Abstract
Legume root nodules develop as a result of a symbiotic relationship between the plant and nitrogen-fixing rhizobia bacteria in soil. Auxin activity is detected in different cell types at different stages of nodule development; as well as an enhanced sensitivity to auxin inhibits, which could affect nodule development. While some transport and signaling mechanisms that achieve precise spatiotemporal auxin output are known, the role of auxin metabolism during nodule development is unclear. Using a soybean root lateral organ transcriptome data set, we identified distinct nodule enrichment of three genes encoding auxin-deactivating GRETCHEN HAGEN 3 (GH3) indole-3-acetic acid (IAA) amido transferase enzymes: GmGH3-11/12, GmGH3-14 and GmGH3-15. In vitro enzymatic assays showed that each of these GH3 proteins preferred IAA and aspartate as acyl and amino acid substrates, respectively. GmGH3-15 showed a broad substrate preference, especially with different forms of auxin. Promoter:GUS expression analysis indicated that GmGH3-14 acts primarily in the root epidermis and the nodule primordium where as GmGH3-15 might act in the vasculature. Silencing the expression of these GH3 genes in soybean composite plants led to altered nodule numbers, maturity, and size. Our results indicate that these GH3s are needed for proper nodule maturation in soybean, but the precise mechanism by which they regulate nodule development remains to be explained.
Highlights
Spatiotemporal auxin output is a combination of tightly regulated biosynthesis, catabolism, inactivation, activation, transport, and signaling [1,2]
Adjacent root segments above and below these organs were used as age- and rhizobium inoculation-appropriate controls to determine GRETCHEN HAGEN 3 (GH3) genes enriched in nodules versus lateral roots at two different stages of development (Table S1)
Auxin appears to play both positive and negative roles during nodule development depending on the level of auxin output, developmental stage, and type of legume nodule
Summary
Spatiotemporal auxin output is a combination of tightly regulated biosynthesis, catabolism, inactivation, activation, transport, and signaling [1,2]. The major form of auxin in plants, indole-3-acetic acid (IAA), is primarily synthesized via the two-step Indole pyruvic acid (IPA) pathway [3]. In this pathway, tryptophan is converted to IPA by TRYPTOPHAN AMINO TRANSFERASE OF ARABIDOPSIS (TAA) and IPA is metabolized to IAA by YUCCA flavin monoxygenases [4,5]. It was recently revealed that 2-oxoindole-3-acetic acid (oxIAA) is the major catabolite of IAA in Arabidopsis and rice [6,7]. Conjugation of IAA is a key regulatory step that dictates the levels of free (active) IAA pools and spatiotemporal auxin output during plant development
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