Abstract
The plant hormone auxin plays a crucial role in plant growth and development. Indole-3-acetic acid (IAA), a natural auxin, is mainly biosynthesized by two sequential enzyme reactions catalyzed by TAA1 and YUCCA (YUC). TAA1 is involved in the conversion of tryptophan to IPA, and YUC catalyzes the conversion of IPA to IAA. We previously demonstrated that yucasin inhibits AtYUC1 enzyme activity and suppress high-auxin phenotype of YUC overexpression plants, although yucasin displayed weak effects on the auxin-related phenotype of wild-type plants. To develop more potent YUC inhibitors, various derivatives of yucasin were synthesized, and their structure–activity relationships were investigated. Yucasin difluorinated analog (YDF) (5-[2,6-difluorophenyl]-2,4-dihydro-[1,2,4]-triazole-3-thione) was identified to be a more potent YUC inhibitor than the original yucasin. YDF caused an auxin-deficient phenotype in Arabidopsis wild-type plants that was restored with auxin application. YDF was found to be highly stable regarding metabolic conversion in vivo, accounting for the potent activity of the inhibition of IAA biosynthesis in planta. Photoaffinity labeling experiments demonstrated that yucasin-type inhibitors bind to the active site of AtYUC1. YDF is a promising auxin biosynthesis inhibitor and is a useful chemical tool for plant biology and agrochemical studies.
Highlights
The plant hormone auxin is a master regulator for plant growth and development
Indole-3-acetic acid (IAA) is mainly biosynthesized from tryptophan by two sequential enzymatic reactions consisting of TRYPTOPHAN AMINOTRANSFERASE of ARABIDOPSIS 1 (TAA1) and YUCCA (YUC) of the indole-3-pyruvic acid (IPA) pathway (Fig. 1)
Genetic studies have demonstrated that YUC functions as the rate-limiting enzyme of the IPA pathway, indicating the YUCs play a crucial role in developmental processes regulated by cellular IAA levels[5,13]
Summary
The plant hormone auxin is a master regulator for plant growth and development. Indole-3-acetic acid (IAA), the predominant naturally occurring auxin, regulates diverse physiological processes in almost every aspect of plant growth and development, including embryo development, vascular differentiation, apical dominance and tropic responses to light and gravity[1,2,3,4]. Genetic studies have demonstrated that YUC functions as the rate-limiting enzyme of the IPA pathway, indicating the YUCs play a crucial role in developmental processes regulated by cellular IAA levels[5,13]. PVM inhibits IAA biosynthesis and results in a severe auxin-deficient phenotype in several plants. Kakei et al demonstrated that the phenyl boronic acid derivatives 4-biphenylboronic acid (BBo) and 4-phenoxyphenylboronic acid (PPBo) displayed very potent inhibition of the YUC enzyme and repressed IAA biosynthesis, conferring an auxin-deficient phenotype to Arabidopsis plants[20]. Yucasin effectively reduces the endogenous IAA level of maize and suppresses high-auxin phenotypes in Arabidopsis YUC overexpression plants. We identified a yucasin difluorinated analog (YDF: 1) (5-[2,6-difluorophenyl]-2,4-dihydro-[1,2,4]-triazole-3-thione) as a potent, reversible YUC inhibitor using YUC-overexpressing transgenic plants harboring an auxin-responsive DR5::GUS reporter system. We simultaneously blocked the IPA and CYP79B auxin biosynthesis pathways using a combination of the inhibitors and mutants for auxin biosynthesis and signaling, and the results suggested auxin null phenotype of Arabidopsis plants
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