Roles of multiple mechanisms in regulating auxin levels during plant growth and development.

Abstract

Auxins, primarily indole-3-acetic acid (IAA), are endogenous plant hormones well known as key regulators of plant growth and development. Both genetic and biochemical studies have demonstrated that plants have developed a complex system to regulate the level of IAA, including biosynthesis of IAA from Trp-dependent and Trp-independent pathways, polar auxin transport, conjugation and hydrolysis of auxin. To accurately measure changes in IAA levels and identify pathways that contribute to the changes, I developed methods for quantitative analyses of auxin levels, auxin biosynthesis, and polar auxin transport. Using radioisotope labeling and stable-isotope dilution, I found that in etiolated tomato seedlings, a brief light exposure increased both IAA biosynthesis in the upper tissue sections and polar IAA transport in hypocotyls in a phytochrome-dependent manner, leading to unchanged free IAA levels in the top section and increased free IAA levels in the lower hypocotyl regions. In addition, using stable-isotope labeling and stable-isotope dilution, I quantified polar auxin transport in Arabidopsis hypocotyls, and I found that the transport of indole-3-butyric acid (IBA), another endogenous auxin, was much lower than IAA and that its transport mechanism was distinct from IAA transport. I also found that a small amount of IBA metabolic products, such as ester-linked IBA and IAA, was transported, while the majority of transported IAA remained as free IAA, suggesting that the polar transport of IAA could directly change the level of IAA while the transport of IBA could be an additional means to regulate IAA. In summary, my studies provide comprehensive views of auxin regulation in plants under different physiological conditions, showing that multiple mechanisms cooperatively regulate local auxin levels.