Functional interactions of lanthanum and phospholipase D with the abscisicacid signaling effecters VP1 and ABI1-1 in rice protoplasts

cis,trans-Abscisic acid (ABA) plays an important role in plant growth and d evelopment, regulation of seed maturation, germination, and adaptation to e nvironmental stresses. Knowledge of ABA mechanisms of action and the intera ctions of components required for ABA signal transduction is far from compl ete. Using transient gene expression in rice protoplasts, we observed addit ive and inhibitory effects between maize VP1 Viviparous-1, a transcriptiona l activator) and a dominant-negative mutant protein phosphatase, ABI1-1 (AB A-insensitive-1-1), from Arabidopsis. Lanthanide ions were shown to be spec ific agonists of ABA-inducible gene expression and to interact synergistica lly with ABA and overexpressed VP1, Both VP1 and lanthanum activities could be antagonized by coexpression of ABI1-1, which demonstrates the specific ABA dependence of these effecters on ABA-regulated gene expression. We obta ined pharmacological evidence that phospholipase D (PLD) functions in ABA-i nducible gene expression in rice, Antagonism of ABA, VP1, and lanthanum syn ergy by 1-butanol, a specific inhibitor of PLD, was similar to the inhibiti on by coexpression of ABI1-1, These results demonstrate that ABA,VP1, lanth anum, PLD, and ABI1 are all involved in ABA-regulated gene expression and a re consistent with an integrated model whereby La3+ acts upstream of PLD.