OsDi19-4 acts downstream of OsCDPK14 to positively regulate ABA response in rice : OsCDPK14 phosphorylates OsDi19-4 in ABA response

Abscisic acid (ABA) is one of the "classical" plant hormones, i.e. discovered at least 50 years ago, that regulates many aspects of plant growth and development. This chapter reviews our current understanding of ABA synthesis, metabolism, transport, and signal transduction, emphasizing knowledge gained from studies of Arabidopsis. A combination of genetic, molecular and biochemical studies has identified nearly all of the enzymes involved in ABA metabolism, almost 200 loci regulating ABA response, and thousands of genes regulated by ABA in various contexts. Some of these regulators are implicated in cross-talk with other developmental, environmental or hormonal signals. Specific details of the ABA signaling mechanisms vary among tissues or developmental stages; these are discussed in the context of ABA effects on seed maturation, germination, seedling growth, vegetative stress responses, stomatal regulation, pathogen response, flowering, and senescence.

Arabidopsis thaliana RD26 cDNA, isolated from dehydrated plants, encodes a NAC protein. Expression of the RD26 gene was induced not only by drought but also by abscisic acid (ABA) and high salinity. The RD26 protein is localized in the nucleus and its C terminal has transcriptional activity. Transgenic plants overexpressing RD26 were highly sensitive to ABA, while RD26-repressed plants were insensitive. The results of microarray analysis showed that ABA- and stress-inducible genes are upregulated in the RD26-overexpressed plants and repressed in the RD26-repressed plants. Furthermore, RD26 activated a promoter of its target gene in Arabidopsis protoplasts. These results indicate that RD26 functions as a transcriptional activator in ABA-inducible gene expression under abiotic stress in plants.

The Arabidopsis abscisic acid (ABA)-insensitive abi5 mutants have pleiotropic defects in ABA response, including decreased sensitivity to ABA inhibition of germination and altered expression of some ABA-regulated genes. We isolated the ABI5 gene by using a positional cloning approach and found that it encodes a member of the basic leucine zipper transcription factor family. The previously characterized abi5-1 allele encodes a protein that lacks the DNA binding and dimerization domains required for ABI5 function. Analyses of ABI5 expression provide evidence for ABA regulation, cross-regulation by other ABI genes, and possibly autoregulation. Comparison of seed and ABA-inducible vegetative gene expression in wild-type and abi5-1 plants indicates that ABI5 regulates a subset of late embryogenesis-abundant genes during both developmental stages.