A novel transcriptional cascade regulating expression of heat stress proteins during seed development of Arabidopsis

Abstract

Within the Arabidopsis thaliana family of 21 heat stress transcription factors (Hsfs), HsfA9 is exclusively expressed in late stages of seed development. Here, we present evidence that developmental expression of HsfA9 is regulated by the seed-specific transcription factor ABSCISIC ACID-INSENSITIVE3 (ABI3). Intriguingly, ABI3 knockout lines leck detectable levels of HsfA9 transcript and protein, and further ectopic expression at ABI3 conferred the ability to accumulate HsfA9 in response to abscisic acid in transgenic plantlets. Consequently, the most abundant heat stress proteins (Hsps) in seeds (Hsp17.4-CI, Hep17.7-CII, and Hsp101) were not detectable in the AB13 knockout lines, but their expression could be detected in planta ectopically expressing HsfA9 in vegetative tissues. Furthermore, this seed-specific transcription factor cascade was reconstructed in transient β-glucuronidese reporter essays in mesophyll protoplasts by showing that ABI3 could activate the HsfA9 promoter, whereas HafA9 in turn was shown to be a potent activator on the promoters of Hsp genes. Thus, our study establishes a genetic framework in which HafA9 operates as a specialized Hsf for the developmental expression of Hsp genes during seed maturation. © 2007 American Society of Plant Biologists.