The ABA INSENSITIVE (ABI) 4 transcription factor is stabilized by stress, ABA and phosphorylation

Abstract

SUMMARY The Arabidopsis transcription factor ABSCISIC ACID INSENSITIVE 4 (ABI4) is a key player in the plant hormone abscisic acid (ABA) signaling pathway. ABI4 is also involved in seed development and germination, the response to abiotic stresses such as drought and salinity, control of lipid reserve mobilization in the embryo, lateral root formation, and redox control. Expression of the ABI4 gene is tightly regulated and basal expression is low. Maximal transcript levels occur during seed maturation and in the early stages of seed germination and are markedly reduced in other developmental stages. ABI4 is an unstable lowly expressed protein, resulting from tight post-transcriptional regulation. Here, we studied factors affecting the stability of the ABI4 protein using transgenic Arabidopsis plants expressing 35S::HA-FLAG-ABI4-eGFP . Despite the expression of eGFP-tagged ABI4 being driven by the highly active 35S CaMV promoter the steady-state levels of ABI4 were extremely low in the roots of seedling grown in optimal conditions. These levels were markedly enhanced upon exposure of the seedlings to abiotic stress and ABA. ABI4 is degraded rapidly by the 26S proteasome and we report on the role of phosphorylation of ABI4-serine 114 in regulating ABI4 stability. Our results indicate that ABI4 is tightly regulated both post-transcriptionally and post-translationally. Moreover, abiotic factors and plant hormones have similar effects on ABI4 transcripts and ABI4 protein levels. This double-check mechanism for controlling ABI4 reflects on its central role in plant development and cellular metabolism. SIGNIFICANCE STATEMENT We show that stabilization of the ABI4 transcription factor by stress and hormones is mediated by phosphorylation of Serine 114 by MAP kinases. Transcription of ABI4 is also modulated by MAP kinases, suggesting that the same signals affect both transcript and protein levels, resulting in tight modulation of ABI4 activity.