Abstract

Plants and microbes coexist in nature by establishing a variety of interactions ranging from the most beneficial, such as symbiotic (Kawaguchi and Minamisawa, 2010), where both the microbe and the plant show enhanced ability to thrive and survive, to the most destructive, such as pathogenic, where the microbe can cause infection or even plant death (Dodds and Rathjen, 2010). The pathogenic interaction between plants and microbes resembles a battle whereby both sides use “ammunition” to attack the other. In the case of the plant, upon pathogen invasion, an array of responses, such as reactive oxygen species (ROS) production, occurs that restrict or prevent pathogen growth (Marcec et al., 2019). In the case of the pathogen, a group of small proteins, called effectors, are directed toward specific targets inside the host to manipulate host immune responses (Jones and Dangl, 2006). Therefore, effectors constitute a key part of the arsenal of the pathogen. Understanding effector functions and targets is crucial for protecting crops from pathogens. In this issue of Plant Physiology, Wan et al. (2022) focus on one of the most destructive pathogens, Puccinia striiformis, which causes the stripe rust disease in wheat (Triticum aestivum). Puccinia striiformis (Pst) is a fungus that infects wheat spikes, stems, and leaves and currently represents a global threat to food security. Understanding how effectors operate inside the host plant cell and their targets is pivotal to design strategies to combat the disease. In this study, the authors investigated the role of the effector Pst27791, which is serine-rich and contains a signal peptide sequence of 17 amino acids that allows the secretion of the effector into the host. The authors first tested whether the effector signal peptide was functional using a signal sequence trap assay in yeast (Saccharomyces cerevisiae) where the Pst27791 signal peptide was fused with invertase, a secreted enzyme involved in sucrose metabolism (Jacobs et al., 1997). The authors found that the invertase fused with the Pst27791 signal peptide was active and concluded that indeed this effector was secreted. However, how this effector contributes toward infection once inside the host was unclear. Puccinia striiformis is a biotrophic pathogen that keeps the host alive during infection and therefore must suppress programmed cell death (PCD) of the host cell (Zheng et al., 2013). To test whether the Pst27791 effector interferes with PCD, the authors expressed Pst27791 in Nicotiana benthamiana leaves along with various other effectors and reporters. They showed Pst27791 can stop PCD when triggered by Pst322 effector (Wang et al., 2012). Pst27791 also suppresses the ROS burst and expression of defense genes in response to flg22, a trigger of pathogen-triggered immunity. The authors next examined Pst27791 in wheat using overexpression and RNA interference (RNAi) in transgenic wheat lines. When Pst27791 was stably overexpressed, ROS accumulation and expression of the salicylic acid-related defense genes pathogenesis-related proteins 1 and 2 (TaPR1/2) was reduced; however, when the effector was silenced in RNAi lines, P. striiformis infection was compromised. These results suggest that Pst27791 plays an important role in pathogenicity by suppressing plant immunity, but how is this orchestrated at the molecular level? To investigate this, the authors conducted a high-throughput yeast-two hybrid (Y2H) assay by using the effector as a bait against a cDNA library of Pst-infected wheat leaves (Yang et al., 2020) with the aim to identify targets from the host. They identified a Raf-like mitogen-activated protein kinase called TaRaf46 with unknown function. By using two approaches, a Y2H assay and constructs expressing TaRaf46 fragments of different domains, the authors determined that N ew s an d V ie w s