Abstract
The gain of function in genes and gene families is a continuous process and is a key factor in understanding gene and genome evolution in plants. TGACG-Binding (TGA) transcription factors (TFs) have long been known for their essential roles in plant defence in Arabidopsis, but their roles in legume symbiosis are yet to be explored. Here, we identified a total of 25 TGA (named GmTGA1-GmTGA25) genes in soybean. Through phylogenetic analysis, we discovered a clade of GmTGA proteins that appear to be legume-specific. Among them, two GmTGAs were unique by possessing the autophagy sequence in their proteins, while the third one was an orphan gene in soybean. GmTGAs were structurally different from AtTGAs, and their expression patterns also differed with the dominant expression of AtTGAs and GmTGAs in aerial and underground parts, respectively. Moreover, twenty-five GmTGAs showed a strong correlation among the gene expression in roots, nodules, and root hairs. The qRT-PCR analysis results revealed that among 15 tested GmTGAs, six were induced and four were suppressed by rhizobia inoculation, while 11 of these GmTGAs were induced by high nitrate. Our findings suggested the important roles of GmTGAs in symbiotic nodulation and in response to nitrogen availability in soybean.
Highlights
To cope with the deficiency of nitrogen (N2) availability in soil, legumes develop specialized symbiotic organs, roots nodules, through association with nitrogen-fixing bacteria, called rhizobia
Since TGA transcription factors belong to the bZIP protein family, we first accessed the browse gene family function of the Soykb database and selected the bZIP TF family to find the members of the bZIP family in the soybean genome
Based on phylogenetic analysis of amino acid (AA) sequences, we found that bZIP TFs were very diverse in structure and formed eight groups in total (Fig. 1a)
Summary
To cope with the deficiency of nitrogen (N2) availability in soil, legumes develop specialized symbiotic organs, roots nodules, through association with nitrogen-fixing bacteria, called rhizobia. This event is started with the release of flavonoids from the plant, which are sensed by rhizobia to trigger the production of node factor molecules[2] These nod factor signals are perceived by the specialized receptors in root hair cells to activate a signal transduction cascade for rhizobial infection and nodule formation. In M. trunctula and Pisum sativum, rhizobial nodulation greatly enhances plant resistance to powdery mildew through inducing systemic resistance and priming for powdery mildew-triggered SA accumulation[14], while in soybean Gr.[3] soil bacteria inhibit nodulation mainly by inducing PR1 and PR5 gene expression[15] These results implicate the central role of SA and the SA signalling pathway in the crosstalk between nodulation and plant defence signalling pathways
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