Distinct Characteristics of Indole-3-Acetic Acid and Phenylacetic Acid, Two Common Auxins in Plants

Abstract

The phytohormone auxin plays a central role in many aspects of plant growth and development. IAA is the most studied natural auxin that possesses the property of polar transport in plants. Phenylacetic acid (PAA) has also been recognized as a natural auxin for >40 years, but its role in plant growth and development remains unclear. In this study, we show that IAA and PAA have overlapping regulatory roles but distinct transport characteristics as auxins in plants. PAA is widely distributed in vascular and non-vascular plants. Although the biological activities of PAA are lower than those of IAA, the endogenous levels of PAA are much higher than those of IAA in various plant tissues in Arabidopsis. PAA and IAA can regulate the same set of auxin-responsive genes through the TIR1/AFB pathway in Arabidopsis. IAA actively forms concentration gradients in maize coleoptiles in response to gravitropic stimulation, whereas PAA does not, indicating that PAA is not actively transported in a polar manner. The induction of the YUCCA (YUC) genes increases PAA metabolite levels in Arabidopsis, indicating that YUC flavin-containing monooxygenases may play a role in PAA biosynthesis. Our results provide new insights into the regulation of plant growth and development by different types of auxins.

Arabidopsis thaliana GH3.5 acyl acid amido synthetase mediates metabolic crosstalk in auxin and salicylic acid homeostasis

“…4 C and D), suggesting either that degradation of this conjugate may not be as efficient as that of IAA-Asp or that the amino acid conjugate serves as a storage form for PAA. Although the results reported here and by others (32,33) highlight the parallels and differences in IAA and PAA metabolism in plants, further investigation is required to understand the roles of PAA and its conjugates in plant growth and development; however, it is likely that AtGH3.5 plays a key role in IAA/PAA homeostasis.…”

“…A recent report describes the formation of PAA-Asp and PAA-Glu in plants overexpressing either AtGH3.6 or AtGH3.9 (31). Although IAA is considered the predominant auxin in plants, PAA functions as a growth-regulating molecule (1,32,33).…”

“…PAA is present at levels similar to that of IAA in plants, it functions through the same TIR1/AFB auxin receptor-mediated pathway as IAA, and it regulates the same auxin-responsive genes as IAA (32). The two auxins show distinct distribution patterns, and, unlike IAA, PAA is not actively transported to form polar gradients in plant cells (32).…”

“…The two auxins show distinct distribution patterns, and, unlike IAA, PAA is not actively transported to form polar gradients in plant cells (32). Given that multiple IAA-conjugating GH3 proteins (7,12,32) as well as AtGH3.5 display significant activity with PAA, the physiological role of these enzymes as mediators of auxin responses likely extends to PAA. The accumulation of PAA-Asp versus IAA-Asp in the Arabidopsis AtGH3.5-overexpressing lines also suggests that the subsequent metabolism of these molecules may differ.…”

Cross Talk between Nitric Oxide and Phytohormones Regulate Plant Development during Abiotic Stresses

“…Interactions between NO and AUX signaling pathways are complex and need to be explored in plants exposed to water-limited environment. It is well established that both NO and AUX interplay during growth and development of plant roots [29,30]. Association of AUX with ET to regulate root morphology and development is considered a key aspect of drought tolerance in plants [31].…”