Transcriptional feedback regulation of YUCCA genes in response to auxin levels in Arabidopsis
The IPyA pathway, the major auxin biosynthesis pathway, is transcriptionally regulated through a negative feedback mechanism in response to active auxin levels.
The phytohormone auxin plays an important role in plant growth and development, and levels of active free auxin are determined by biosynthesis, conjugation, and polar transport. Unlike conjugation and polar transport, little is known regarding the regulatory mechanism of auxin biosynthesis. We discovered that expression of genes encoding indole-3-pyruvic acid (IPyA) pathway enzymes is regulated by elevated or reduced active auxin levels. Expression levels of TAR2, YUC1, YUC2, YUC4, and YUC6 were downregulated in response to synthetic auxins [1-naphthaleneacetic acid (NAA) and 2,4-dichlorophenoxyacetic acid (2,4-D)] exogenously applied to Arabidopsis thaliana L. seedlings. Concomitantly, reduced levels of endogenous indole-3-acetic acid (IAA) were observed. Alternatively, expression of these YUCCA genes was upregulated by the auxin biosynthetic inhibitor kynurenine in Arabidopsis seedlings, accompanied by reduced IAA levels. These results indicate that expression of YUCCA genes is regulated by active auxin levels. Similar results were also observed in auxin-overproduction and auxin-deficient mutants. Exogenous application of IPyA to Arabidopsis seedlings preincubated with kynurenine increased endogenous IAA levels, while preincubation with 2,4-D reduced endogenous IAA levels compared to seedlings exposed only to IPyA. These results suggest that in vivo conversion of IPyA to IAA was enhanced under reduced auxin levels, while IPyA to IAA conversion was depressed in the presence of excess auxin. Based on these results, we propose that the IPyA pathway is transcriptionally regulated through a negative feedback mechanism in response to active auxin levels.
KeywordsAuxin biosynthesis Auxin homeostasis Indole-3-acetic acid Indole-3-pyruvic acid Transcriptional regulation YUCCA
We thank Ms. Emi Ishida for technical assistance and Dr. Shozo Fujioka for helpful discussion. A part of this work was supported by The Science and Technology Research Promotion Program for Agriculture, Forestry, Fisheries and Food Industry (to Y. S. and K. S.), and a Grant-in-Aid for Scientific Research (nos. 23580144 and 26506015 to M. S. and no. 25514004 to C. Y.) from the Japan Society for the Promotion of Science. This paper is contribution no. 1015 from the Kihara Institute for Biological Research, Yokohama City University.
Conflict of interest
The authors declare that they have no conflict of interest.
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