Auxin Biosynthesis by the <em>YUCCA</em> Genes in Rice^{<a href="#fn1" rid="fn1" class=" fn">1</a>,}^{<a href="#fn2" rid="fn2" class=" fn">[W]</a>}^{<a href="#fn3" rid="fn3" class=" fn">[OA]</a>}

Abstract

Indole-3-acetic acid (IAA), the predominant auxin in plants, plays a critical role in many plant growth and developmental processes, including cell division, differentiation, and elongation; flower and vascular development; and tropism (for review, see Teale et al., 2006). During the last decade, plant scientists have made significant advances in understanding the molecular mechanisms of IAA-mediated gene expression, signaling, and transport in the model dicot Arabidopsis (Arabidopsis thaliana; e.g. Leyser, 2002; Dharmasiri and Estelle, 2004; Dharmasiri et al., 2005; Kepinski and Leyser, 2005; Paponov et al., 2005). However, the biosynthesis and regulation of IAA are still not well understood.

Two major pathways for IAA biosynthesis have been proposed: the Trp-dependent and Trp-independent pathways (Zazimalova and Napier, 2003). A large body of evidence indicates that plants convert Trp to IAA by several routes (Fig. 1); this evidence includes results from in vivo and in vitro labeling experiments using radiolabeled Trp and the identification of enzymes for each of the proposed steps. Furthermore, recent studies using gain-of-function approaches in Arabidopsis have revealed that two distinct routes lead from Trp to the intermediate indole-3-acetaldoxime (IAOx; Fig. 1). The YUCCA gene, which encodes a flavin monooxygenase-like enzyme, was identified by isolation of a dominant mutant with elevated levels of IAA (Zhao et al., 2001). Because YUCCA can convert tryptamine (TAM) to N-hydroxytryptamine (NHT) in vitro, Zhao et al. (2001) suggested that YUCCA catalyzes the N-oxygenation of TAM, a rate-limiting step in auxin biosynthesis in many plants. Involvement of YUCCA in IAA synthesis is also supported by molecular analysis of a petunia (Petunia hybrida) mutant, floozy (fzy; Tobena-Santamaria et al., 2002). The fzy mutant was first isolated as a flower mutant in which the formation of the outermost three floral whorl formations and one of the two bracts is blocked at an early stage. FZY encodes a flavin monooxygenase-like enzyme homologous to Arabidopsis YUCCA, and its overexpression results in increased IAA levels and in an auxin overproduction phenotype (Tobena-Santamaria et al., 2002). More recently, Cheng et al. (2006) reported that double, triple, and quadruple mutants of four Arabidopsis YUCCA genes display severe defects in floral patterning, vascular formation, and the other developmental processes. The defects in the Arabidopsis yuc mutants were rescued by expression of the bacterial auxin biosynthesis gene iaaM, indicating that these YUCCA genes are important for auxin biosynthesis in Arabidopsis (Cheng et al., 2006).

Figure 1.

Proposed Trp-dependent pathway for IAA biosynthesis. Genes or mutants that have been identified with particular enzymatic steps are specified boxes. The steps indicated by dotted arrows are shared with the glucosinolate pathway, which is restricted to a few plant families but not in rice.

On the other hand, Zhao et al. (2002) reported evidence that the multifunctional cytochrome P450 enzymes CYP79B2 and CYP79B3, which were previously identified as enzymes that catalyze conversion of Trp to IAOx in vitro (Hull et al., 2000; Mikkelsen et al., 2000), are critical enzymes in auxin biosynthesis in vivo. Arabidopsis plants overproducing CYP79B2 contained increased levels of free auxin and exhibited auxin overproduction phenotypes, and double knockout of the CYP79B2 and CYP79B3 genes caused a reduction in IAA levels and induced growth defects related to partial auxin deficiency. These findings concerning YUCCA and CYP79B2/CYP79B3 strongly suggest that a Trp-dependent pathway occurring via an IAOx intermediate is a major source of IAA in Arabidopsis.

Although recent molecular genetic studies have successfully identified a few genes involved in the Trp-dependent IAA biosynthetic pathway in Arabidopsis, auxin biosynthesis mechanisms in monocots have not been defined (Zhao et al., 2002). As a model monocot, rice (Oryza sativa) has been extensively studied. The entire rice genome sequence is known, and full-length cDNAs, transformation systems, and many mutant collections are available. Using these tools, we have systematically screened and characterized the genes involved in biosynthesis of gibberellin and brassinosteroid (e.g. Sakamoto et al., 2004; Hong et al., 2005). As a result, we have concluded that the mechanisms of gibberellin and brassinosteroid biosynthesis differ somewhat in rice and Arabidopsis.

By analogy to our previous work, and because of the lack of obvious CYP79B2/CYP79B3 genes in rice, we have postulated that the mechanism of IAA synthesis in rice may be different from that in Arabidopsis. In this study, we identified and characterized a YUCCA-like gene in rice, OsYUCCA1. Plants overexpressing OsYUCCA1 exhibited increased IAA levels and auxin overproduction phenotypes, whereas plants suppressing antisense OsYUCCA1 cDNA showed abnormal phenotypes similar to auxin-insensitive rice. Moreover, OsYUCCA1 is not expressed ubiquitously; rather, its expression is restricted to discrete places such as the tips of leaves and roots and parenchyma cells surrounding large vascular stem tissues. Based on these observations, we propose a molecular mechanism for IAA synthesis in rice.

Acknowledgments

Notes

^{This work was supported by the Center of Excellence and the Japan Rice Genome Project of the Ministry of Agriculture, Forestry and Fisheries (grant-in-aid).}

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Makoto Matsuoka (pj.ca.u-ayogan.rga@otokam).

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www.plantphysiol.org/cgi/doi/10.1104/pp.106.091561