A Shelter for the Future: How WUSCHEL Protects the Shoot Apical Meristem from Viral Infection

Abstract

Plant development in its majority occurs post-embryonically through the activity of local meristems that provide daughter cells for the development of new organs. It has long been acknowledged that the shoot apical meristem (SAM), which holds the stem cells that will form above-ground organs, is recalcitrant to infection by multiple pathogens, a crucial strategy to safeguard normal development and subsequent generations. However, the molecular mechanisms underlying SAM immunity remain largely unknown. Recently, Wu et al., 2020Wu H. Qu X. Dong Z. Luo L. Shao C. Forner J. Lohmann J.U. Su M. Xu M. Liu X. et al.WUSCHEL triggers innate antiviral immunity in plant stem cells.Science. 2020; 370: 227-231Crossref PubMed Scopus (19) Google Scholar identified the stem cell regulator WUSCHEL (WUS) as a key player in the regulation of SAM responses to viral infection. In the SAM of non-infected plants, WUS is produced in the organizing center (OC) and moves to the upper layers of the central zone (CZ), where it plays a key role in the maintenance of the stem cell niche (Mayer et al., 1998Mayer K.F.X. Schoof H. Haecker A. Lenhard M. Jürgens G. Laux T. Role of WUSCHEL in regulating stem cell fate in the Arabidopsis shoot meristem.Cell. 1998; 95: 805-815Abstract Full Text Full Text PDF PubMed Scopus (1205) Google Scholar; Yadav et al., 2011Yadav R.K. Perales M. Gruel J. Girke T. Jönsson H. Reddy G.V. WUSCHEL protein movement mediates stem cell homeostasis in the Arabidopsis shoot apex.Genes Dev. 2011; 25: 2025-2030Crossref PubMed Scopus (350) Google Scholar; Figure 1A). However, upon infection by Cucumber mosaic virus (CMV), the WUS-containing region expands to the outer layers of the peripheral zone (PZ) and young floral primordia (Figure 1B), perfectly overlapping with the region of viral exclusion in the SAM. The authors found that WUS binds to and represses the expression of genes encoding S-adenosyl-l-methionine-dependent methyltransferases involved in ribosomal RNA methylation, thus impairing ribosomal assembly and global protein synthesis (Figure 1C). Consequently, in the WUS-containing region, viral protein accumulation is reduced, limiting the spreading of the virus in the SAM. The work by Wu et al., 2020Wu H. Qu X. Dong Z. Luo L. Shao C. Forner J. Lohmann J.U. Su M. Xu M. Liu X. et al.WUSCHEL triggers innate antiviral immunity in plant stem cells.Science. 2020; 370: 227-231Crossref PubMed Scopus (19) Google Scholar provides new insights into SAM immune responses to viral infection. This property has been widely used in horticulture for the propagation of healthy plants through meristem tip culture. Therefore, these findings are particularly relevant to the plant breeding industry. Given that WUS inhibits global protein synthesis, it will be important to further understand the impact of such defense mechanisms in stem cell maintenance and differentiation, particularly in cases of prolonged viral exposure. Moreover, the questions remain of how WUS is induced by viral infection and which/if additional factors are at play. Follow-up work should thus unveil exciting new aspects of SAM innate immunity. No conflict of interest declared.