ArabidopsisABA INSENSITIVE4 Regulates Lipid Mobilization in the Embryo and Reveals Repression of Seed Germination by the Endosperm

Abstract

Regulation of seed germination requires coordinate action by the embryo and surrounding endosperm. We used Arabidopsis thaliana to establish the relative roles of embryo and endosperm in the control of seed germination and seedling establishment. We previously showed that endospermic oil reserves are used postgerminatively via gluconeogenesis to fuel seedling establishment and that lipid breakdown is repressed by abscisic acid (ABA) in embryo but not endosperm tissues. Here, we use RNA amplification to describe the transcriptome of the endosperm and compare the hormone responses of endosperm and embryo tissues. We show that the endosperm responds to both ABA and gibberellin but that ABA in particular regulates nuclear but not plastid-encoded photosynthetic gene expression in the embryo. We also show that ABA INSENSITIVE4 (ABI4) expression is confined to the embryo, accounts for the major differences in embryo response to ABA, and defines a role for ABI4 as a repressor of lipid breakdown. Furthermore, ABI5 expression in the endosperm defines a second region of altered ABA signaling in the micropylar endosperm cap. Finally, embryo and endosperm ABA signaling mutants demonstrate the spatial specificity of ABA action in seed germination. We conclude that the single cell endosperm layer plays an active role in the regulation of seed germination in Arabidopsis.

The Putative E3 Ubiquitin Ligase ECERIFERUM9 Regulates Abscisic Acid Biosynthesis and Response during Seed Germination and Postgermination Growth in Arabidopsis

“…The transcription factors ABI3, ABI4, and ABI5 are well known positive regulators of the ABA responses during seed germination (Finkelstein and Lynch, 2000;Lopez-Molina et al, 2002;Penfield et al, 2006). Our study showed that the expression levels of ABI3, ABI4, and ABI5 were significantly increased in the cer9 mutant.…”

“…We also noted changes in the levels for proteins involved in lipid metabolism. Lipid is an important storage reserve, providing energy for radicle protrusion and seedling growth, and lipid mobilization is affected by ABA treatment during seed germination (Penfield et al, 2006). Previous reports indicate that oleosin proteins are involved in seed germination, enlargement of intracellular oil bodies, and total lipid and protein accumulation (Siloto et al, 2006;Shimada et al, 2008).…”

“…This result indicates that the dominant mutation of abi1-1 could completely block ABA signaling in the cer9 mutant. The ABI3, ABI4, and ABI5 transcription factors positively regulate the expression of certain ABA-responsive genes in seeds (Finkelstein and Lynch, 2000;Lopez-Molina et al, 2002;Penfield et al, 2006). ABI3 acts upstream of ABI5, and both are located downstream of ABI1 (LopezMolina et al, 2002).…”

ABI4 Activates DGAT1 Expression in Arabidopsis Seedlings during Nitrogen Deficiency

“…ABI4 encodes an Activator Protein-2/Ethylene-Responsive Factor transcription factor that binds the CE1-like element (CACCG) present in many ABA-and sugarresponsive promoters (Finkelstein et al, 1998;Niu et al, 2002;Acevedo-Hernández et al, 2005). ABI4 is a crucial determinant of ABA sensitivity during TAG breakdown in the embryo (Penfield et al, 2006). ABI3 and ABI5, which encode B3 and basic-region Leu zipper-type transcription factors, respectively, participate in the repression of TAG degradation by close association with the N-end rule components PRT6 and ATE (Holman et al, 2009).…”

The N-end rule pathway promotes seed germination and establishment through removal of ABA sensitivity in Arabidopsis

“…In particular, the transcription factors ABI3, ABI4, and ABI5 enhance sensitivity to ABA (12). ABI3 has been shown to act upstream of ABI5 (16), and ABI4 has been implicated in the repression of seedling lipid degradation (17). The mechanisms through which ABA desensitization occurs are not well understood.…”