Rhizobia bacteroid transcription helps to coordinate the soybean nodule drought response

Soybean (Glycine max L. [Merr.]) nitrogen fixation is sensitive to soil drying and is an important target for improving soybean drought resilience. Transcriptional regulation of the nodule drought response and the influence of symbiotic rhizobia are not well understood. We measured whole-plant responses to mild and moderate soil moisture deficit (41% and 20% of field capacity) alongside the nodule transcriptome, including soybean and Bradyrhizobium diazoefficiens transcripts, in two soybean genotypes ('Benning' and PI 471938) that were previously found to have diverging nitrogen fixation drought responses. The transcriptomes of the two genotypes responded very similarly to soil moisture deficit, with only eight genes responding to soil moisture deficit differently between genotypes. Mild soil moisture deficit reduced stomatal conductance and increased stem and petiole ureide while altering transcript abundance of genes involved in nitrogen assimilation-related pathways in both plant genotypes and in the bacteroid. Gene regulatory network inference identified two B. diazoefficiens loci, BAC49768 and BAC47090, as major regulators in the whole-nodule transcriptome during soil moisture deficit. Several soybean network nodes were located within quantitative trait loci previously associated with relevant drought responses. This work revealed new soybean and rhizobia candidate genes with important roles in the soybean nodule drought response.