Interplay between positive and negative regulation by B3-type transcription factors is critical for the accurate expression of the ABA INSENSITIVE 4 gene.

The ABA-INSENSITIVE 4 transcription factor is key for the regulation of diverse aspects of plant development and environmental responses, including proper perception of hormonal and nutritional signals. ABI4 activity is highly regulated at the transcriptional and post-transcriptional levels leading to precise expression mainly in the developing seed and early seedling development. Based on genetic and molecular approaches in the current study we provide new insights into the central mechanism underpinning the transcriptional regulation of ABI4 during both seed and vegetative development. We identified a complex interplay between the LEC2 and ABI3 transcriptional activators and the HSI/VAL repressors that is critical for proper ABI4 expression. Interestingly, the regulation by these proteins relies on the two RY cis-acting motifs present two kb upstream of the ABI4 gene. Our analysis also shows that the chromatin landscape of the ABI4 loci is highly dependent on the LEC2 and HSI2/VAL proteins. LEC2 regulation extends to the vegetative development and the absence of this factor results in ABA- and sugar-insensitive signaling in the developing plant. This regulatory circuit functions as a major control module for the correct spatial-temporal expression of ABI4 and prevents its ectopic accumulation that is harmful to the plant.