Nodule-Enriched GRETCHEN HAGEN 3 Enzymes Have Distinct Substrate Specificities and Are Important for Proper Soybean Nodule Development

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.

The Roles of Auxin Biosynthesis YUCCA Gene Family in Plants

“…Although it seems that auxin signaling is crucial for nodulation [113], it was found that rhizobia infection and nodule organogenesis were closely associated with GmYUC2a, an ortholog of Arabidopsis YUC2, to regulate local auxin biosynthesis in legumes [34]. In line with this, GH3s were also found to play a role in regulating proper nodule maturation in soybean [112]. These results highlight the importance of auxin metabolism, besides auxin transport, in legume nodulation.…”

“…YUC genes also play a role in the interactions of plant-microbes or plant-plant by regulating auxin levels. Root nodules are a unique type of lateral organ on the roots of most legumes that house nitrogen-fixing bacteria [112]. Although it seems that auxin signaling is crucial for nodulation [113], it was found that rhizobia infection and nodule organogenesis were closely associated with GmYUC2a, an ortholog of Arabidopsis YUC2, to regulate local auxin biosynthesis in legumes [34].…”

Genome-wide identification of GH3 genes in Brassica oleracea and identification of a promoter region for anther-specific expression of a GH3 gene

“…Over-expression of AtGH3.5 ( WES1 ), which is induced by treatment of abscisic acid and SA, as well as auxin, leads to auxin resistant phenotypes [ 17 ]. In various plants, important roles played by plant GH3 enzymes have also been demonstrated: nodule numbers and sizes in soybean [ 18 ], resistance to Xanthomonas bacteria in citrus [ 19 ], drought and salt tolerance in cotton [ 20 ], and fruit softening in kiwi [ 21 ], were shown to be affected by GH3 gene expressions.…”

Genome-Wide Identification of GH3 Genes in Brassica Oleracea and Identification of a Promoter Region for Anther-Specific Expression of a GH3 Gene

“…Over-expression of WES1, which is induced by treatment of abscisic acid and SA, as well as auxin, leads to auxin resistant phenotypes [17]. In various plants, important roles played by plant GH3 enzymes have also been demonstrated: nodule numbers and sizes in soybean [18], resistance to Xanthomonas bacteria in citrus [19], drought and salt tolerance in cotton [20], and fruit softening in kiwi and tomato [21], were shown to be affected by GH3 gene expressions.…”

“…However, only Group and GH3 genes have been identi ed in Gramineae genomes [23][24][25]. Using GH3 enzymes in various plant species, preferential substrates of GH3 enzymes in terms of acyl acids and amino acids have been determined [8,14,18,22,[26][27][28][29][30]. In addition, a systematic evaluation of sixty GH3 enzymes from Arabidopsis, grape, rice, Physcomitrella, and Sellaginella also revealed that not all the enzymes in Group are involved in JA signaling and twelve out of sixteen subgroup GH3 enzymes tested displayed clear substrate preferences for IAA using three acyl acid substrates -jasmonate, IAA, and 4 hydroxybenzoate (4-HBA) [31].…”