Believe your GFP: cytoplasmic RNA granules play a role in plant defence.

Abstract

Eukaryotic cells contain a variety of RNA granules, membrane-less structures composed of RNA–protein assemblies. Although these granules are involved in many biological processes, their roles are not fully understood. New research in The Plant Journal by Tabassum et al. (2020) shows that plant defence responses against pathogens rely on certain types of RNA granules to modulate gene expression post-transcriptionally. Cytoplasmic granules, which include stress granules and processing bodies (PBs), respond to different environmental or gene expression cues. For example, PBs are present under normal conditions, while stress granules only form under conditions that disrupt translation initiation and lead to ribosomes running off the mRNA (Van Treeck and Parker, 2019). When pathogens attack, plants respond via Pathogen-Associated Molecular Pattern (PAMP)-triggered immunity, in which receptors on the plasma membrane recognize conserved molecular features in the pathogen. These receptors trigger a signalling cascade of mitogen-activated protein kinases (MAPK, MAPKK, and MAPKKK) that phosphorylate distinct targets within the plant cell. In 2014, Maldonado-Bonilla and colleagues discovered that an Arabidopsis protein found in PBs, the tandem zinc finger TZF9, is a phosphorylation target of the MAPK cascade (Maldonado-Bonilla et al., 2014). In their follow-up study (Tabassum et al., 2020), the group identifies the major phosphorylation sites in TZF9 and shows that these sites regulate PAMP-induced TZF9 protein destabilization, reduction in PB structures in the cytosol, and TZF9–RNA binding. Using comparative transcriptomics of total and polysome-associated mRNAs, they also show that many genes are deregulated and translationally biased in the tzf9 mutant. Finally, they demonstrate that TZF9 also interacts with the stress granule component PAB2 (PolyA-binding protein 2). These results suggest that TZF9 mediates defence responses through post-transcriptional regulation (see Figure). The results contribute to the growing body of research suggesting that RNA granules play many important roles. The work comes from the laboratory of Justin Lee, who leads the Cellular Signaling research group at the Leibniz Institute of Plant Biochemistry (Halle, Germany). Naheed Tabassum, a Ph.D. student at the time the research was performed, and Lennart Eschen-Lippold, a Senior Postdoctoral Scientist, are the co-lead authors of the article. Luis Maldonado-Bonilla, a former Postdoctoral Fellow, laid the foundation for this research when he demonstrated that TZF9 could bind to RNA (Maldonado-Bonilla et al., 2014). The group first identified TZF9 as a putative MAPK substrate, and their initial hypothesis was that it would function as a transcription factor, but Maldonado-Bonilla's discovery proved otherwise. The observation that TZFs localize in cytoplasmic foci faced scepticism from colleagues, who argued that the ‘spots’ observed under the microscope might be non-functional aggregates of the protein fused to a fluorescent marker. However, as Tabassum et al. (2020) demonstrated, these structures are dynamic and can change upon elicitation. RNA granules were first morphologically defined as dark-staining, polar structures in insect embryo cells (Metschnikoff, 1865). The next description of RNA granules came more than a century later, coincidentally, from the same institute as Tabassum et al. (2020). Lutz Nover demonstrated that heat shock treatment led to the formation of cytoplasmic stress granules in tomato cells (Nover et al., 1983). Lee, who worked in Lutz Nover's group in the early 1990s, says he did not anticipate that RNA granule research would return to the institute. In humans, these structures are increasingly being recognized as key players in several diseases, including neurodegenerative disorders and cancer (Van Treeck and Parker, 2019). According to Lee, the results of this study will open many new avenues of investigation. For instance, how are the dynamic changes of these membrane-less organelles controlled? Does this process involve protein–protein or protein–RNA interactions? Which mRNA subpopulations are associated to TZF9 or to TZF9-interacting proteins? Other studies have suggested that the translational control of defence responses may also occur through PAB2–mRNA complexes and raise new hypotheses for future research. For instance, Xu et al. (2017) showed that a distinct microbial elicitor leads to preferential translation of specific mRNAs that carry a specific R-motif in the 5′UTR. The motif regulates translational control through an interaction with PABs, including PAB2. Lee and his group are eager to understand a possible connection between their results and those described by Xu et al. (2017). Further research will enable scientists to understand the selectivity processes that determine which mRNAs are translated and which are recycled. In the meantime, if you see cytoplasmic ‘spots’ under the microscope, it may not just be an artefact, but an interesting new discovery.