Vanishing tassel2 encodes a grass-specific tryptophan aminotransferase required for vegetative and reproductive development in maize.

Kimberly A Phillips,Jerry D Cohen,Solmaz Barazesh,Xing Liu,Simon Malcomber,Christopher Hudson,Thomas L Slewinski,Ashley Christensen,Paula Mcsteen,Andrea L Skirpan

doi:10.1105/tpc.110.075267

Abstract

Auxin plays a fundamental role in organogenesis in plants. Multiple pathways for auxin biosynthesis have been proposed, but none of the predicted pathways are completely understood. Here, we report the positional cloning and characterization of the vanishing tassel2 (vt2) gene of maize (Zea mays). Phylogenetic analyses indicate that vt2 is a co-ortholog of TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS1 (TAA1), which converts Trp to indole-3-pyruvic acid in one of four hypothesized Trp-dependent auxin biosynthesis pathways. Unlike single mutations in TAA1, which cause subtle morphological phenotypes in Arabidopsis thaliana, vt2 mutants have dramatic effects on vegetative and reproductive development. vt2 mutants share many similarities with sparse inflorescence1 (spi1) mutants in maize. spi1 is proposed to encode an enzyme in the tryptamine pathway for Trp-dependent auxin biosynthesis, although this biochemical activity has recently been questioned. Surprisingly, spi1 vt2 double mutants had only a slightly more severe phenotype than vt2 single mutants. Furthermore, both spi1 and vt2 single mutants exhibited a reduction in free auxin levels, but the spi1 vt2 double mutants did not have a further reduction compared with vt2 single mutants. Therefore, both spi1 and vt2 function in auxin biosynthesis in maize, possibly in the same pathway rather than independently as previously proposed.

Highlights

Auxin has been shown to play a critical role in all stages of plant development
We show that the vt2 plays a significant role in axillary meristem formation during inflorescence development in maize
Vt2 does not play a role in axillary meristem formation but functions in leaf initiation during the adult phase of growth. vt2 encodes an enzyme with similarity to TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS1 (TAA1), which has been demonstrated to convert Trp to indole-3-pyruvic acid (IPA) in Arabidopsis (Stepanova et al, 2008; Tao et al, 2008; Yamada et al, 2009)

Summary

INTRODUCTION

Auxin has been shown to play a critical role in all stages of plant development. Through its functions in cell division and cell expansion, auxin is required for the initiation of lateral roots, vascular strands, leaves, flowers, and floral organs (Benjamins and Scheres, 2008). Taa tar tar triple mutants lack roots and are seedling lethal, similar to the phenotype of plants containing quadruple knockouts of the YUC gene family (Cheng et al, 2007a; Stepanova et al, 2008) Both the TAA1 and YUC gene families exhibit genetic redundancy and function in auxin biosynthesis, which raises the question of why neither pathway can compensate for the other. Positional cloning and phylogenetic analysis indicate that vt encodes a co-ortholog of the TAA1/TAR1/TAR2 genes of Arabidopsis, which function in the IPA pathway for auxin biosynthesis (Stepanova et al, 2008; Tao et al, 2008; Yamada et al, 2009). Due to the reduced redundancy of the vt and spi genes in maize, we were able to test the relative contributions of the IPA and TAM pathways to auxin biosynthesis

RESULTS

DISCUSSION

Findings

METHODS