The group of Prof. Zhao-Jun Ding at School of Life Sciences, Key Laboratory of Plant Development and Environmental Adaptation Biology, Ministry of education, Shandong University, published a paper entitled "Antagonistic interaction between auxin and SA signaling pathways regulations bacterial infection through lateral root in Arabidopsis" in Cell Reports. The co-first authors are Research Associate Xiang-Pei Kong and postdoc Chun-Lei Zhang, while Prof. Zhao-Jun Ding and Research Associate Xiang-Pei Kong are the co-corresponding authors of this article.

Plant pathogens deploy a variety of sophisticated strategies to invade their hosts and then access nutrients and water from hosts for their growth. Plant surfaces, such as stomata, provide entry sites for pathogens. Previous studies have shown that when plants are infected by pathogenic bacteria, Pattern Recognition Receptors (PRRs) on the surface of leaf cells detect Microbe-Associated Molecular Patterns (MAMP) and induce stomatal defense, which promotes the closure of stomata and prevents the invasion of pathogenic bacteria.

Besides anchoring plants into the soil, plant roots are important for the uptake of water and nutrients and therefore play a fundamental role in plant growth and development. The rhizosphere of plant root is densely populated by microorganisms, including fungi, bacteria, and nematodes. It has been reported that the flagellin 22 (flg22) can trigger pattern-triggered immunity (PTI) responses in root, indicating that the root acts as an important part of plant innate immune system. However, little is known on the molecular mechanism of pathogen invasion in plant roots.

Lateral root primordia (LRP) emerges through the overlaying endodermal, cortical and epidermal cell layers to form a lateral root (LR), which provides space and entrance for the invasion of external pathogens. Here Zhao lab used a GFP-labeled Pto DC3000 for live imaging of pathogen infection and found that a large amount of Pto DC3000 accumulated in the LR emergence (LRE). These results suggested that the LR is a potential invasion site of pathogens. They also observed that Pto DC3000 strongly induced the formation and development of LR, a process dependent on IAA14-ARF7/19-LBD16/18 mediated-auxin signaling. In addition, the phytotoxins coronatine (COR) and type-III-secretion-system effectors (T3SEs) are involved in P. syringae-triggered LR development. Flg22 could induce LR defense and inhibit LRE. On the other hand, the plant defense hormone SA could strongly interfere with LR formation through ARF7. Overexpression of PR1 and PR2 partially blocked the Pto DC3000-induced LRE. Furthermore, the ARF7 proteins directly bound to the promoters of PR1 and PR2 and repressed the transcription of these two genes.

Taken together, these results highlight the antagonistic interaction between auxin and SA signaling pathways in LR development and their roles in controlling bacterial infection through LR, and also provide insight into the understanding of the interaction between plant roots and rhizosphere microorganisms.

This work is supported by the National Natural Science Foundation of China, the Shandong Province Natural Science Foundation of Major Basic Research Program, the Young Scholars Program of Shandong University, the Key Research and Development Projects of Shandong Province, and by the Youth Interdisciplinary Science and Innovative Research Groups of Shandong University.

Paper link：

https://doi.org/10.1016/j.celrep.2020.108060