Function and regulation of ABI4, an integrator of nutritional, stress and hormonal signaling

Function and regulation of ABI4, an integrator and nutritional, stress and hormonal signaling Prof. Dudy Bar-Zvi, Ben-Gurion University , Beer-Sheva, Israel.

Prof. Ruth R Finkelstein, University of California, Santa Barbara , CA, USA.

The transcription factor ABI4 integrates responses to abiotic and nutrient stresses that regulate agronomically important features of plant growth. These processes include seed maturation, germination, lipid mobilization, seedling establishment, root growth and branching, pathogen response, growth control by redox balance, and communication with organelles affecting cellular energy balance. However, many of the mechanism(s) by which ABI4 integrates these responses are still unclear. We hypothesize that ABI4 expression is controlled by combinatorial action of signaling components from many pathways, that ABI4 activity is regulated by changes in phosphorylation state, and that ABI4 accumulation is inhibited by mechanisms leading to its degradation.

ABI4 is a low expressing gene, and its transcript levels are known to be tightly regulated. We have analyzed the ABI4-promoter activity using serial deletions of the promoter fused to a GUS reporter gene. Promoter sequences harboring NaCl-responsive and repressor-binding DNA sequences were identified. However, post-transcriptional regulation due to stress and hormone effects on protein stability further limits ABI4 protein accumulation. By assaying ABI4 protein fusion accumulation in various E3 ligase mutant backgrounds, we have found multiple mechanisms regulating stability under different growth conditions.

We have found that the MAP kinases can phosphorylate ABI4 and modulate its activity. Using complementation of abi4-mutant plants, we found that the phosphorylated form is the active whereas a phosphorylation-null form of the ABI4 failed to restore function.

A yeast two-hybrid screen identified multiple ABI4-interacting proteins, including members of two distinct classes of transcription factors that also affect salt and glucose sensitivity.