Investigation of Auxin Biosynthesis and Action Using Auxin Biosynthesis Inhibitors
Auxin plays important roles in almost all aspects of plant growth and development. Chemical genetics is an effective approach to understand auxin action, especially in nonmodel plant species, in which auxin-related mutants are not yet available. Among aux
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Introduction Indole-3-acetic acid (IAA) is a natural auxin that regulates almost all aspects of plant growth and development. In classical studies, auxin action was studied mainly with exogenous application of IAA to plants. However, exogenous auxin has been known to be ineffective or less effective, especially when applied to intact plants, e.g., in hypocotyls or stems. The main reason is because the endogenous auxin level necessary to support plant growth is saturated or maximum. Overdose of exogenous auxin also results in inhibition of growth. One of the main reasons is evolution of ethylene in response to exogenous auxin. To solve these problems and to better understand physiological functions of auxin, it is ideal to knock out or knock down the endogenous auxin level or activity using chemical tools. Chemical genetic approaches using micromolecules have been widely employed to analyze the function of auxin [1, 2]. Auxinole and PEO-IAA were developed as potent inhibitors of auxin signaling [3]. On the other hand, there are several types of auxin efflux transport inhibitors, such as 1-N-naphthylphthalamic acid (NPA)
Glenn R. Hicks and Chunhua Zhang (eds.), Plant Chemical Genomics: Methods and Protocols, Methods in Molecular Biology, vol. 2213, https://doi.org/10.1007/978-1-0716-0954-5_12, © Springer Science+Business Media, LLC, part of Springer Nature 2021
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N H
Indole
? O OH
Trp
?
O
OH OH
NH2 N H
?
TAM IAOx IAM
O
O
TAA1/TARs
N H
IPyA
N H
YUCCAs
IAA
Fig. 1 Auxin biosynthesis pathways
and 2,3,5-triiodobenzoic acid (TIBA). However, specificities of known transport inhibitors to auxin action are questionable. For example, TIBA inhibits vesicle trafficking process [4] and NPA inhibits actin cytoskeleton remodeling [5]. These inhibitors are effective in nonplant species. Among auxin inhibitors, use of auxin biosynthesis inhibitor is advantageous. The specificity to auxin action is well established in recent studies, since these inhibitors directly inhibit auxin biosynthesis enzymes. In addition, it is also advantageous that the specificity of the inhibitor can be confirmed with co-application of the inhibitor with auxin, when growth inhibition can be recovered by the co-treatment. The indole-3-pyruvic acid (IPyA) pathway has been proposed as the main pathway for IAA biosynthesis in Arabidopsis. In the IPyA pathway, TAA1/TARs synthesize IPyA from L-tryptophan [6, 7], and YUCCAs (YUCs) convert IPyA to IAA [8] (see Fig. 1). Evidence from chemical biology approaches supported the function of the IPyA pathway in Arabidopsis. The TAA1/TAR inhibitors L-α-aminooxy-phenylpropionic acid (L-AOPP) [9], L-kynurenine (L-Kyn) [10], and pyruvamine (PVM) [11] effectively blocked IPyA production and subsequent auxin biosynthesis in vivo and in vitro. In addition, the YUC inhibitor yucasin [12], YDF [13], and aromatic borate [14] effectively inhibited auxin biosynthesis in vivo and in vitro. The well-established auxin biosynthesis inhibitors are listed in Table 1. In this chapter, we
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