Sugar rush: Glucosylation of IPyA attenuates auxin levels

The phytohormone auxin regulates plant growth and development. Proper spatiotemporal distribution of active auxin (indole-3-acetic acid [IAA]), via formation of auxin gradients, ensures precise organ morphogenesis and physiology. Because plants are sessile organisms, they are exquisitely attuned to fluctuating environmental conditions, such as alterations in ambient temperature and light conditions. Response to environmental changes often incurs modulation of auxin gradients to coordinate appropriate growth responses (such as hypocotyl elongation in response to shade). Auxin’s role as a key hormonal regulator to translate environmental stimuli into an adaptive growth response requires optimal auxin levels and response throughout the plant. Thus, auxin is tightly regulated through multiple processes including metabolism (biosynthesis, conjugation, degradation), transport, and signaling. Despite significant progress toward understanding the mechanism and role of auxin biosynthesis in growth and environmental responses, unanswered questions regarding regulation of auxin levels by distinct inputs remain. In PNAS, Chen et al. (1) uncover an additional layer of complexity to the seemingly simple indole-3-pyruvic acid (IPyA) pathway of auxin biosynthesis.

Although several parallel IAA biosynthesis pathways have been suggested (2⇓–4), the IPyA pathway has emerged as the predominant source of auxin in the plant. This pathway consists of two steps: tryptophan (Trp)-to-IPyA conversion via the TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS (TAA) family of Trp aminotransferases, followed by IPyA-to-IAA conversion via the YUCCA (YUC) family of flavin monooxygenases (Fig. 1) (5, 6). In Arabidopsis, the importance of IPyA pathway contributions is highlighted by significant reductions in endogenous levels of free …

↵¹To whom correspondence may be addressed. Email: strader{at}wustl.edu.