The Arabidopsis ABA-INSENSITIVE (ABI) 4 factor acts as a central transcription activator of the expression of its own gene, and for the induction of ABI5 and SBE2.2 genes during sugar signaling.

The transcription factor ABA INSENSITIVE 4 (ABI4), discovered nearly 10 years ago, plays a central role in a variety of functions in plants, including sugar responses. However, not until very recently has its mechanism of action begun to be elucidated. Modulating gene expression is one of the primary mechanisms of sugar regulation in plants. Nevertheless, the transcription factors involved in regulating sugar responses and their role(s) during the signal transduction cascade remain poorly defined. In this paper we analyzed the participation of ABI4, as it is one of the main transcription factors implicated in glucose signaling during early seedling development. Our studies show that ABI4 is an essential activator of its own expression during development, in ABA signaling and in sugar responses. It is also important for the glucose-mediated expression of the genes ABI5 and SBE2.2. We demonstrate that ABI4 binds directly to the promoter region of all three genes and activates their expression in vivo through at CE1-like element. Previous studies found that ABI4 also functions as a transcriptional repressor of sugar-regulated genes, therefore this transcription factor is a versatile protein with dual functions for modulating gene expression.

WRINKLED1 specifies the regulatory action of LEAFY COTYLEDON2 towards fatty acid metabolism during seed maturation in Arabidopsis.

The WRINKLED1 (WRI1) transcription factor has been shown to play a role of the utmost importance during oil accumulation in maturing seeds of Arabidopsis thaliana. However, little is known about the regulatory processes involved. In this paper, comprehensive functional analyses of three new mutants corresponding to null alleles of wri1 confirm that the induction of WRI1 is a prerequisite for fatty acid synthesis and is important for normal embryo development. The strong expression of WRI1 specifically detected at the onset of the maturation phase in oil-accumulating tissues of A. thaliana seeds is fully consistent with this function. Complementation experiments carried out with various seed-specific promoters emphasized the importance of a tight regulation of WRI1 expression for proper oil accumulation, raising the question of the factors controlling WRI1 transcription. Interestingly, molecular and genetic analyses using an inducible system demonstrated that WRI1 is a target of LEAFY COTYLEDON2 and is necessary for the regulatory action of LEC2 towards fatty acid metabolism. In addition to this, quantitative RT-PCR experiments suggested that several genes encoding enzymes of late glycolysis, the fatty acid synthesis pathway, and the biotin and lipoic acid biosynthetic pathways are targets of WRI1. Taken together, these results indicate new relationships in the regulatory model for the control of oil synthesis in maturing A. thaliana seeds. In addition, they exemplify how metabolic and developmental processes affecting the developing embryo can be coordinated at the molecular level.