s multiple comparison ANOVA, = 0.05). In conclusion, the kiwifruit genes were able to functionally complement the corresponding Arabidopsis branching mutants.

image

Figure 2.  Complementation of Arabidopsis branching mutants with AcCCD7 and AcCCD8. Number of rosette branches in five independent transgenic lines of 35S-AcCCD7 in max3 (a) and six independent transgenic lines of 35S-AcCCD8 in max4 (b) compared with wild-type plants (Col), plants containing a control vector (pHEX2) and max3 or max4. Values are mean ± SE, = 6–12. Representative plants from lines containing 35S-AcCCD7 in max3 (c) or 35S-AcCCD8 in max4 (d). Differences in the number of rosette branches are most obvious at the base of the plants.

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The AcCCD7 and AcCCD8 genes are predominantly expressed in roots

To determine the tissues where AcCCD7 and AcCCD8 are expressed in wild-type A. chinensis plants qRT-PCR was performed on RNA isolated from a range of plant tissue: root, expanding leaf, stem segments from current season shoots, vegetative buds at bud break, flowers at anthesis, young fruit 13 d after anthesis (DAA), mature fruit pre-harvest (266 DAA) and seeds extracted from ripe fruit.

The highest level of expression of both AcCCD7 and AcCCD8 was detected in the root tissue of A. chinensis plants (Fig. 3a), and AcCCD7 was expressed at a substantially lower (42-fold) level than AcCCD8 in the roots. A low level of expression (1000-fold lower) was also detected for both genes in young fruit and seeds from ripe fruit. Expression of AcCCD7 and AcCCD8 was not detected in the leaf, stem, vegetative bud, flowers or mature fruit.

image

Figure 3.  Expression of CAROTENOID CLEAVAGE DIOXYGENASE (CCD) genes in kiwifruit (Actinidia chinensis). (a) Expression of AcCCD7 and AcCCD8 in wild-type kiwifruit. Real-time qRT-PCR expression of AcCCD7 (grey bars) and AcCCD8 (black bars) in a range of tissues relative to AcCCD8 expression in root. Values are means of three technical replicates ± SE, normalized to internal control genes. Tissues without visible bars had expression levels below the threshold of detection. (b) Expression of AcCCD8 in roots of transgenic kiwifruit plants. Real-time qRT-PCR expression in RNAi (black bars) and control (grey bars) lines at three time points after transfer of plants to the glasshouse, relative to the expression in control line C3 at 13 months after transfer. Values are normalized to internal control genes and are means ± SE (= 8 for RNAi and 5–6 for control).

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Identification of kiwifruit RNAi lines with altered phenotype

To investigate the role of AcCCD8 in controlling branch development in kiwifruit, transgenic AcCCD8 RNAi lines were generated. Owing to the relatively slow growth rate of woody perennials compared with annual species, all of the 11 RNAi lines and six control lines were maintained and analysed over the 15 month growth period. During this growth period, the expression of AcCCD8 was determined at three time points, the chlorophyll content of leaves were determined and branch development over two growing seasons was recorded (Supporting Information, Fig. S1). A PCA on all 17 transgenic lines was used to investigate the relationship between the expression data, chlorophyll content and number of branches (Fig. 4). The PCA reduced the number of variables in the data set to two principal components; dimension 1 combines the negative correlation of two of the variables (branch number and chlorophyll content) with the expression level of AcCCD8 and accounts for 49% of the variation in the data, while dimension 2 indicates the variation in the expression data over the three time points and accounts for 25% of the variation (Fig. 4).

image

Figure 4.  Principal components analysis and nonhierarchical cluster analysis of AcCCD8 expression, number of branches and chlorophyll content in transgenic kiwifruit (Actinidia chinensis). Values for individual RNAi lines are shown as closed squares and labelled R1–R11. Values for control lines are shown as open squares and labelled C1–C6. Mean values are shown as a closed circle for RNAi lines and an open circle for control lines. Vectors (black arrows with grey text) indicate the AcCCD8 expression at three time points (4, 8 and 13 months) after transfer (e4, e8 and e13), the number of branches and the chlorophyll content (chlor). The length of a vector relative to the unit circle (dotted circle) indicates the level that the vector is represented in the two-dimensional graph. Vectors that are close to the unit circle are well represented and can be reliably interpreted. Vectors in opposite directions indicate a negative correlation, while acute angles between vectors signify positive correlations. Variance explained by dimension 1 = 49% and dimension 2 = 25%. Two clusters of the transgenic plants are shown by the shaded ovals and were determined by nonhierarchical cluster analysis.

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The PCA plot also highlighted the variation between the different RNAi lines and suggested that some RNAi lines had not been altered in their AcCCD8 expression levels or phenotype, compared with the control plants. A nonhierarchical clustering analysis identified three RNAi lines (R5, R10 and R11) that were clustered with the control plants (Fig. 4). These three lines were excluded from subsequent analysis.

Expression of AcCCD8 in transgenic A. chinensis roots

The expression of AcCCD8 in the roots of the transgenic plants was determined at three time points after the plants were transferred to the glasshouse and compared between the RNAi and control plants (Fig. 3b). Analysis of the expression data using the Mann–Whitney U-test revealed that at 4 months after transfer there was no significant difference in the relative level of AcCCD8 expression between the control plants and the RNAi plants. However, there was a significant difference in AcCCD8 expression between the two populations of plants at the two later time points analysed, with the RNAi plants having a significantly reduced level of expression (8 months, = 0.005, and 13 months, = 0.008; Fig. 3b). The individual RNAi lines varied considerably in the level of AcCCD8 expression (Fig. S1a–c). For example, at 13 months after transfer, no AcCCD8 expression could be detected in line R8 and a very low level was detected for line R9 compared with RNAi line R6, which had levels of expression equivalent to the control lines (Fig. S1a–c).

Branch development in transgenic A. chinensis plants

The total number of primary and higher-order branches that developed on the transgenic plants was recorded during the two spring/summer seasons (Fig. 5). The RNAi plants produced significantly more branches than the control plants (Mann–Whitney U-test, = 0.005; Fig. 5a). The RNAi lines R8 and R9 developed the highest number of branches (59 and 66 branches), double that of control line C5 which had the highest branch number among the control plants (29 branches; Fig. S1d). As the main axis of the plants was pruned to a maximum of 1.7 m at the end of the first spring/summer season, fixing the maximum number of nodes for primary branch development, the difference in the branch number reflects the development of secondary and higher order branches as shown in Fig. 5(c) and (d).

image

Figure 5.  Branch development on transgenic kiwifruit (Actinidia chinensis). (a) Total number of branches formed on RNAi and control plants over 2 yr of growth. (b) Length of branches on transgenic kiwifruit. Branches formed on RNAi (black bars) or control (grey bars) transgenic plants over 2 yr of growth were divided into three different length categories: < 5 cm, between 5 and 100 cm and > 100 cm. Values are means ± SE; = 8 for RNAi and = 6 for control. (c) Branch of RNAi line R9 showing four secondary branches, including three branches < 5 cm in length (*) and one branch between 5 and 100 cm in length (#). (d) Branch of control line C4 showing one secondary branch > 100 cm (arrow) at node 10. No other secondary branches developed on this branch.

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The total number of branches produced over 2 yr of growth was divided into three different length categories: short (< 5 cm), medium (between 5 and 100 cm) and long (> 100 cm) (Fig. 5b). The RNAi plants showed a large increase in the number of short branches compared with the control plants, while there was less of an increase in the number of medium and long branches (Fig. 5b).

The length of 10 internodes on the main stem was measured on all transgenic plants (starting 20 cm above the base of the plant) and there was no significant difference in internode length between the RNAi and control groups of plants (data not shown).

Leaf senescence in transgenic A. chinensis plants

Kiwifruit are deciduous with leaves senescing each autumn. At the end of the first growth season, it was observed that the leaves on some of the RNAi plants were slower to senesce than the control plants (Fig. 6a,b). Chlorophyll degradation is an early indicator of leaf senescence, so to quantify our observation we analysed the amount of chlorophyll in leaf discs collected from the transgenic plants. The total chlorophyll (Ca+b) detected in the leaves from the group of RNAi plants was significantly higher than in the control plants (Mann–Whitney U-test, < 0.001; Fig. 6c). Two of the RNAi lines (R1 and R8) contained the highest amounts of chlorophyll (89 and 73 μg per disc, respectively), while the highest amount of chlorophyll detected in the control plants was 47 μg per disc in line C5 (Fig. S1e).

image

Figure 6.  Leaf senescence of transgenic kiwifruit (Actinidia chinensis). Example of leaf appearance at the end of the first growing season on RNAi plant R4 (a) and control plant C1 (b). Bar, 42 mm. (c) Total chlorophyll (Ca+b) in leaves of RNAi or control transgenic plants. Values are means ± SE; = 8 for RNAi and = 6 for control.

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To determine if there is a correlation between the AcCCD8 expression and the phenotype (total branch number and chlorophyll content) of the transgenic plants, a Spearman’s rank correlation coefficient analysis was performed using data from the 14 transgenic kiwifruit plants (Table 2). The analysis confirmed a significant negative correlation between the total number of branches and the relative expression levels of AcCCD8 at each of the three time points after transfer to the glasshouse. A significant negative correlation was also shown between AcCCD8 expression levels at 8 and 13 months and the chlorophyll content in the leaves, and the positive correlation between the chlorophyll content and the branch number in the kiwifruit plants was confirmed (Table 2). Therefore, the reduction of AcCCD8 expression resulted in an increase in branching and a delay in leaf senescence.

Table 2.   Spearman’s rank correlation coefficient and P-values (in brackets) for AcCCD8 expression at three time points (4, 8 or 13 months after transfer of plants to the glasshouse) and phenotype (number of branches and chlorophyll content) of transgenic kiwifruit (Actinidia chinensis) plants
 BranchesChlorophyll
  1. *Significant at P < 0.05, = 13.

AcCCD8 expression
 4 months−0.521 (0.017*)−0.286 (0.084)
 8 months−0.559 (0.012*)−0.621 (0.006*)
 13 months−0.579 (0.010*)−0.571 (0.011*)
 Chlorophyll0.769 (0.001*)

The PCA plot and nonhierarchical clustering analysis (Fig. 4) highlight the RNAi lines that are most dramatically altered in their AcCCD8 expression level and phenotype (R8 and R9) and these lines will be propagated for further investigation.

Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information

Mutants in the CCD7 and CCD8 genes resulting in an increased branching phenotype have been identified in petunia, Arabidopsis, pea and rice, indicating that there is a conserved pathway that controls branch development in both monocots and eudicots. Recently, the reduction of CCD7 expression in tomato has been shown to increase branching (Vogel et al., 2010) and the tomato mutant Sl-ORT1, which is deficient in strigolactones, has been shown to have reduced CCD7 expression and increased branching (Koltai et al., 2010). Genome sequencing from evolutionary diverse species such as the moss P. patens, M. truncatula, black cottonwood P. trichocarpa, and grape V. vinifera has revealed the presence of predicted CCD7 and CCD8 genes. However, proof of function of these genes in the control of branch development is yet to be determined in these species.

We have isolated the CCD7 and CCD8 genes from A. chinensis, a deciduous woody perennial, and have demonstrated that a reduction in expression of AcCCD8 correlates with an increase in branch development. The AcCCD7 and AcCCD8 genes, under the control of the CaMV 35S promoter, were also sufficient to restore the branching of the corresponding Arabidopsis mutants to a wild-type phenotype. This provides functional evidence that the CCD pathway controlling branch development is conserved in a woody perennial even though kiwifruit exhibits quite a different plant form to previously studied species.

The AcCCD8 gene has been mapped to the linkage group 3 using a microsatellite marker (Fraser et al., 2009). In the future it would be worthwhile determining if architectural traits, such as branch number, branch length and bud burst, co-segregate with the marker for AcCCD8.

The expression of the AcCCD7 and AcCCD8 genes predominantly in roots of A. chinensis is consistent with the patterns of expression observed in petunia (Snowden et al., 2005; Drummond et al., 2009) and Arabidopsis (Sorefan et al., 2003; Booker et al., 2004). The significantly lower level of expression detected for AcCCD7 than AcCCD8 in kiwifruit roots is also similar to that found for PhCCD7 in petunia (Drummond et al., 2009). Reciprocal grafting experiments in petunia with the dad1 and dad3 mutants and wild-type plants have shown that both CCD7 and CCD8 are required in the roots for the generation of a signal controlling branching (Simons et al., 2007). Therefore, the co-localization of expression of these genes in kiwifruit roots is reasonable. In rice and pea, the expression of CCD7 and CCD8 is required for the production of strigolactones (Gomez-Roldan et al., 2008; Umehara et al., 2008), which have been shown to be involved in the inhibition of branch development. In tomato plants with reduced CCD7 expression, there was also a reduction in the concentration of strigolactones detected in root exudates (Koltai et al., 2010; Vogel et al., 2010). The isolation and identification of strigolactones have been carried out in a wide range of plants and have revealed a diverse range and mixtures of known and unknown strigolactones (Yoneyama et al., 2008). Strigolactones have been shown to be involved in a range of activities, such as interaction with parasitic plants and arbuscular mycorrhizal fungi, nutrient acquisition and the control of branch development. However, it is yet to be determined if the different strigolactones isolated vary in their specificity (or efficacy) in these different activities.

The expression of the AcCCD7 and AcCCD8 genes in the young fruit and seeds of wild-type A. chinensis suggests that these genes may have a role in fruit and seed development. The effect of reducing AcCCD8 expression on fruit development and flavour will be determined when the transgenic plants reach reproductive maturity.

The reduction of AcCCD8 expression in transgenic A. chinensis plants is correlated with an increase in the total number of branches produced by the plants over two growing seasons. The increase in the total number of branches reflected the large increase in branches < 5 cm in length. Increasing the number of short branches could increase the number of flowers produced on kiwifruit vines by increasing the proportion of nodes with the potential to flower (normally nodes one to nine) (Snowball & Considine, 1986). It will be interesting to determine if there is an increase in the number of flowers and fruit produced on the AcCCD8 RNAi plants when they reach flowering maturity. Different Actinidia rootstocks, such as Actinidia hemsleyana, have been shown to increase the percentage bud burst and the number of flowers produced per shoot on the scion (Wang et al., 1994). Low-vigour rootstocks (Actinidia polygama and Actinidia kolomikta) have also increased the proportion of short shoots on the scion and increased the fruit load relative to total leaf area compared with the scions on high-vigour rootstocks (Clearwater et al., 2004, 2006). The mechanism of this effect is not clear, but a recent investigation in cherry has revealed differential expression of genes involved in brassinosteroid signalling, flavonoid metabolism and cell wall biosynthesis between the scions grafted to dwarfing or semivigorous rootstock (Prassinos et al., 2009).

The delayed leaf senescence observed in RNAi plants with reduced levels of CCD8 expression is similar to that seen in the dad1 (ccd8) mutant of petunia (Snowden et al., 2005), showing a conserved link between CCD8 expression, branch development and leaf senescence in both perennial and annual species. Another Arabidopsis branching mutant max2 (Stirnberg et al., 2002) was originally isolated as the ore9 mutant through a screen for delayed onset of age-dependent leaf senescence (Oh et al., 1997) and was also shown to have delayed hormone-induced leaf senescence (Woo et al., 2001). The ORE9 gene was shown to be a member of the F-box leucine-rich repeat gene family and involved in ubiquitin-dependent proteolysis (Woo et al., 2001). A similar relationship between leaf senescence and branch development was observed in the rice tillering dwarf mutant d3, an orthologue of the Arabidopsis mutant max2/ore9 (Yan et al., 2007). However, the pea orthologue rms4 does not display delayed leaf senescence (Johnson et al., 2006), suggesting that the interaction between leaf senescence and branch development is not conserved in all plant species.

An inherent difficulty in the study of the regulation of branching in woody perennial plants is the length of time required for plants to develop branches. Consequently the length of time for screening transgenic plants for altered phenotypes is also an extended process. The first time point (4 months after transfer) selected for screening the transgenic kiwifruit plants for the expression of CCD8 provided an indication that there had been a reduction in the expression in the RNAi lines compared with the control lines; however, it was not until the two later time points that a significant reduction could be shown. An understanding of the timing of expression of CCD8 over the growth of a perennial plant and the relationship between the timing of CCD8 expression and the visible growth of a branch would assist in selecting the best time for screening RNAi transgenic plants as well as providing further insights into the role of CCD8 in the control of branch development.

We have demonstrated that the CCD pathway controlling branch development and leaf senescence observed in model plant systems is also conserved in a woody perennial plant. The effect of reducing root expression of AcCCD8 on fruit production is yet to be determined. However, the ability of low-vigour rootstocks (A. polygama and A. kolomikta) to increase the number of short branches on the scion, and consequently increase the fruit load (Clearwater et al., 2004, 2006), suggests that this aspect of plant development will be worth investigating further when the plants reach reproductive maturity.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information

We thank Marcela Martínez-Sánchez for assistance with Arabidopsis transformation, Mark Wohlers for assistance with statistics and Tim Holmes for photography of the kiwifruit leaves. We thank Revel Drummond and Marion Wood for critical reading and discussion of the manuscript.

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  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information
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Supporting Information

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  9. Supporting Information

Fig. S1 Expression and phenotype data for individual transgenic kiwifruit plants.

FilenameFormatSizeDescription
NPH_3394_sm_fS1.pdf89KSupporting info item