The role of Arabidopsis thaliana YUCCA6 in plant development and growth

Auxin regulates a variety of physiological and developmental processes including cell elongation, cell division, cell differentiation, cell cycle, tropisms, and root and vascular development (Taiz and Zeiger, 2006). Although a number of auxin biosynthetic pathways have been identified their overlapping nature has prevented a clear elucidation of auxin biosynthesis. Independent groups reported Arabidopsis mutants with supernormal auxin phenotypes. These mutants exhibit hyperactivation of genes belonging to the YUCCA family encoding putative flavin monooxygenase enzymes that result in increased endogenous auxin levels. We isolated and characterized fertile dominant Arabidopsis thaliana hyt1-1D and hyt1-2D (yucca6-1D, -2D) mutants that exhibit typical auxin overproduction phenotypic alterations such as epinastic cotyledons, increased apical dominance, and curled leaves. However, unlike other auxin overproduction mutants, yucca6 plants do not display short or hairy root phenotypes and lack morphological changes under dark conditions. In addition, yucca6-1D and yucca6-2D have extremely tall (over one meter) infloresences with extreme apical dominance and twisted cauline leaves. Microarray analyses revealed that expression of several IAA-inducible genes including Aux/IAA, SAUR, and GH3 are several-fold higher in yucca6 mutants than in wild type. Catalytic activity assays with recombinant YUCCA6 indicate that YUCCA6 is involved in a Trp-dependent auxin biosynthesis pathway. YUCCA6:GFP and YUCCA6:RFP fusion proteins indicate YUCCA6 protein exhibits a non-plastidial subcellular localization in an unidentified intracellular compartment. Based on these data, we conclude that Arabidopsis thaliana YUCCA6 as a member of the YUCCA family, functions in auxin biosynthesis. Furthermore, the unique phenotype of yucca6-1D, such as extremely delayed life span proposes possible unique roles of YUCCA6 in plant growth and development. Since YUCCA6 plays important roles in auxin biosynthesis, dramatically delayed senescence phenotype of yuc6-1D could be from elevated auxin levels in yuc6-1D. Next, I analyzed delayed senescence phenotypes in yuc6-1D. Although the first report for the involvement of auxin in controlling senescence was over fifty years ago, the role of auxin in plant senescence is still not clear. Both Arabidopsis thaliana yuc6-1D and 35S:YUC6 mutants that overexpress YUCCA6 contain elevated free IAA level compared to wild type plants and display dramatic delayed senescence phenotypes. Detached rosette leaves of YUCCA6 overexpressers exhibit delayed dark-induced senescence as well as delayed hormone-induced senescence. The monooxygenase enzyme activity of YUCCA6 is required for delaying senescence. Another hyper auxin mutant, Arabidosis thaliana yucca1-ox, also displays delayed dark-induced senescence as well as hormone-induced senescence. When excess free IAA from yuc6-1D was reduced, yuc6-1D leaves senesced normally implying that elevated free IAA retards leaf senescence in yuc6-1D. Free IAA measurements and auxin response gene expression profiles revealed that auxin homeostasis is actively down-regulated during senescence. The expression of GH3 family members involved in auxin amino acid conjugation was dramatically up-regulated during senescence, which suggests that plants tightly control auxin homeostasis during senescence possibly by conjugating free IAA. ANAC21/NAC1 and ANAC92/ATNAC6 expression is reduced in yuc6-1D during dark induced senescence compared to WT. Expression of miR164 in yuc6-1D is increased compared to WT whereas NAC1 expression is decreased in yuc6-1D. Our findings indicate that plants tightly control auxin levels which play critical roles in senescence, at least in part, through deactivation of auxin by conjugation