In vitro control of floral transition in tomato (Lycopersicon esculentum Mill.), the model plant for autonomously flowering species has been investigated using the late flowering mutant uniflora (uf). Apices collected from truly vegetative plants were cultivated on solid media supplemented with different combinations of growth regulators and chemicals. Several chemical factors implicated in the promotion of floral transition of the uf mutant have been identified: sucrose, cytokinins and nitrogenous nutrients have all to be supplied at optimal concentrations. In contrast, gibberellic acid was found to be inhibitory. These results are discussed in relation to knowledge accumulated on the nature of the flowering signals circulating, at floral transition, in other plants, especially in photoperiodic species. This study suggests that tomato could constitute an adequate model to investigate the genetic and physiological control of floral transition and contribute in unravelling pathways which are constitutively regulating this important step of plant life cycle.
The impact of the season on flowering time and the organization and morphogenesis of the reproductive structures are described in three tomato mutants: compound inflorescence (s), single flower truss (sft), and jointless (j), respectively, compared with their wild-type cultivars Ailsa Craig (AC), Platense (Pl), and Heinz (Hz). In all environmental conditions, the sft mutant flowered significantly later than its corresponding Pl cultivar while flowering time in j was only marginally, but consistently, delayed compared with Hz. The SFT gene and, to a lesser extent, the J gene thus appear to be constitutive flowering promoters. Flowering in s was delayed in winter but not in summer compared with the AC cultivar, suggesting the existence of an environmentally regulated pathway for the control of floral transition. The reproductive structure of tomato is a raceme-like inflorescence and genes regulating its morphogenesis may thus be divided into inflorescence and floral meristem identity genes as in Arabidopsis. The s mutant developed highly branched inflorescences bearing up to 200 flowers due to the conversion of floral meristems into inflorescence meristems. The S gene appears to be a floral meristem identity gene. Both sft and j mutants formed reproductive structures containing flowers and leaves and reverting to a vegetative sympodial growth. The SFT gene appears to regulate the identity of the inflorescence meristem of tomato and is also involved, along with the J gene, in the maintenance of this identity, preventing reversion to a vegetative identity. These results are discussed in relation to knowledge accumulated in Arabidopsis and to domestication processes.
• Flowering of uniflora (uf), a tomato (Lycopersicon esculentum) mutant which consistently produces solitary flowers instead of inflorescences, is late and highly asynchronous in winter. This puzzling behaviour prompted us to further investigate flowering regulation in this mutant to improve our understanding of UNIFLORA gene function. • Growing plants under different daylengths and light intensities revealed that flowering time in uf is dependent on daily light energy integral. Transferring plants from low to high light energy integrals at different times after sowing showed that the light-conditions effect was stage dependent, suggesting that interactions between light energy integrals and endogenous regulatory pathways affect meristem sensitivity to flowering signals. • Carbohydrate analyses suggested that one of these signals could be sucrose, but other interacting factors are probably generated by the root system, as indicated by grafting experiments. • The UNIFLORA gene thus appears to have a dual role in tomato: floral transition regulation and the maintenance of inflorescence meristem identity.
The plasma membrane H+-ATPase builds up a pH and potential gradient across the plasma membrane, thus activating a series of secondary ion and metabolite transporters. pma4 (for plasma membrane H+-ATPase 4), the most widely expressed H+-ATPase isogene in Nicotiana plumbaginifolia, was overexpressed in tobacco. Plants that overexpressed PMA4 showed no major changes in plant growth under normal conditions. However, two transformants were identified by their stunted growth, slow leaf initiation, delayed stem bolting and flowering, and male sterility. Protein gel blot analysis showed that expression of the endogenous and transgenic pma4 was cosuppressed. Cosuppression was developmentally regulated because PMA4 was still present in developing leaves but was not detected in mature leaves. The glucose and fructose content increased threefold, whereas the sucrose content remained unchanged. The rate of sucrose exudation from mature leaves was reduced threefold and the sugar content of apical buds was reduced twofold, suggesting failure of sucrose loading and translocation to the sink tissues. Cosuppression of PMA4 also affected the guard cells, stomatal opening, and photosynthesis in mature leaves. These results show that a single H+-ATPase isoform plays a major role in several transport-dependent physiological processes.
• Different tomato (Solanum lycopersicum) mutants, affected in flowering time, reproductive structure or plant architecture, were crossed to produce double mutants in order to investigate gene interactions in flowering regulation in this autonomous species with a sympodial growth habit. • The compound inflorescence: uniflora, uniflora: self pruning, uniflora: blind, and jointless: uniflora double mutants all produced solitary flowers like their uniflora parent, instead of inflorescences. • All double mutants were late flowering. uniflora: blind and uniflora: self pruning had flowering times intermediate between those of their two parents. jointless: uniflora and compound inflorescence: uniflora flowered later than uniflora, the mutant with the most delayed flowering. All double mutants developed strong lateral shoots at node levels approximately corresponding to the level at which their parent cultivars initiated their first reproductive structure, which is a typical trait of uniflora. • These results suggest that the UNIFLORA gene acts upstream of the other investigated genes in controlling flowering in tomato, and that floral transition of the primary shoot and floral transition of sympodial segments are regulated differently.
