CONICET | Buscador de Institutos y Recursos Humanos

Hahb-4 belongs to the sunflower subfamily I of HD-Zip proteins and is involved in drought tolerance response and ethylene mediated senescence. A cross-talk between these two processes by the action of this transcription factor was recently described. Here we report that the expression of Hahb-4 is strongly induced in darkness and postulate that its function is to down-regulate the biogenesis of the photosynthetic machinery. Activation by darkness is immediately reverted when sunflower plants are illuminated. This transcriptional regulation was corroborated obtaining transgenic Arabidopsis plants bearing constructs in which different segments of the Hahb-4 promoter including deletions, point mutations and chimeras were fused with the reporter gene gus. These experiments together with EMSA done with sunflower nuclear proteins allowed us to localize cis-acting elements responsible for dark response. At least one co-activator and one co-repressor of a GT-1 like protein interact with this segment to achieve dark/light response. Microarray analysis done on transgenic plants overexpressing the Hahb-4 cDNA led us to observe a down-regulation of a considerable group of genes related to chloroplast components biogenesis. Accordingly, chlorophyll content and CO2 fixation are diminished in Arabidopsis transgenic plants. Transient transformation of sunflower leaves allowed us to propose a role for this gene as a down-regulator of photosynthetic proteins. We conclude that this transcription factor acts as a negative switch of the photosynthetic machinery in order to save energy during the night, what consequently reduces the photoxidative stress in drought conditions.