GmTOC1b inhibits nodulation by repressing GmNIN2a and GmENOD40-1 in soybean

Evening complex component ELF3 interacts with LUX proteins to repress soybean root nodulation

Formation of root nodules is a unique hallmark of the symbiotic interaction between legume host plants and rhizobia and is governed by a complex regulatory framework that balances the appropriate orchestration of rhizobial infection and subsequent nodule organogenesis. In contrast to prominent model species such as Medicago truncatula and Lotus japonicus, research on symbiotic signal transduction in the staple-crop soybean Glycine max remains relatively insufficient. Here, we identified a soybean mutant with ~25% additional root nodules over wild-type, designated as increased number of nodules 1 (inn1). Through map-based cloning, INN1 encodes the EARLY FLOWERING 3a (ELF3a) protein component of the soybean Evening Complex, together with LUX1 and LUX2. INN1 is co-expressed with LUX1 and LUX2 in roots, and knockout of INN1 or knockdown of LUX1 and LUX2 enhances root nodulation. The function of INN1 in negatively regulating nodulation is genetically and biochemically dependent upon LUXs, as the INN1-LUX complex binds to the promoter of the downstream pro-nodulation target ENOD40, repressing its expression. ELF3a/INN1's repression of root-nodule formation extends beyond its established roles in diverse above-ground developmental and physiological processes and offers a theoretical basis for enhancing the biological-nitrogen fixation capacity of soybean.

Soybean hypocotyl elongation is regulated by a MYB33-SWEET11/21-GA2ox8c module involving long-distance sucrose transport

The length of hypocotyl affects the height of soybean and lodging resistance, thus determining the final grain yield. However, research on soybean hypocotyl length is scarce, and the regulatory mechanisms are not fully understood. Here, we identified a module controlling the transport of sucrose, where sucrose acts as a messenger moved from cotyledon to hypocotyl, regulating hypocotyl elongation. This module comprises four key genes, namely MYB33, SWEET11, SWEET21 and GA2ox8c in soybean. In cotyledon, MYB33 is responsive to sucrose and promotes the expression of SWEET11 and SWEET21, thereby facilitating sucrose transport from the cotyledon to the hypocotyl. Subsequently, sucrose transported from the cotyledon up-regulates the expression of GA2ox8c in the hypocotyl, which ultimately affects the length of the hypocotyl. During the domestication and improvement of soybean, an allele of MYB33 with enhanced abilities to promote SWEET11 and SWEET21 has gradually become enriched in landraces and cultivated varieties, SWEET11 and SWEET21 exhibit high conservation and have undergone a strong purified selection and GA2ox8c is under a strong artificial selection. Our findings identify a new molecular pathway in controlling soybean hypocotyl elongation and provide new insights into the molecular mechanism of sugar transport in soybean.

Timely symbiosis: circadian control of legume-rhizobia symbiosis

Legumes house nitrogen-fixing endosymbiotic rhizobia in specialised polyploid cells within root nodules. This results in a mutualistic relationship whereby the plant host receives fixed nitrogen from the bacteria in exchange for dicarboxylic acids. This plant-microbe interaction requires the regulation of multiple metabolic and physiological processes in both the host and symbiont in order to achieve highly efficient symbiosis. Recent studies have showed that the success of symbiosis is influenced by the circadian clock of the plant host. Medicago and soybean plants with altered clock mechanisms showed compromised nodulation and reduced plant growth. Furthermore, transcriptomic analyses revealed that multiple genes with key roles in recruitment of rhizobia to plant roots, infection and nodule development were under circadian control, suggesting that appropriate timing of expression of these genes may be important for nodulation. There is also evidence for rhythmic gene expression of key nitrogen fixation genes in the rhizobium symbiont, and temporal coordination between nitrogen fixation in the bacterial symbiont and nitrogen assimilation in the plant host may be important for successful symbiosis. Understanding of how circadian regulation impacts on nodule establishment and function will identify key plant-rhizobial connections and regulators that could be targeted to increase the efficiency of this relationship.

NopC/T/L Signal Crosstalk Gene GmPHT1-4

Symbiotic nodulation between leguminous plants and rhizobia is a critical biological interaction. The type III secretion system (T3SS) employed by rhizobia manipulates the host's nodulation signaling, analogous to mechanisms used by certain bacterial pathogens for effector protein delivery into host cells. This investigation explores the interactive signaling among type III effectors HH103ΩNopC, HH103ΩNopT, and HH103ΩNopL from SinoRhizobium fredii HH103. Experimental results revealed that these effectors positively regulate nodule formation. Transcriptomic analysis pinpointed GmPHT1-4 as the key gene facilitating this effector-mediated signaling. Overexpression of GmPHT1-4 enhances nodulation, indicating a dual function in nodulation and phosphorus homeostasis. This research elucidates the intricate regulatory network governing Rhizobium-soybean (Glycine max (L.) Merr) interactions and the complex interplay between type III effectors.