Abstract
The capacity to discriminate variations in day length allows plants to align flowering with the most favourable season of the year. This capacity has been altered by artificial selection when cultivated varieties became adapted to environments different from those of initial domestication. Rice flowering is promoted by short days when HEADING DATE 1 (Hd1) and EARLY HEADING DATE 1 (Ehd1) induce the expression of florigenic proteins encoded by HEADING DATE 3a (Hd3a) and RICE FLOWERING LOCUS T 1 (RFT1). Repressors of flowering antagonize such induction under long days, maintaining vegetative growth and delaying flowering. To what extent artificial selection of long day repressor loci has contributed to expand rice cultivation to Europe is currently unclear. This study demonstrates that European varieties activate both Hd3a and RFT1 expression regardless of day length and their induction is caused by loss-of-function mutations at major long day floral repressors. However, their contribution to flowering time control varies between locations. Pyramiding of mutations is frequently observed in European germplasm, but single mutations are sufficient to adapt rice to flower at higher latitudes. Expression of Ehd1 is increased in varieties showing reduced or null Hd1 expression under natural long days, as well as in single hd1 mutants in isogenic backgrounds. These data indicate that loss of repressor genes has been a key strategy to expand rice cultivation to Europe, and that Ehd1 is a central node integrating floral repressive signals.
Highlights
Photoperiod and temperature are crucial cues that plants use to monitor diurnal and seasonal time (Amasino, 2010)
Rice flowering is promoted by short days when HEADING DATE 1 (Hd1) and EARLY HEADING DATE 1 (Ehd1) induce the expression of florigenic proteins encoded by HEADING DATE 3a (Hd3a) and RICE FLOWERING LOCUS T 1 (RFT1)
To address the molecular mechanisms involved, a working panel composed of 16 temperate japonica varieties cultivated in Italy was selected to be representative of the diversity of heading dates observed under natural field conditions (Fig. 1)
Summary
Photoperiod (or day length) and temperature are crucial cues that plants use to monitor diurnal and seasonal time (Amasino, 2010). Rice (Oryza sativa) is a facultative short day (SD) plant in which flowering is accelerated upon exposure to photoperiods falling below a critical threshold, and has been extensively used as model system to understand the genetic and molecular basis of flowering time (Brambilla and Fornara, 2013). Recent molecular data based on re-sequencing of several hundred accessions of cultivated and wild rice, indicate that the centre of rice domestication can be traced to the middle area of.
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