In soybean (Glycine max)-rhizobium interactions, the type III secretion system (T3SS) of rhizobium plays a key role in regulating host specificity. However, the lack of information on the role of T3SS in signaling networks limits our understanding of symbiosis. Here, we conducted an RNA sequencing analysis of three soybean chromosome segment substituted lines, one female parent and two derived lines with different chromosome-substituted segments of wild soybean and opposite nodulation patterns. By analyzing chromosome-linked differentially expressed genes in the substituted segments and quantitative trait loci (QTL)-assisted selection in the substituted-segment region, genes that may respond to type III effectors to mediate plant immunity–related signaling were identified. To narrow down the number of candidate genes, QTL assistant was used to identify the candidate region consistent with the substituted segments. Furthermore, one candidate gene, GmDRR1, was identified in the substituted segment. To investigate the role of GmDRR1 in symbiosis establishment, GmDRR1-overexpression and RNA interference soybean lines were constructed. The nodule number increased in the former compared with wild-type soybean. Additionally, the T3SS-regulated effectors appeared to interact with the GmDDR1 signaling pathway. This finding will allow the detection of T3SS-regulated effectors involved in legume-rhizobium interactions.
Rhizobia can fine‐tune activating key genes involved in background regulation network of nodulation in effective symbiosis processes. Detecting the mechanism of the soybean‐rhizobium interaction plays a pivotal role in aiding the agricultural environment. Here, we report that LysR family transcriptional factor rluD is a novel putative gene that supports symbiosis of broad host‐range strain Sinorhizobium fredii HH103 (S. fredii HH103) during soybean nodulation. The nodule numbers (NN) and nodule dry weight (NDW) of ‘Charleston’ and ‘Dongnong594’ (parents of recombinant inbred line [RIL] population) were reduced after inoculation with S. fredii HH103ΩrluD compared to HH103. To further elucidate the mechanism of rluD in the establishment of symbiosis, unconditional and conditional quantitative trait loci (QTLs) underlying NN and NDW were detected via a high‐generation RILs inoculated with HH103ΩrluD and wild‐type HH103. A major locus of the overlapping region between unconditional and conditional QTLs on Chromosome 6 was identified. The candidate gene Glyma.06g166800 encodes a novel sugar transporter, which might interact with rluD based on qRT‐PCR analysis. This result is valuable for elucidating the mechanism of symbiosis.
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