Heat-shock protein 40 is the key farnesylation target in meristem size control, abscisic acid signaling, and drought resistance

Protein farnesylation is central to molecular cell biology. In plants, protein farnesyl transferase mutants are pleiotropic and exhibit defective meristem organization, hypersensitivity to the hormone abscisic acid, and increased drought resistance. The precise functions of protein farnesylation in plants remain incompletely understood because few relevant farnesylated targets have been identified. Here, we show that defective farnesylation of a single factor-heat-shock protein 40 (HSP40), encoded by the J2 and J3 genes-is sufficient to confer ABA hypersensitivity, drought resistance, late flowering, and enlarged meristems, indicating that altered function of chaperone client proteins underlies most farnesyl transferase mutant phenotypes. We also show that expression of an abiotic stress-related microRNA (miRNA) regulon controlled by the transcription factor SPL7 requires HSP40 farnesylation. Expression of a truncated SPL7 form mimicking its activated proteolysis fragment of the membrane-bound SPL7 precursor partially restores accumulation of SPL7-dependent miRNAs in farnesyl transferase mutants. These results implicate the pathway directing SPL7 activation from its membrane-bound precursor as an important target of farnesylated HSP40, consistent with our demonstration that HSP40 farnesylation facilitates its membrane association. The results also suggest that altered gene regulation via select miRNAs contributes to abiotic stress-related phenotypes of farnesyl transferase mutants.