Auxin metabolism rates and implications for plant development.

Eric M Kramer,Ethan M Ackelsberg

doi:10.3389/fpls.2015.00150

Abstract

Studies of auxin metabolism rarely express their results as a metabolic rate, although the data obtained would often permit such a calculation to be made. We analyze data from 31 previously published papers to quantify the rates of auxin biosynthesis, conjugation, conjugate hydrolysis, and catabolism in seed plants. Most metabolic pathways have rates in the range 10 nM/h–1 μM/h, with the exception of auxin conjugation, which has rates as high as ~100 μM/h. The high rates of conjugation suggest that auxin metabolic sinks may be very small, perhaps as small as a single cell. By contrast, the relatively low rate of auxin biosynthesis requires plants to conserve and recycle auxin during long-distance transport. The consequences for plant development are discussed.

Highlights

The plant hormone auxin plays a role in virtually every aspect of plant growth and development (Davies, 2004)
Studies of auxin metabolism routinely measure the concentrations of auxin and related metabolites, but these results are rarely expressed as a rate that would permit a comparison with the effects of auxin transport [ see (Epstein et al, 1980) for an interesting early example]
We find that the oxindole-3-acetic acid (oxIAA) and indole-3-butyric acid (IBA) pathways have rates comparable to the biosynthesis rates described above, with values between 10 and 40 nM/h

Summary

Introduction

The plant hormone auxin plays a role in virtually every aspect of plant growth and development (Davies, 2004). To better understand the complex interplay of auxin metabolism and transport in plant development, we are obligated to consider not just which gene families play a role in auxin metabolism, but how much auxin they can synthesize or catabolize per unit time. This is the time it would take biosynthesis to replace the entire endogenous auxin content already present in the tissue, calculated by dividing the auxin concentration by the rate.

Results

Conclusion