Just as humans get sick, plants get diseased from attacks by various phytopathogens such as bacteria, fungi and nematodes. During my Ph.D. years in graduate school, I worked on a bacterial pathogen called Pseudomonas syringae to study the functions of type III “effector” proteins, which are essential virulence factors translocated, via the bacterial type III secretion system, into the plant host cell to promote infection. It was a “hot” and highly competitive research area, and many labs across the world published beautiful papers demonstrating how different type III effectors target diverse host components to cause disease. One common theme emerged--many type III effectors suppress plant immune responses to dampen plant defense and ultimately be able to colonize host tissue. While this makes a lot of sense and parallels the virulence strategy utilized by many animal pathogens, it’s unclear if immune suppression is the only function of bacterial effectors in plants.
Toward the end of my Ph.D. study, I was trying to push out the final paper from my graduate study and planning to move on to postdoc position. However, in one of the final sets of bacterial infection assays I was performing, I noticed a huge effect of environmental humidity on disease outcome. In particular, I inoculated Arabidopsis plants with two P. syringae strains, one wild type and the other a mutant that has two type III effector genes (avrE and hopM1) deleted, and placed inoculated plants under high and low humidity conditions. Interestingly, while wild type strain caused severe disease under high humidity and little disease under low humidity, the avrE1-/hopM1- deletion mutant almost completely lost sensitivity to humidity and caused little disease regardless of external humidity settings. This accidental finding intrigued me and I discussed with my advisor Dr. Sheng Yang He. We quickly agreed that the result was very striking and further experiments had potential to provide insight into the enigmatic basis of the profound effect of high humidity on many bacterial diseases. Excited by this possibility, I decided to stay longer in my Ph.D. lab and, after convincing another lab member, Dr. Kinya Nomura, to join the new project, we formed a very supportive team to further pursue.
The key experiment, we thought, is to figure out how AvrE1 and HopM1, which acts inside plant cell, could “sense” external humidity. After different types of experiments, we eventually realized that, even though AvrE1 and HopM1 are completely different proteins, they both cause dramatic accumulation of liquid in the otherwise air-filled leaf intercellular space (apoplast), where bacteria live. Maintaining this “watery” apoplast requires high humidity, and under low external humidity, liquid accumulated in the apoplast quickly evaporates through numerous microscopic pores (stomata) dotted on the leaf surface. Supplementing water to the apoplast could restore the virulence of the avrE1-/hopM1- deletion mutant. These results not only provide an explanation for the high humidity dependence of P. syringae infection, but also show that not all type III effectors are involved in suppression of host immunity. Instead, some effectors, like AvrE1 and HopM1, are involved in establishing an aqueous living space inside infected leaves.
Further experiments let us discover that immune suppression and creating a watery apoplast are two principal processes necessary and sufficient for basic P. syringae pathogenesis in Arabidopsis leaves. We also found an intriguing connection between immunity, humidity and leaf commensal microbiome in leaves. I can feel that this project is opening up several unexpected new directions, which is quite exciting.
I feel very lucky to have the generous support of my coworkers, especially Kinya Nomura, and my advisor Sheng Yang He, who helped me to push the project forward in an efficient manner. I will surely miss the insightful weekly “bacterial effector” group discussions about the project. Some questions, such as how exactly AvrE1 and HopM1 cause “water soaking”, remain unaddressed. More needs to be done, but I am happy that we set a framework for future progress.
Xiufang Xin, Ph.D.
Postdoc, Plant Research Laboratory, Michigan State University, USA
To see the full article, please go to Nature website http://www.nature.com/nature/journal/v539/n7630/fu...
Images: Left, an Arabidopsis plant infected by the bacterial pathogen Pseudomonas syringae. Typical of phyllosphere-infecting bacteria, this pathogen causes water-soaking symptoms (dark-colored patches) required for aggressive bacterial multiplication. Right, pseudo-colored bioluminescence (pink; labeling Pseudomonas syringae bacteria tagged with a luciferase reporter) emitted from infected Arabidopsis leaves. The bioluminescence image was superimposed upon a stylized image of the infected Arabidopsis plant (outlined in green). Credit: Kyaw Aung and Caitlin Thireault.