The wheat abscisic acid-responsive protein kinase mRNA, PKABA1, Is up-regulated by dehydration, cold temperature, and osmotic stress. Plant Physiol

kinases also have roles in changing environmental condi-tions. For example, osmotic stress results in the phosphor-ylation of unique sites on SUC phosphate synthase in spin-ach leaves (Huber et al., 1992), and low temperature and salt stress result in the accumulation of phosphorylated proteins (Garbarino et 1991; M~~~~ ~ et 1993). The plant hormone ABA is a signal common to the initiation of many environmental stress responses and there is increasing evidence that protein kinases have a role in ABA-mediated responses. ABA affects protein phos-PhorJ'lation of k ~ ~ a chromatin Proteins (ChaPman et a1.t 1975) and carrot embryonic proteins (Koontz and Choi, 1993). Additionally, an Arabidopsis mutant with a defec-tive protein phosphatase 2C is insensitive to ABA and has reduced ABA-mediated responses ( L ~ ~ ~ ~ et 1994; M~~~ ~ et al,, 1 9 ~ ~ ). In wheat (Triticum aestivum L.) we have identified a nove1 protein kinase mRNA, PKABA1, that accumulates in embryos treated with ABA and in severely dehydrated shoots (Anderberg and Walker-Simmons, 1992). The de-duced amino acid sequence of PKABAl has extensive ho-mology to a11 12 conserved catalytic subdomains character-istic of Ser/Thr protein kinases but has a unique stretch of Asp residues near the carboxyl terminus. The up-regula-tion of P M B A ~ at the transcriptional leve1 is unusual because protein kinases are usually regulated at the post-translational level. The accumulation of PKABAl mRNA may be part of the initial stress response to environmental signals mediated by ABA that ultimately result in the accumulation of osmoprotectants~ ABA increases in plants exposed to severa1 types of environmental stress and acts as a signal, although ter-tainly not the only signal, to promote tissue acclimation to stress (reviewed by Chandler and Robertson, 1994). Ele-vated ABA levels have been correlated with increased tol-erance to dehydration (Bartels et al., 1990) and sometime