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Drought is the most significant abiotic stress affecting plant growth and limiting crop yields (Jones & Corlett, 1992; Beltrano et al., 1999; Jiang & Zhang, 2004). In cereals such as wheat (Triticum aestivum), stress caused by drought at the time of grain-filling usually shortens the grain-filling period and reduces the grain-filling rate, which leads to a reduction in grain yield (Aggarwal & Sinha, 1984; Nicolas et al., 1985; Kobata et al., 1992; Zhang et al., 1998). This process is suggested to be mediated by hormones and their interactions in plants (Brenner & Cheikh, 1995; Sharp & LeNoble, 2002; Yang & Zhang, 2006).
Ethylene and abscisic acid (ABA) are two of the major phytohormones induced in response to stress (Davies & Zhang, 1991; Cheng & Lur, 1996; Gazzarrini & McCourt, 2001; Wilkinson & Davies, 2002; Davies, 2004). Various types of stresses have been reported to promote ethylene production in different tissues of a number of plant species (Narayana et al., 1991; Morgan & Drew, 1997). An overproduction of ethylene induced by drought has frequently been related to fruit abortion in cotton (Gossypium hirsutum) (Guinn, 1976) and grain weight reduction in wheat (Xu et al., 1995; Beltrano et al., 1999) and in maize (Zea Mays) (Chen & Lur, 1996). Although water deficit is one of the most commonly encountered stresses causing an increase in the release of ethylene, there are many reports that drought stress reduces, rather than increases, ethylene production (e.g. Morgan & Drew, 1997).
The concentration of ABA, which is generally regarded as an inhibitory growth hormone (Walton, 1980; Trewavas & Jones, 1991), increases markedly in leaves (Ober & Setter, 1990; Westgate et al., 1996), floral organs (Saini & Aspinall, 1982; Lee et al., 1988; Ober et al., 1991), and developing grains (Goldbach & Goldbach, 1977; Ober et al., 1991; Ahmadi & Baker, 1999; Yang et al., 2001) in responses to soil drying. Water stress-induced reductions in grain set and kernel growth in wheat (Morgan, 1980; Saini & Aspinall, 1982; Ahmadi & Baker, 1999) and a decreased rate of endosperm cell division in water-stressed maize (Myers et al., 1990; Ober et al., 1991) have been observed to be associated with elevated concentrations of ABA. It has frequently been observed, however, that ABA can promote dry matter accumulation in the sink organ and that ABA concentration is correlated with the growth rate of fruits or seeds (e.g. Eeuwens & Schwabe, 1975; Browning, 1980; Berüter, 1983; Schussler et al., 1984, 1991; Wang et al., 1987; Ross & McWha, 1990; Kato et al., 1993; Wang et al., 1998; Yang et al., 2001). Recent studies on maize (Spollen et al., 2000), tomato (Lycopersicon esculentum) (Hussain et al., 2000; Sharp et al., 2000; Sharp & LeNoble, 2002; Sharp, 2002), and rice (Oryza sativa) (Yang et al., 2004b) indicate that an increased concentration of ABA is necessary to prevent excess ethylene production under water stress, and that as a result of this interaction ABA may often function to maintain, rather than inhibit, appropriate plant growth.
Our early work (Yang et al., 2000; Yang & Zhang, 2006) showed that mild soil drying imposed during the grain-filling period in wheat can increase carbon remobilization from vegetative tissues to grains and accelerate the grain-filling rate. However, little is known about whether and how ABA and ethylene are involved in these processes. The purpose of this study was to test the hypothesis that the interaction between ABA and ethylene is involved in mediating the effects of soil drying on grain filling. The changing patterns of ABA and ethylene concentrations in wheat grains subjected to soil drying during grain filling and their relations with grain-filling rate were investigated. The effects of chemical regulators on the concentrations of ABA and ethylene in the grains were also studied to verify the roles of the two hormones.
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Few previous studies have described the interaction between ethylene and ABA in superior and inferior grains of wheat and the relationship between this interaction and the grain-filling rate. Here, we found that the grain-filling rates of both superior and inferior grains, under either the WW or the MD treatment, were closely associated with ABA concentrations in grains (Figs 2b, 3a, 5a). The superior grains had higher ABA concentrations and higher grain-filling rates than the inferior grains. The MD treatment increased the grain-filing rate in both superior and inferior grains, and this increase was accompanied by an increase in ABA concentration in these grains. Application of ABA to WW spikes significantly increased the ABA concentration in grains, and grain-filling rates and grain weights were significantly increased for both superior and inferior grains. ABA applied to SD spikes also significantly increased the grain-filling rate and grain weight of the inferior grains. Application of fluridone, an inhibitor of ABA biosynthesis, had the opposite effect (Tables 1, 2, 3), indicating that a higher concentration of ABA is required to maintain a faster grain-filling rate, consistent with reports that normal concentrations of endogenous ABA are required to maintain appropriate shoot and root growth in well-watered and water-stressed tomato plants (Sharp et al., 2000; LeNoble et al., 2004).
The mechanism by which ABA facilitates grain filling is not understood. It has been proposed that ABA has a major role in relation to sugar-signaling pathways and enhances the ability of plant tissues to respond to subsequent sugar signals (Rook et al., 2001; Davies, 2004). There are many reports that ABA can enhance the movement of photosynthetic assimilates towards to developing seeds (Dewdney & McWha, 1979; Ackerson, 1985; Brenner & Cheikh, 1995). We found that the activities of three key enzymes involved in the sucrose-to-starch pathway in the grains, SuSase, AGPase and SSSase (Hawker & Jenner, 1993; Ahmadi & Baker, 2001; Hurkman et al., 2003), were significantly enhanced by application of ABA, but substantially reduced by application of fluridone, to WW spikes at the early grain-filling stage (Table 2). This result suggests that ABA may promote grain filling through an increase of sink activity by regulation of the key enzymes involved.
We found that, in contrast to ABA, ethylene concentration in grains was rather high at the early grain-filling stage (Fig. 3b), and its temporal pattern was opposite to that of ABA under WW and MD treatments. The ethylene evolution rate was correlated with the grain-filling rate, the relationship being one of exponential decay (Fig. 5b). Application of an inhibitor of ethylene synthesis (cobalt ion) increased, while application of an ethylene-releasing agent (ethephon) reduced, the activities of SuSase, AGPase and SSSase in grains, the grain-filling rate and grain weight (Tables 2, 3). These results suggest that, in contrast to ABA, ethylene plays a role in inhibiting grain filling, consistent with speculation that ethylene may be a negative regulator of ABA action in the seed (Ghassemian et al., 2000).
The effect of water stress on ethylene production has remained a matter of debate (Morgan & Drew, 1997). Our results showed that the concentrations of ethylene and ACC in grains were reduced under the MD treatment and greatly increased under the SD treatment (Fig. 3b,c), indicating that the production of ethylene in wheat grains may depend on the severity and duration of soil drying. Our data also demonstrated that the temporal pattern of ACC concentration was very similar to that of ethylene (Fig. 3c), and that they were very significantly corrected, suggesting that the increase in ethylene production may be attributable to the increase in ACC concentration in grains under the SD treatment.
We found that the ABA concentration was greatly increased, whereas the grain-filling rate was decreased, under the SD treatment (Figs 2b, 3a). A probable explanation is that ethylene production outperforms ABA accumulation under such conditions. Davies (1995) proposed that plant hormones can act either synergistically or antagonistically and it is the balance between promoting and inhibiting agents that ultimately determines the path of plant growth and development. Our results showed that the grain-filling rate was not only correlated with the concentrations of ABA and ethylene, but also correlated with the ratio of ABA to ACC (Fig. 5c). Under the SD treatment, concentrations of both ABA and ethylene were increased in the grains (Fig. 3a,b). However, the ratio of ABA to ACC was greatly reduced (Fig. 4), suggesting that soil drying had a greater effect on ethylene release than on ABA accumulation in these grains. When these spikes were treated with cobalt ion, ethylene production was reduced and grain-filling rate and grain weight were increased. In contrast, grain-filling rate and grain weight were decreased when application of fluridone resulted in a reduction in ABA and an increase in ethylene production (Table 1). On the basis of these results, we speculate that the reduction in the grain-filling rate and grain weight under the SD treatment was mainly attributable to an increase in ethylene production, and that antagonistic interactions between ABA and ethylene mediate the effects of soil drying on grain-filling rate in wheat.
It is noteworthy that the correlation between ABA concentration and grain-filling rate showed a polynomial relationship (Fig. 5a), and the regression between the ratio of ABA to ACC and the grain-filling rate exhibited a hyperbolic curve (Fig. 5c). Such curves suggest that the maximum grain-filling rate (3.4–3.50 mg per kernel d−1) is achieved only when the ABA concentration in grains is 3.41 nmol g−1 DW and the ratio of ABA to ACC is 0.13. This result indicates that a higher ABA concentration and a higher ratio of ABA to ACC are necessary to increase the grain-filling rate. Such enhancement would be most effective when the ABA concentration is low or the ethylene (ACC) concentration is high. However, when soil drying is too severe, a large increase in water stress-induced ABA and consequently a very high ABA concentration and a very high ABA to ACC ratio will not produce a high grain-filling rate. Other factors, such as inhibited photosynthesis and phloem translocation, might limit grain filling. In addition, very severe soil drying and very large amounts of ABA may have an adverse effect on grain filling as a result of a shortened grain-filling period.
In conclusion, the slower grain-filling rate of the inferior grains compared with the superior grains was associated with a lower ABA concentration and higher concentrations of ethylene and ACC in the grains. It was found that moderate soil drying imposed during the grain-filling period can accelerate the grain-filling rate. The accelerated grain-filling rate was attributable, at least in part, to an increased ABA concentration and decreased ethylene production in the grains. Under severe soil drying, a greater increase in ethylene production than in ABA accumulation contributed to a reduction in the grain-filling rate and grain weight. A higher ratio of ABA to ethylene in wheat grains is required to increase the grain-filling rate. Further investigation is needed to confirm the interactions between ABA and ethylene using mutant or transgenic plants with an attenuated capacity to respond to or synthesize plant hormones or with targeted down-regulation of hormone action.