Modifications of plant immune complexes by secreted pathogen effectors can trigger strong immune responses mediated by the action of nucleotide binding-leucine-rich repeat immune receptors. Although some strains of the pathogen Pseudomonas syringae harbor effectors that individually can trigger immunity, the plant's response may be suppressed by other virulence factors. This work reveals a robust strategy for immune suppression mediated by HopZ3, an effector in the YopJ family of acetyltransferases. The suppressing HopZ3 effector binds to and can acetylate multiple members of the RPM1 immune complex, as well as two P. syringae effectors that together activate the RPM1 complex. These acetylations modify serine, threonine, lysine, and/or histidine residues in the targets. Through HopZ3-mediated acetylation, it is possible that the whole effector-immune complex is inactivated, leading to increased growth of the pathogen.

Salicylic acid (SA) is a major defense signal in plants. In Arabidopsis (Arabidopsis thaliana), the chloroplast-localized isochorismate pathway is the main source of SA biosynthesis during abiotic stress or pathogen infections. In the first step of the pathway, the enzyme ISOCHORISMATE SYNTHASE1 (ICS1) converts chorismate to isochorismate. An unknown enzyme subsequently converts isochorismate to SA. Here, we show that ICS1 protein levels increase during UV-C stress. To identify proteins that may play roles in SA production by regulating ICS1, we analyzed proteins that coimmunoprecipitated with ICS1 via mass spectrometry. The ICS1 complexes contained a large number of peptides from the PROHIBITIN (PHB) protein family, with PHB3 the most abundant. PHB proteins have diverse biological functions that include acting as scaffolds for protein complex formation and stabilization. PHB3 was reported previously to localize to mitochondria. Using fractionation, protease protection, and live imaging, we show that PHB3 also localizes to chloroplasts, where ICS1 resides. Notably, loss of PHB3 function led to decreased ICS1 protein levels in response to UV-C stress. However, ICS1 transcript levels remain unchanged, indicating that ICS1 is regulated posttranscriptionally. The phb3 mutant displayed reduced levels of SA, the SA-regulated protein PR1, and hypersensitive cell death in response to UV-C and avirulent strains of Pseudomonas syringae and, correspondingly, supported increased growth of P. syringae The expression of a PHB3 transgene in the phb3 mutant complemented all of these phenotypes. We suggest a model in which the formation of PHB3-ICS1 complexes stabilizes ICS1 to promote SA production in response to stress.

The plant pathogen Pseudomonas syringae secretes multiple effectors that modulate plant defenses. Some effectors trigger defenses due to specific recognition by plant immune complexes, whereas others can suppress the resulting immune responses. The HopZ3 effector of P. syringae pv. syringae B728a (PsyB728a) is an acetyltransferase that modifies not only components of plant immune complexes, but also the Psy effectors that activate these complexes. In Arabidopsis, HopZ3 acetylates the host RPM1 complex and the Psy effectors AvrRpm1 and AvrB3. This study focuses on the role of HopZ3 during tomato infection. In Psy-resistant tomato, the main immune complex includes PRF and PTO, a RIPK-family kinase that recognizes the AvrPto effector. HopZ3 acts as a virulence factor on tomato by suppressing AvrPto1Psy-triggered immunity. HopZ3 acetylates AvrPto1Psy and the host proteins PTO, SlRIPK and SlRIN4s. Biochemical reconstruction and site-directed mutagenesis experiments suggest that acetylation acts in multiple ways to suppress immune signaling in tomato. First, acetylation disrupts the critical AvrPto1Psy-PTO interaction needed to initiate the immune response. Unmodified residues at the binding interface of both proteins and at other residues needed for binding are acetylated. Second, acetylation occurs at residues important for AvrPto1Psy function but not for binding to PTO. Finally, acetylation reduces specific phosphorylations needed for promoting the immune-inducing activity of HopZ3's targets such as AvrPto1Psy and PTO. In some cases, acetylation competes with phosphorylation. HopZ3-mediated acetylation suppresses the kinase activity of SlRIPK and the phosphorylation of its SlRIN4 substrate previously implicated in PTO-signaling. Thus, HopZ3 disrupts the functions of multiple immune components and the effectors that trigger them, leading to increased susceptibility to infection. Finally, mass spectrometry used to map specific acetylated residues confirmed HopZ3's unusual capacity to modify histidine in addition to serine, threonine and lysine residues.

The COVID-19 pandemic shut down undergraduate research programs across the United States. A group of 23 colleges, universities, and research institutes hosted remote undergraduate research programs in the life sciences during Summer 2020. Given the unprecedented offering of remote programs, we carried out a study to describe and evaluate them. Using structured templates, we documented how programs were designed and implemented, including who participated. Through focus groups and surveys, we identified programmatic strengths and shortcomings as well as recommendations for improvements from students' perspectives. Strengths included the quality of mentorship, opportunities for learning and professional development, and a feeling of connection with a larger community. Weaknesses included limited cohort building, challenges with insufficient structure, and issues with technology. Although all programs had one or more activities related to diversity, equity, inclusion, and justice, these topics were largely absent from student reports even though programs coincided with a peak in national consciousness about racial inequities and structural racism. Our results provide evidence for designing remote Research Experiences for Undergraduates (REUs) that are experienced favorably by students. Our results also indicate that remote REUs are sufficiently positive to further investigate their affordances and constraints, including the potential to scale up offerings, with minimal concern about disenfranchising students.

Reader Comments | Submit a Comment The white paper reports the deliberations of a workshop focused on biotic challenges to plant health held in Washington, D.C. in September 2016. Ensuring health of food plants is critical to maintaining the quality and productivity of crops and for sustenance of the rapidly growing human population. There is a close linkage between food security and societal stability; however, global food security is threatened by the vulnerability of our agricultural systems to numerous pests, pathogens, weeds, and environmental stresses. These threats are aggravated by climate change, the globalization of agriculture, and an over-reliance on nonsustainable inputs. New analytical and computational technologies are providing unprecedented resolution at a variety of molecular, cellular, organismal, and population scales for crop plants as well as pathogens, pests, beneficial microbes, and weeds. It is now possible to both characterize useful or deleterious variation as well as precisely manipulate it. Data-driven, informed decisions based on knowledge of the variation of biotic challenges and of natural and synthetic variation in crop plants will enable deployment of durable interventions throughout the world. These should be integral, dynamic components of agricultural strategies for sustainable agriculture.