The zinc finger transcription factor CONSTANS has a well-established central role in the mechanism for photoperiod sensing in Arabidopsis, integrating light and circadian clock signals to upregulate the florigen gene FT under long-day but not short-day conditions. Although CONSTANS-LIKE (COL) genes in other species have also been shown to regulate flowering time, it is not clear how widely this central role in photoperiod sensing is conserved. Legumes are a major plant group and various legume species show significant natural variation for photoperiod responsive flowering. Orthologs of several Arabidopsis genes have been shown to participate in photoperiodic flowering in legumes, but the possible function of COL genes as integrators of the photoperiod response has not yet been examined in detail. Here we characterize the COL family in the temperate long-day legume Medicago truncatula, using expression analyses, reverse genetics, transient activation assays and Arabidopsis transformation. Our results provide several lines of evidence suggesting that COL genes are unlikely to have a central role in the photoperiod response mechanism in this species.

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The stay-green trait is recognised as a key drought adaptation mechanism in cereals worldwide. Stay-green sorghum plants exhibit delayed senescence of leaves and stems, leading to prolonged growth, a reduced risk of lodging and higher grain yield under end-of-season drought stress. More than 45 quantitative trait loci (QTL) associated with stay-green have been identified, including two major QTL (Stg1 and Stg2). However, none of the contributing genes that regulate functional stay-green are known. Here we show that the PIN FORMED family of auxin efflux carriers induce some of the causal mechanisms driving the stay-green phenotype in sorghum, with SbPIN4 and SbPIN2 located in Stg1 and Stg2, respectively. We found that 9 of 11 sorghum PIN genes aligned with known stay-green QTL. In transgenic studies, we demonstrated that PIN genes located within the Stg1 (SbPIN4), Stg2 (SbPIN2) and Stg3b (SbPIN1) QTL regions acted pleiotropically to modulate canopy development, root architecture and panicle growth in sorghum, with SbPIN1, SbPIN2 and SbPIN4 differentially expressed in various organs relative to the non stay-green control. The emergent consequence of such modifications in canopy and root architecture is a stay-green phenotype. Crop simulation modelling shows that the SbPIN2 phenotype can increase grain yield under drought.