Activation of a flavin monooxygenase gene <Emphasis Type="Italic">YUCCA7</Emphasis> enhances drought resistance in <Emphasis Type="Italic">Arabidopsis</Emphasis>

Auxin regulates diverse molecular and physiological events at the cellular and organismal levels during plant growth and development in response to environmental stimuli. It acts either through distinct signaling pathways or in concert with other growth hormones. Its biological functions are adjusted by modulating biosynthesis, conjugate formation, and polar transport and distribution. Several tryptophan-dependent and -independent auxin biosynthetic pathways have been proposed. Recent studies have shown that a few flavin monooxygenase enzymes contribute to the tryptophan-dependent auxin biosynthesis. Here, we show that activation of a flavin monooxygenase gene YUCCA7 (YUC7), which belongs to the tryptophan-dependent auxin biosynthetic pathway, enhances drought resistance. An Arabidopsis activation-tagged mutant yuc7-1D exhibited phenotypic changes similar to those observed in auxin-overproducing mutants, such as tall, slender stems and curled, narrow leaves. Accordingly, endogenous levels of total auxin were elevated in the mutant. The YUC7 gene was induced by drought, primarily in the roots, in an abscisic acid (ABA)-dependent manner. The yuc7-1D mutant was resistant to drought, and drought-responsive genes, such as RESPONSIVE TO DESSICATION 29A (RD29A) and COLD-REGULATED 15A (COR15A), were up-regulated in the mutant. Interestingly, whereas stomatal aperture and production of osmoprotectants were not discernibly altered, lateral root growth was significantly promoted in the yuc7-1D mutant when grown under drought conditions. These observations support that elevation of auxin levels in the roots enhances drought resistance possibly by promoting root growth.