SHADE MODIFIES PLANT ARCHITECTURE AND DELAYS FLOWERING IN ALFALFA (MEDICAGO SATIVA)

Shade intolerant plants respond to a reduction in the red (R) to far-red light (FR) ratio (R:FR) by elongating stems and petioles, re-positioning leaves and accelerating flowering. These strategies are known collectively as the shade avoidance syndrome (SAS). As of date, not much is known about the physiological and molecular mechanisms of SAS in alfalfa (Medicago sativa), an important perennial forage species. Therefore, we exposed alfalfa plants to simulated shade to analyze morphological changes, coupled with a RNA sequencing analysis to study genes involved in SAS. For RNA seq, two samples were analyzed for both conditions, each comprised of two leaves belonging to independent plants. For statistics analysis Edge R was used for p-value and false discovery rate (FDR) correction. Low R:FR produced a classical SAS, such as increased internode, petiole length and reduced chlorophyll biosynthesis. On the contrary to most shade intolerant species, flowering onset was delayed. In terms of genes involved, the SAS was likely mediated by upregulation of msPIF3 and msHB2 in low R:FR, whose constitutive expression in Arabidopsis thaliana led to a complete SAS phenotype. On the other hand, delayed flowering was likely to be mediated by downregulation of msSPL3.We propose that shade-delayed flowering in alfalfa may be an important response to extend the vegetative phase under sub-optimal light conditions and thus assure an accumulation of reserves to resume growth after the next season.