Involvement of Pyridoxine/Pyridoxamine 5'-Phosphate Oxidase (PDX3) in Ethylene-Induced Auxin Biosynthesis in the Arabidopsis Root.

Abstract

As sessile organisms, plants have evolved to adjust their growth and development to environmental changes. It has been well documented that the crosstalk between different plant hormones plays important roles in the coordination of growth and development of the plant. Here, we describe a novel recessive mutant, <i>mildly insensitive to ethylene</i> (<i>mine</i>), which displayed insensitivity to the ethylene precursor, ACC (1-aminocyclopropane-1-carboxylic acid), in the root under the dark-grown conditions. By contrast, <i>mine</i> roots exhibited a normal growth response to exogenous IAA (indole-3-acetic acid). Thus, it appears that the growth responses of <i>mine</i> to ACC and IAA resemble those of <i>weak ethylene insensitive</i> (<i>wei</i>) mutants. To understand the molecular events underlying the crosstalk between ethylene and auxin in the root, we identified the <i>MINE</i> locus and found that the <i>MINE</i> gene encodes the pyridoxine 5'-phosphate (PNP)/pyridoxamine 5'-phosphate (PMP) oxidase, PDX3. Our results revealed that MINE/PDX3 likely plays a role in the conversion of the auxin precursor tryptophan to indole-3-pyruvic acid in the auxin biosynthesis pathway, in which <i>TAA1</i> (<i>TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS 1</i>) and its related genes (<i>TRYPTOPHAN AMINOTRANSFERASE RELATED 1</i> and <i>2</i>; <i>TAR1</i> and <i>TAR2</i>) are involved. Considering that TAA1 and TARs belong to a subgroup of PLP (pyridoxal-5'-phosphate)-dependent enzymes, we propose that PLP produced by MINE/PDX3 acts as a cofactor in TAA1/TAR-dependent auxin biosynthesis induced by ethylene, which in turn influences the crosstalk between ethylene and auxin in the Arabidopsis root.