RNA interference (RNAi) in plants can move from cell to cell, allowing for systemic spread of an antiviral immune response. How this cell-to-cell spread of silencing is regulated is currently unknown. Here, we describe that the C4 protein from Tomato yellow leaf curl virus can inhibit the intercellular spread of RNAi. Using this viral protein as a probe, we have identified the receptor-like kinase (RLK) BARELY ANY MERISTEM 1 (BAM1) as a positive regulator of the cell-to-cell movement of RNAi, and determined that BAM1 and its closest homolog, BAM2, play a redundant role in this process. C4 interacts with the intracellular domain of BAM1 and BAM2 at the plasma membrane and plasmodesmata, the cytoplasmic connections between plant cells, interfering with the function of these RLKs in the cell-to-cell spread of RNAi. Our results identify BAM1 as an element required for the cell-to-cell spread of RNAi and highlight that signaling components have been coopted to play multiple functions in plants. Significance In plants, RNA interference (RNAi) is the main antiviral defense mechanism. RNAi moves cell-to-cell through cytoplasmic channels called plasmodesmata, spreading ahead of the viral infection to immunize tissues before arrival of the virus. How this movement is regulated has been elusive. Here, we identify a plant protein, BAM1, localized at plasmodesmata and promoting RNAi spread; both BAM1 and its closest homolog, BAM2, are required for this process. In agreement with their role in promoting RNAi movement, we found that BAM1 and BAM2 are targeted by a viral effector, which acts as a suppressor of RNAi spread. Our work offers insight into the regulation of cell-to-cell spread of RNAi and provides another example of the arms race between pathogens and hosts.

Geminiviruses are plant viruses with limited coding capacity. Geminivirus-encoded proteins are traditionally identified by applying a 10-kDa arbitrary threshold; however, it is increasingly clear that small proteins play relevant roles in biological systems, which calls for the reconsideration of this criterion. Here, we show that geminiviral genomes contain additional ORFs. Using tomato yellow leaf curl virus, we demonstrate that some of these small ORFs are expressed during the infection, and that the encoded proteins display specific subcellular localizations. We prove that the largest of these additional ORFs, which we name V3, is required for full viral infection, and that the V3 protein localizes in the Golgi apparatus and functions as an RNA silencing suppressor. These results imply that the repertoire of geminiviral proteins can be expanded, and that getting a comprehensive overview of the molecular plant-geminivirus interactions will require the detailed study of small ORFs so far neglected.

The plant pathogenic bacterium Ralstonia solanacearum, causal agent of the devastating bacterial wilt disease, is a soil-borne microbe that infects host plants through their roots. The initial mutual recognition between host plants and bacteria and the ensuing invasion of root tissues by R. solanacearum are critical steps in the establishment of the infection, and can determine the outcome of the interaction between plant and pathogen. In this minireview, we will focus on the early stages of the bacterial invasion, offering an overview of the defence mechanisms deployed by the host plants, the manipulation exerted by the pathogen in order to promote virulence, and the alterations in root development concomitant to bacterial colonization.

Pathogens modulate plant cell structure and function by secreting effectors into host tissues. Effectors typically function by associating with host molecules and modulating their activities. This study aimed to identify the host processes targeted by the RXLR class of host-translocated effectors of the potato blight pathogen Phytophthora infestans. To this end, we performed an in planta protein–protein interaction screen by transiently expressing P. infestans RXLR effectors in Nicotiana benthamiana leaves followed by coimmunoprecipitation and liquid chromatography-tandem mass spectrometry. This screen generated an effector–host protein interactome matrix of 59 P. infestans RXLR effectors x 586 N. benthamiana proteins. Classification of the host interactors into putative functional categories revealed over 35 biological processes possibly targeted by P. infestans. We further characterized the PexRD12/31 family of RXLR-WY effectors, which associate and colocalize with components of the vesicle trafficking machinery. One member of this family, PexRD31, increased the number of FYVE positive vesicles in N. benthamiana cells. FYVE positive vesicles also accumulated in leaf cells near P. infestans hyphae, indicating that the pathogen may enhance endosomal trafficking during infection. This interactome dataset will serve as a useful resource for functional studies of P. infestans effectors and of effector-targeted host processes.

Summary: It is increasingly clear that chloroplasts play a central role in plant stress responses. Upon activation of immune responses, chloroplasts are the source of multiple defensive signals, including reactive oxygen species (ROS). Intriguingly, it has been described that chloroplasts establish physical contact with the nucleus, through clustering around it and extending stromules, following activation of effector‐triggered immunity (ETI). However, how prevalent this phenomenon is in plant–pathogen interactions, how its induction occurs, and what the underlying biological significance is are important questions that remain unanswered. Here, we describe that the chloroplast perinuclear clustering seems to be a general plant response upon perception of an invasion threat. Indeed, activation of pattern‐triggered immunity, ETI, transient expression of the Rep protein from geminiviruses, or infection with viruses or bacteria all are capable of triggering this response in Nicotiana benthamiana . Interestingly, this response seems non‐cell‐autonomous, and exogenous treatment with H ₂ O ₂ is sufficient to elicit this relocalization of chloroplasts, which appears to require accumulation of ROS. Taken together, our results indicate that chloroplasts cluster around the nucleus during plant–pathogen interactions, suggesting a fundamental role of this positioning in plant defence, and identify ROS as sufficient and possibly required for the onset of this response.