GmARP is Related to the Type III Effector NopAA to Promote Nodulation in Soybean (Glycine max)

NopAA and NopD Signaling Association-Related Gene GmNAC27 Promotes Nodulation in Soybean (Glycine max)

Rhizobia secrete effectors that are essential for the effective establishment of their symbiotic interactions with leguminous host plants. However, the signaling pathways governing rhizobial type III effectors have yet to be sufficiently characterized. In the present study, the type III effectors, NopAA and NopD, which perhaps have signaling pathway crosstalk in the regulation of plant defense responses, have been studied together for the first time during nodulation. Initial qRT-PCR experiments were used to explore the impact of NopAA and NopD on marker genes associated with symbiosis and defense responses. The effects of these effectors on nodulation were then assessed by generating bacteria in which both NopAA and NopD were mutated. RNA-sequencing analyses of soybean roots were further utilized to assess signaling crosstalk between NopAA and NopD. NopAA mutant and NopD mutant were both found to repress GmPR1, GmPR2, and GmPR5 expression in these roots. The two mutants also significantly reduced nodules dry weight and the number of nodules and infection threads, although these changes were not significantly different from those observed following inoculation with double-mutant (HH103ΩNopAA&NopD). NopAA and NopD co-mutant inoculation was primarily found to impact the plant-pathogen interaction pathway. Common differentially expressed genes (DEGs) associated with both NopAA and NopD were enriched in the plant-pathogen interaction, plant hormone signal transduction, and MAPK signaling pathways, and no further changes in these common DEGs were noted in response to inoculation with HH103ΩNopAA&NopD. Glyma.13G279900 (GmNAC27) was ultimately identified as being significantly upregulated in the context of HH103ΩNopAA&NopD inoculation, serving as a positive regulator of nodulation. These results provide new insight into the synergistic impact that specific effectors can have on the establishment of symbiosis and the responses of host plant proteins.

Effector-Dependent and -Independent Molecular Mechanisms of Soybean-Microbe Interaction

Soybean is a pivotal staple crop worldwide, supplying the main food and feed plant proteins in some countries. In addition to interacting with mutualistic microbes, soybean also needs to protect itself against pathogens. However, to grow inside plant tissues, plant defense mechanisms ranging from passive barriers to induced defense reactions have to be overcome. Pathogenic but also symbiotic micro-organisms effectors can be delivered into the host cell by secretion systems and can interfere with the immunity system and disrupt cellular processes. This review summarizes the latest advances in our understanding of the interaction between secreted effectors and soybean feedback mechanism and uncovers the conserved and special signaling pathway induced by pathogenic soybean cyst nematode, Pseudomonas, Xanthomonas as well as by symbiotic rhizobium.

The Rhizobial Type 3 Secretion System: The Dr. Jekyll and Mr. Hyde in the Rhizobium–Legume Symbiosis

Rhizobia are soil bacteria that can establish a symbiotic association with legumes. As a result, plant nodules are formed on the roots of the host plants where rhizobia differentiate to bacteroids capable of fixing atmospheric nitrogen into ammonia. This ammonia is transferred to the plant in exchange of a carbon source and an appropriate environment for bacterial survival. This process is subjected to a tight regulation with several checkpoints to allow the progression of the infection or its restriction. The type 3 secretion system (T3SS) is a secretory system that injects proteins, called effectors (T3E), directly into the cytoplasm of the host cell, altering host pathways or suppressing host defense responses. This secretion system is not present in all rhizobia but its role in symbiosis is crucial for some symbiotic associations, showing two possible faces as Dr. Jekyll and Mr. Hyde: it can be completely necessary for the formation of nodules, or it can block nodulation in different legume species/cultivars. In this review, we compile all the information currently available about the effects of different rhizobial effectors on plant symbiotic phenotypes. These phenotypes are diverse and highlight the importance of the T3SS in certain rhizobium–legume symbioses.