Shade delays flowering in Medicago sativa.

Shade-intolerant plants respond to the decrease in the red (R) to far-red (FR) light ratio (R:FR) occurring under shade by elongating stems and petioles and by re-positioning leaves, in a race to outcompete neighbors for the sunlight resource. In some annual species, the shade avoidance syndrome (SAS) is accompanied by the early induction of flowering. Anticipated flowering is viewed as a strategy to set seeds before the resources become severely limiting. Little is known about the molecular mechanisms of SAS in perennial forage crops like alfalfa (Medicago sativa). To study SAS in alfalfa, we exposed alfalfa plants to simulated shade by supplementing with FR light. Low R:FR light produced a classical SAS, with increased internode and petiole lengths, but unexpectedly also with delayed flowering. To understand the molecular mechanisms involved in uncoupling SAS from early flowering, we used a transcriptomic approach. The SAS is likely to be mediated by increased expression of msPIF3 and msHB2 in low R:FR light. Constitutive expression of these genes in Arabidopsis led to SAS, including early flowering, strongly suggesting that their roles are conserved. Delayed flowering was likely to be mediated by the downregulation of msSPL3, which promotes flowering in both Arabidopsis and alfalfa. Shade-delayed flowering in alfalfa may be important to extend the vegetative phase under suboptimal light conditions, and thus assure the accumulation of reserves necessary to resume growth after the next season.

Hahb-4, a sunflower homeobox-leucine zipper gene, is a developmental regulator and confers drought tolerance to Arabidopsis thaliana plants.

Homeodomain-leucine zipper proteins constitute a family of transcription factors found only in plants. Hahb-4 is a member of Helianthus annuus (sunflower) subfamily I. It is regulated at the transcriptional level by water availability and abscisic acid. In order to establish if this gene plays a functional role in drought responses, transgenic Arabidopsis thaliana plants that overexpress Hahb-4 under the control of the 35S Cauliflower Mosaic Virus promoter were obtained. Transformed plants show a specific phenotype: they develop shorter stems and internodes, rounder leaves and more compact inflorescences than their non-transformed counterparts. Shorter stems and internodes are due to a lower rate in cell elongation rather than to a stop in cell division. Transgenic plants were more tolerant to water stress conditions, showing improved development, a healthier appearance and higher survival rates than wild-type plants. Indeed, either under normal or drought conditions, they produce approximately the same seed weight per plant as wild-type plants under normal growth conditions. Plants transformed with a construct that bears the Hahb-4 promoter fused to gusA show reporter gene expression in defined cell-types and developmental stages and are induced by drought and abscisic acid. Since Hahb-4 is a transcription factor, we propose that it may participate in the regulation of the expression of genes involved in developmental responses of plants to desiccation.