Deletion of the SACPD-C Locus Alters the Symbiotic Relationship Between Bradyrhizobium japonicum USDA110 and Soybean, Resulting in Elicitation of Plant Defense Response and Nodulation Defects

Legumes form symbiotic associations with soil-dwelling bacteria collectively called rhizobia. This association results in the formation of nodules, unique plant-derived organs, within which the rhizobia are housed. Rhizobia-encoded nitrogenase facilitates the conversion of atmospheric nitrogen into ammonia, which is utilized by the plants for its growth and development. Fatty acids have been shown to play an important role in root nodule symbiosis. In this study, we have investigated the role of stearoyl-acyl carrier protein desaturase isoform C (SACPD-C), a soybean enzyme that catalyzes the conversion of stearic acid into oleic acid, which is expressed in developing seeds and in nitrogen-fixing nodules. In-depth cytological investigation of nodule development in sacpd-c mutant lines M25 and MM106 revealed gross anatomical alteration in the sacpd-c mutants. Transmission electron microscopy observations revealed ultrastructural alterations in the sacpd-c mutants that are typically associated with plant defense response to pathogens. In nodules of two sacpd-c mutants, the combined jasmonic acid (JA) species (JA and the isoleucine conjugate of JA) were found to be reduced and 12-oxophytodienoic acid (OPDA) levels were significantly higher relative to wild-type lines. Salicylic acid levels were not significantly different between genotypes, which is divergent from previous studies of sacpd mutant studies on vegetative tissues. Soybean nodule phytohormone profiles were very divergent from those of roots, and root profiles were found to be almost identical between mutant and wild-type genotypes. The activities of antioxidant enzymes, ascorbate peroxidase, and superoxide dismutase were also found to be higher in nodules of sacpd-c mutants. PR-1 gene expression was extremely elevated in M25 and MM106, while the expression of nitrogenase was significantly reduced in these sacpd-c mutants, compared with the parent ‘Bay’. Two-dimensional gel electrophoresis and matrix-assisted laser desorption-ionization time of flight mass spectrometry analyses confirmed sacpd-c mutants also accumulated higher amounts of pathogenesis-related proteins in the nodules. Our study establishes a major role for SACPD-C activity as essential for proper maintenance of soybean nodule morphology and physiology and indicates that OPDA signaling is likely to be involved in attenuation of nodule biotic defense responses.

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The bZIP Transcription Factor LtAP1 Modulates Oxidative Stress Tolerance and Virulence in the Peach Gummosis Fungus Lasiodiplodia theobromae

Frontiers in Microbiology ◽

10.3389/fmicb.2021.741842 ◽

2021 ◽

Vol 12 ◽

Author(s):

He Zhang ◽

Wanqi Shen ◽

Dongmei Zhang ◽

Xingyi Shen ◽

Fan Wang ◽

...

Keyword(s):

Oxidative Stress ◽

Transcription Factor ◽

Plant Defense ◽

Defense Response ◽

Type Strain ◽

Deletion Mutant ◽

Wild Type Strain ◽

Plant Defense Response ◽

Wild Type ◽

Lasiodiplodia Theobromae

Lasiodiplodia theobromae is one of the primary causal agents in peach gummosis disease, leading to enormous losses in peach production. In our previous study, a redox-related gene, LtAP1, from the fungus was significantly upregulated in peach shoots throughout infection. Here, we characterized LtAP1, a basic leucine zipper transcription factor, during peach gummosis progression using the CRISPR-Cas9 system and homologous recombination. The results showed that LtAP1-deletion mutant had slower vegetative growth and increased sensitivity to several oxidative and nitrosative stress agents. LtAP1 was highly induced by exogenous oxidants treatment in the L. theobromae wild-type strain. In a pathogenicity test, the deletion mutant showed decreased virulence (reduced size of necrotic lesions, less gum release, and decreased pathogen biomass) on infected peach shoots compared to the wild-type strain. The mutant showed severely reduced transcription levels of genes related to glutaredoxin and thioredoxin in L. theobroame under oxidative stress or during infection, indicating an attenuated capacity for reactive oxygen species (ROS) detoxification. When shoots were treated with an NADPH oxidase inhibitor, the pathogenicity of the mutant was partially restored. Moreover, ROS production and plant defense response were strongly activated in peach shoots infected by the mutant. These results highlight the crucial role of LtAP1 in the oxidative stress response, and further that it acts as an important virulence factor through modulating the fungal ROS-detoxification system and the plant defense response.

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