UNIFLORA and the regulation of flowering transition in tomato
Investigating flowering time in uf is not straightforward given the variability of response between experiments, although every effort was made to reproduce experimental conditions. This behaviour was more obvious when plants were grown under conditions that were neither fully favourable nor fully detrimental to flowering. This inconsistency of the absolute flowering response, which has been repeatedly observed from our early work on this mutant (Dielen et al., 1998, 2001), suggests that UNIFLORA is a pivotal gene regulating floral transition in tomato. When mutated, passing the step controlled by UF becomes dependent on environmental parameters, and subtle variations in culture conditions, which are difficult to control and, hence, to reproduce from one experiment to another, may be critical to convey the decision whether an individual will remain vegetative or not. That UNIFLORA may be playing a critical role during floral transition is supported by the observation that, in plants that remained vegetative, lateral branches were developing at node levels where flowering normally occur on wild-type plant (AC cultivar). Release from apical dominance is known to be an early and essential event, always associated with floral evocation (Bernier et al., 1981). Its occurrence in the uf mutant strongly suggests that vegetative plants underwent a partial evocation but were unable to complete the process. Afterwards, the shoot apical meristem returned to a vegetative functioning and apical dominance was re-established as axillary outgrowth was inhibited at upper nodes (Fig. 2). Thus, the shoot apical meristem of uf plants appears to have the capacity to swing between successive vegetative and partly florally evoked states.
Discontinued light experiments clearly revealed that, during an extended period of vegetative growth, uf plants underwent changes in their developmental programme although maintaining a monopodial mode of growth. These changes affected the competence of the meristem to respond to signals generated by external cues, such as the level of daily light energy integral. This observation is reminiscent of results obtained in works showing that flowering of many plants has distinct phases of sensitivity to the environment, especially to the photoperiod (Ellis et al., 1992; Adams et al., 2001). In many species, seedlings are incapable of responding to inductive conditions, exhibiting a so-called ‘juvenile’ phase. When this initial phase is completed, plants become capable of responding to inductive conditions and are considered to be ‘ripe to flower’, a condition that is generally stable (Thomas & Vince-Prue, 1984). Interestingly, the situation in uf is different as young plants are first sensitive to light conditions promoting flowering and enter thereafter into a phase of reduced sensitivity. According to Pnueli et al., 1998), the gene SELF PRUNING acts, in indeterminate tomatoes, as part of a system which prevents early flowering in each of the de novo developing sympodial shoot meristems. This system should be downregulated in a step-like manner, with each internode of every new sympodial segment, to allow transition to flowering after having initiated a limited number of leaves. The question thus arises as to whether SELF PRUNING is implicated in the alternation of phases of higher and lower sensitivity to light conditions, exhibited by the monopodial shoot of uf.
Measurement of sucrose content in the apical exudates indicated that carbohydrate supply to the apex was lower in uf than in AC plants. This reduced sugar availability appeared to be independent of the physiological stage of the uf plants, suggesting that their apical bud is permanently undersupplied compared with their wild-type counterpart. This finding, associated with the fact that flowering is strongly delayed in uf plants, could suggest that sucrose is one of the signals implicated in the regulation of floral transition in tomato and that daily light energy integral may interact with the endogenous developmental programme through the supply of adequate levels of sugars to the meristem. That sucrose is required for flowering in tomato was suggested by in vitro studies (Dielen et al., 2001) and by work with transgenic plants which have an increased capacity for sucrose synthesis (Micallef et al., 1995).
We showed also that floral transition in uf was effectively triggered by grafting as flowering occurred in all grafted plants despite the fact that light conditions were unfavourable, as revealed by the poor response of intact control uf plants. The mechanisms by which grafting stimulated flowering under unfavourable light conditions are far from clear. Grafting is considered to be a valuable tool to investigate long distance signalling pathways (Turnbull et al., 2002). Our results suggest that a root signal may be affecting the fate of the shoot meristem and that the efficiency of this signal is not dependent on the stock genotype. The stimulatory effect of graft per se, could thus be caused by the fact that it brings the shoot apical meristem closer to the root system, a situation at odds with what was reported for Nicotiana tabacum, where the roots sustained vegetative growth when in close proximity to the apical meristem (McDaniel, 1996). However, a positive effect of roots upon floral transition in tomato was also suggested by previous experiments on uf, using topping and cutting treatments (Dielen et al., 1998), and by the observation that plants that were growing vegetatively for a long period of time before being submitted to light conditions conducive to flowering, produced flowers later than young individuals (V. Dielen & J. M. Kinet, unpubl. data). It is tempting to envisage that the potential stimulatory role of the root system, as revealed by grafting experiments, could be mediated by cytokinins as cytokinins are required for in vitro flowering of tomato (Dielen et al., 2001).
UNIFLORA and the regulation of reproductive morphogenesis in tomato
Throughout the years we have submitted uf plants to various growing conditions and have not found a single treatment capable of modifying the single-flower phenotype of this mutant, which is thus remarkably stable. Furthermore, work in progress in our laboratory aiming at generating double mutants suggests that UNIFLORA may be acting during floral transition upstream of genes affecting the structure of the tomato inflorescence, such as COMPOUND INFLORESCENCE (S) or BLIND (BL). Earlier observations (Dielen et al., 1998) led us to postulate, in agreement with Allen and Sussex (1996), that the reproductive structure of tomato is a raceme-like inflorescence; consequently, the uf mutation may be affecting a gene that controls inflorescence meristem identity. TERMINAL FLOWER (TFL) and CENTRORADIALIS (CEN) are genes that control the identity of the inflorescence meristem in Arabidopsis thaliana and in Antirrhinum majus, respectively (Bradley et al., 1997). Their orthologue in tomato is SELF PRUNING (Pnueli et al., 1998), a gene that is distinct from UNIFLORA as revealed by the phenotype of the F1 progeny of a cross between self pruning and uf mutants (M. Quinet et al. unpubl. data). Thus, the precise function of UNIFLORA and the way it interacts with other genes await further studies.
UNIFLORA, a key gene regulating flowering in tomato
To date, all the information at our disposal coming from segregation analyses in F2 populations issued from crosses between different mutants and backcrosses to wild-type plants indicate that the uf phenotypes are under the control of a single recessive mutation (M. Quinet et al. unpubl. data). The UF gene thus appears to act in both the promotion of floral transition and the regulation of reproductive morphogenesis in tomato. It appears to be involved in the control of inflorescence meristem identity, operating upstream of various genes regulating inflorescence edification. The gene UNIFLORA has not yet been mapped in the tomato genome. Cloning and sequencing this pivotal gene will allow an in-depth investigation of its functions and improve our knowledge of the genetic and molecular control of flowering in tomato.