GmSGT1 is differently required for soybean Rps genes-mediated and basal resistance to Phytophthora sojae

Functional Diversification Analysis of Soybean Malectin/Malectin-Like Domain-Containing Receptor-Like Kinases in Immunity by Transient Expression Assays

Plants have responded to microbial pathogens by evolving a two-tiered immune system, involving pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) and effector-triggered immunity (ETI). Malectin/malectin-like domain-containing receptor-like kinases (MRLKs) have been reported to participate in many biological functions in plant including immunity and resistance. However, little is known regarding the role of MRLKs in soybean immunity. This is a crucial question to address because soybean is an important source of oil and plant proteins, and its production is threatened by various pathogens. Here, we systematically identified 72 Glycine max MRLKs (GmMRLKs) and demonstrated that many of them are transcriptionally induced or suppressed in response to infection with microbial pathogens. Next, we successfully cloned 60 GmMRLKs and subsequently characterized their roles in plant immunity by transiently expressing them in Nicotiana benthamiana, a model plant widely used to study host-pathogen interactions. Specifically, we examined the effect of GmMRLKs on PTI responses and noticed that a number of GmMRLKs negatively regulated the reactive oxygen species burst induced by flg22 and chitin, and cell death triggered by XEG1 and INF1. We also analyzed the microbial effectors AvrB- and XopQ-induced hypersensitivity response and identified several GmMRLKs that suppressed ETI activation. We further showed that GmMRLKs regulate immunity probably by coupling to the immune receptor complexes. Furthermore, transient expression of several selected GmMRLKs in soybean hairy roots conferred reduced resistance to soybean pathogen Phytophthora sojae. In summary, we revealed the common and specific roles of GmMRLKs in soybean immunity and identified a number of GmMRLKs as candidate susceptible genes that may be useful for improving soybean resistance.

GmWRKY40, a member of the WRKY transcription factor genes identified from Glycine max L., enhanced the resistance to Phytophthora sojae

BACKGROUND

The WRKY proteins are a superfamily of transcription factors and members play essential roles in the modulation of diverse physiological processes, such as growth, development, senescence and response to biotic and abiotic stresses. However, the biological roles of the majority of the WRKY family members remains poorly understood in soybean relative to the research progress in model plants.

RESULTS

In this study, we identified and characterized GmWRKY40, which is a group IIc WRKY gene. Transient expression analysis revealed that the GmWRKY40 protein is located in the nucleus of plant cells. Expression of GmWRKY40 was strongly induced in soybean following infection with Phytophthora sojae, or treatment with methyl jasmonate, ethylene, salicylic acid, and abscisic acid. Furthermore, soybean hairy roots silencing GmWRKY40 enhanced susceptibility to P. sojae infection compared with empty vector transgenic roots. Moreover, suppression of GmWRKY40 decreased the accumulation of reactive oxygen species (ROS) and modified the expression of several oxidation-related genes. Yeast two-hybrid experiment combined with RNA-seq analysis showed that GmWRKY40 interacted with 8 JAZ proteins with or without the WRKY domain or zinc-finger domain of GmWRKY40, suggesting there were different interaction patterns among these interacted proteins.

CONCLUSIONS

Collectively, these results suggests that GmWRKY40 functions as a positive regulator in soybean plants response to P. sojae through modulating hydrogen peroxide accumulation and JA signaling pathway.

Genome-Wide Analysis Reveals the Role of Mediator Complex in the Soybean-Phytophthora sojae Interaction

The mediator complex is an essential link between transcription factors and RNA polymerase II, and mainly functions in the transduction of diverse signals to genes involved in different pathways. Limited information is available on the role of soybean mediator subunits in growth and development, and their participation in defense response regulation. Here, we performed genome-wide identification of the 95 soybean mediator subunits, which were unevenly localized on the 20 chromosomes and only segmental duplication events were detected. We focused on GmMED16-1, which is highly expressed in the roots, for further functional analysis. Transcription of GmMED16-1 was induced in response to Phytophthora sojae infection. Agrobacterium rhizogenes mediated soybean hairy root transformation was performed for the silencing of the GmMED16-1 gene. Silencing of GmMED16-1 led to an enhanced susceptibility phenotype and increased accumulation of P. sojae biomass in hairy roots of transformants. The transcript levels of NPR1, PR1a, and PR5 in the salicylic acid defense pathway in roots of GmMED16-1-silenced transformants were lower than those of empty-vector transformants. The results provide evidence that GmMED16-1 may participate in the soybean-P. sojae interaction via a salicylic acid-dependent process.

GmDAD1, a Conserved Defender Against Cell Death 1 (DAD1) From Soybean, Positively Regulates Plant Resistance Against Phytophthora Pathogens

Initially identified as a mammalian apoptosis suppressor, defender against apoptotic death 1 (DAD1) protein has conserved plant orthologs acting as negative regulators of cell death. The potential roles and action mechanisms of plant DADs in resistance against Phytophthora pathogens are still unknown. Here, we cloned GmDAD1 from soybean and performed functional dissection. GmDAD1 expression can be induced by Phytophthora sojae infection in both compatible and incompatible soybean varieties. By manipulating GmDAD1 expression in soybean hairy roots, we showed that GmDAD1 transcript accumulations are positively correlated with plant resistance levels against P. sojae. Heterologous expression of GmDAD1 in Nicotiana benthamiana enhanced its resistance to Phytophthora parasitica. NbDAD1 from N. benthamiana was shown to have similar role in conferring Phytophthora resistance. As an endoplasmic reticulum (ER)-localized protein, GmDAD1 was demonstrated to be involved in ER stress signaling and to affect the expression of multiple defense-related genes. Taken together, our findings reveal that GmDAD1 plays a critical role in defense against Phytophthora pathogens and might participate in the ER stress signaling pathway. The defense-associated characteristic of GmDAD1 makes it a valuable working target for breeding Phytophthora resistant soybean varieties.

The bHLH transcription factor GmPIB1 facilitates resistance to Phytophthora sojae in Glycine max

Phytophthora sojae Kaufmann and Gerdemann causes Phytophthora root rot, a destructive soybean disease worldwide. A basic helix-loop-helix (bHLH) transcription factor is thought to be involved in the response to P. sojae infection in soybean, as revealed by RNA sequencing (RNA-seq). However, the molecular mechanism underlying this response is currently unclear. Here, we explored the function and underlying mechanisms of a bHLH transcription factor in soybean, designated GmPIB1 (P. sojae-inducible bHLH transcription factor), during host responses to P. sojae. GmPIB1 was significantly induced by P. sojae in the resistant soybean cultivar 'L77-1863'. Analysis of transgenic soybean hairy roots with elevated or reduced expression of GmPIB1 demonstrated that GmPIB1 enhances resistance to P. sojae and reduces reactive oxygen species (ROS) accumulation. Quantitative reverse transcription PCR and chromatin immunoprecipitation-quantitative PCR assays revealed that GmPIB1 binds directly to the promoter of GmSPOD1 and represses its expression; this gene encodes a key enzyme in ROS production. Moreover, transgenic soybean hairy roots with GmSPOD1 silencing through RNA interference exhibited improved resistance to P. sojae and reduced ROS generation. These findings suggest that GmPIB1 enhances resistance to P. sojae by repressing the expression of GmSPOD1.

Overexpression of gma-miR1510a/b suppresses the expression of a NB-LRR domain gene and reduces resistance to Phytophthora sojae

MicroRNAs (miRNAs) are universal regulators that repress target gene expression in eukaryotes and play essential roles in plant immune responses. miRNAs were recently found to be involved in soybean and Phytophthora sojae interactions. Here, we screened miR1510, which was repressed in soybean during infection with P. sojae, indicating that it might be involved in soybean response to pathogens. To further uncover the roles of miRNAs in soybean, gma-miR1510a/b was overexpressed in the hairy roots of soybean using an Arabidopsis miR319a precursor as the backbone. The gma-miR1510a/b-overexpressing hairy roots showed enhanced susceptibility to P. sojae, and the results showed that miR1510 guides the cleavage of the Glyma.16G135500 gene, which encodes a classic type of plant disease resistance-associated gene that harbors the Toll-interleukin-like receptor (TIR) domain and nucleotide-binding site-leucine-rich repeat (NB-LRR) domain. Noticeably, several biotic stresses and hormone-responsive cis-regulatory elements were found to be present in the promoters of gma-MIR1510a and the target gene. Collectively, the results obtained in the current study reveal that gma-miR1510 regulates the target NB-LRR immune receptor gene Glyma.16G135500 and thus plays a crucial role in regulating the resistance of soybean to P. sojae.

SGT1 is required in PcINF1/SRC2-1 induced pepper defense response by interacting with SRC2-1

PcINF1 was previously found to induce pepper defense response by interacting with SRC2-1, but the underlying mechanism remains uninvestigated. Herein, we describe the involvement of SGT1 in the PcINF1/SRC2-1-induced immunity. SGT1 was observed to be up-regulated by Phytophthora capsici inoculation and synergistically transient overexpression of PcINF1/SRC2-1 in pepper plants. SGT1-silencing compromised HR cell death, blocked H₂O₂ accumulation, and downregulated HR-associated and hormones-dependent marker genes’ expression triggered by PcINF1/SRC2-1 co-overexpression. The interaction between SRC2-1 and SGT1 was found by the yeast two hybrid system and was further confirmed by bimolecular fluorescence complementation and co-immunoprecipitation analyses. The SGT1/SRC2-1 interaction was enhanced by transient overexpression of PcINF1 and Phytophthora capsici inoculation, and SGT1-silencing attenuated PcINF1/SRC2-1 interaction. Additionally, by modulating subcellular localizations of SRC2-1, SGT1, and the interacting complex of SGT1/SRC2-1, it was revealed that exclusive nuclear targeting of the SGT1/SRC2-1 complex blocks immunity triggered by formation of SGT1/SRC2-1, and a translocation of the SGT1/SRC2-1 complex from the plasma membrane and cytoplasm to the nuclei upon the inoculation of P. capsici. Our data demonstrate that the SGT1/SRC2-1 interaction, and its nucleocytoplasmic partitioning, is involved in pepper’s immunity against P. capsici, thus providing a molecular link between Ca²⁺ signaling associated SRC2-1 and SGT1-mediated defense signaling.