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Molecular mechanisms of plant reproductive adaptation to aridity gradients

Annual plants have developed a range of different mechanisms to avoid flowering (exposure of reproductive organs to the environment) under adverse environmental conditions. Seasonal environmental events such as gradual changes in day length and temperature affect the timing of transition to flowering in many annual and perennial plants. Research in Arabidopsis and additional species suggest that some environmental signals converge on transcriptional regulation of common floral integrators such as FLOWERING LOCUS T (FT). Here we studied environmental induction of flowering in the model legume Medicago truncatula. Similarly to Arabidopsis, the transition to flowering in M. truncatula is hastened by long photoperiods and long periods of vernalization (4°C for 2-3 weeks). Ecotypes collected in Israel retain a vernalization response even though winter temperatures are way above 4°C. Here we show that this species is also highly responsive (flowers earlier) to mild ambient temperatures up to 19°C simulating winter conditions in its natural habitat. Physiological experiments allowed us to time the transition to flowering due to low temperatures, and to compare it to vernalization. We have made use of natural variation, and induced mutants to identify key genes involved in this process, and we provide here data suggesting that an FT gene in M.truncatula is transcriptionally regulated by different environmental cues. Flowering time was found to be correlated with MtFTA and MtFTB expression levels. Mutation in the MtFTA gene showed a late flowering phenotype, while over-expressing MtFTA in Arabidopsis complemented the ft- phenotype. We found that combination of 4°C and 12°C resulted in a synergistic increase in MtFTB expression, while combining 4°C and long photoperiods caused a synergistic increase in MtFTA expression. These results suggest that the two vernalization temperatures work through distinct mechanisms. The early flowering kalil mutant expressed higher levels of MtFTA and not MtFTB suggesting that the KALIL protein represses MtFTA specifically. The desert ecotype Sde Boker flowers earlier in response to short treatments of 8-12oc vernalization and expresses higher levels of MtFTA. This suggests a possible mechanism this desert ecotype developed to flower as fast as possible and finish its growth cycle before the dry period. MtFTA and FT expression are induced by common environmental cues in each species, and expression is repressed under short days. Replacing FT with the MtFTA gene (including regulatory elements) caused high MtFTA expression and early flowering under short days suggesting that the mechanism used to repress flowering under short days has diversified between the two species.The circadian regulated gene, GIGANTEA (GI) encodes a unique protein in Arabidopsis that is involved in flowering mechanism. In this research we characterized how the expression of the M.truncatula GI ortholog is regulated by light and temperature in comparison to its regulation in Arabidopsis. In Arabidopsis GI was found to be involved in temperature compensation to the clock. In addition, GI was found to be involved in mediating the effect of temperature on flowering time. We tested the influence of cold temperature on the MtGI gene in M.truncatula and found correlation between MtGI levels and extended periods of 12°C treatment. MtGI elevation that was found mostly after plants were removed from the cold influence preceded the induction of MtFT expression. This data suggests that MtGI might be involved in 12°C cold perception with respect to flowering in M.truncatula. GI seems to integrate diverse environmental inputs and translates them to the proper physiological and developmental outputs, acting through several different pathways. These research enabled to correlate between temperature and circadian clock in M.truncatula and achieved a better understanding of the flowering mechanism of this species.  