Involvement of Arabidopsis thaliana phospholipase D zeta 2 in root hydrotropism through the suppression of root gravitropism

Root hydrotropism is the phenomenon of directional root growth toward moisture under water-deficient conditions. Although physiological and genetic studies have revealed the involvement of the root cap in the sensing of moisture gradients, and those of auxin and abscisic acid (ABA) in the signal transduction for asymmetric root elongation, the overall mechanism of root hydrotropism is still unclear. We found that the promoter activity of the Arabidopsis phospholipase D zeta 2 gene (PLD zeta 2) was localized to epidermal cells in the distal root elongation zone and lateral root cap cells adjacent to them, and that exogenous ABA enhanced the activity and extended its area to the entire root cap. Although pld zeta 2 mutant root caps did not exhibit a morphological phenotype in either the absence or presence of exogenous ABA, the inhibitory effect of ABA on gravitropism, which was significant in wild-type roots, was not observed in pld zeta 2 mutant roots. In root hydrotropism experiments, pld zeta 2 mutations significantly retarded or disturbed root hydrotropic responses. A drought condition similar to that used in a hydrotropism experiment enhanced the PLD zeta 2 promoter activity in the root cap, as did exogenous ABA. These results suggest that PLD zeta 2 responds to drought through ABA signaling in the root cap and accelerates root hydrotropism through the suppression of root gravitropism.