Indole-3-acetic acid biosynthesis and its regulation in plant-associated bacteria
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MINI-REVIEW
Indole-3-acetic acid biosynthesis and its regulation in plant-associated bacteria Daiana R. Duca 1 & Bernard R. Glick 1 Received: 6 January 2020 / Revised: 19 August 2020 / Accepted: 26 August 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Numerous studies have reported the stimulation of plant growth following inoculation with an IAA-producing PGPB. However, the specific mode of IAA production by the PGPB is rarely elucidated. In part, this is due to the overwhelming complexity of IAA biosynthesis and regulation. The promiscuity of the enzymes implicated in IAA biosynthesis adds another element of complexity when attempting to decipher their role in IAA biosynthesis. To date, the majority of research on IAA biosynthesis describes three separate pathways classified in terms of their intermediates—indole acetonitrile (IAN), indole acetamide (IAM), and indole pyruvic acid (IPA). Each of these pathways is mediated by a set of enzymes, many of which are traditionally assumed to exist for that specific catalytic role. This lends the possibility of missing other, novel, enzymes that may also incidentally serve that function. Some of these pathways are constitutively expressed, while others are inducible. Some enzymes involved in IAA biosynthesis are known to be regulated by IAA or by IAA precursors, as well as by a multitude of environmental cues. This review aims to provide an update to our current understanding of the biosynthesis and regulation of IAA in bacteria. Key points • IAA produced by PGPB improves bacterial stress tolerance and promotes plant growth. • Bacterial IAA biosynthesis is convoluted; multiple interdependent pathways. • Biosynthesis of IAA is regulated by IAA, IAA-precursors, and environmental factors. Keywords Indole-3-acetic acid . IAA . Plant growth-promoting bacteria . PGPB
Introduction Most, if not all, of the physiological activities of a plant are regulated by one or more plant hormones including auxin, cytokinin, gibberellin, abscisic acid, ethylene, salicylic acid, jasmonic acid, and brassinosteroids. Auxin includes a group of important molecules found in plants, humans, animals, fungi, and bacteria. Indole-3-acetic acid (IAA) is the predominant and most indispensable auxin in plants (Teale et al. 2006). Its far-reaching effects go beyond regulating plant growth; IAA also plays a role in the regulation of bacterial physiology, adaptation to stress conditions, and microbe-microbe
* Daiana R. Duca [email protected] 1
Department of Biology, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada
communication. As Paque and Weijers refer to it, IAA is the molecule that “influences almost anything” (Paque and Weijers 2016). The mechanism that is most often used to explain the positive effects of plant growth-promoting bacteria (PGPB) is the production of plant hormones, and the one that has received most of the attention is IAA. IAA orchestrates precise phenotypic effects in plants including the response of roots and shoots to light and gravity, d
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