September 2013

Plants use a network of modifying enzymes to control hormone action

plant stock artAnyone who has placed a ripe banana in a paper bag with hard fruit understands the importance of plant hormones: The volatile hormone ethylene diffuses from the banana and binds to ethylene receptors on the unripe fruit, hastening its ripening.
The effects of plant hormones, such as ethylene, auxins or gibberellins, are crucial to the proper growth and development of plants. Equally important, however, is the biochemical regulation of plant hormones in synthesis and modification. In a recent minireview published in The Journal of Biological Chemistry, Corey S. Westfall and colleagues at Washington University in St. Louis highlight the key enzymatic players in hormone regulation, noting the remarkable evolutionary conservation of families of regulatory enzymes as well as the intricate network needed to turn hormones on and off at just the right time.
The first regulated steps in hormone action are at the biosynthetic level, where amino-acid and lipid metabolites are the precursors to most plant hormones. Once synthesized, all hormones are subject to various modifications that alter their chemical activity. These modifications include inactivating methylation by the SABATH family of methyltransferases and activating demethylation by MES methylesterases.
Highlighting the importance of these enzymes’ roles in hormone regulation, the authors note that all plants encode multiple SABATH and MES enzymes; within these enzyme families, the active sites are highly conserved, but the overall sequences are divergent, reflecting the widespread use of these modifications on a number of substrates.
An increasing number of crystal structures are adding to the understanding of substrate selection and reaction mechanisms.
Another type of modification the authors discuss is amino-acid conjugation or hydrolysis performed by the GH3 family of acyl acid amido synthetases and the M20 family of peptidases, respectively. These modifications lead to activation, inactivation, targeting for degradation or anti-hormone activity depending on the hormone and the amino acid conjugate.
The authors note that the conserved enzyme families make prediction and discovery of modifying enzymes relatively simple, yet the substrates of these enzymes remain elusive in many cases. Additionally, the identification of more key players and a better understanding of the chemical mechanisms in hormone regulation will help lead to a clearer picture of the network of hormone action that leads to proper plant growth and development.

Sarah PerdueSarah Perdue ( received her Ph.D. in microbiology from Cornell University in 2011 and has spent the past two years teaching at different colleges as a visiting professor. She is currently arranging a postdoctoral fellowship.

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