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Abstract

Aiming to bloom where they are planted, plants are capable of coordinating and reprograming their growth and development in response to environmental signals. This remarkable plasticity owes to pools of pluripotent stem cells at their growth points. In the shoot apical meristem (SAM) of Arabidopsis thaliana, the maintenance of stem cells is orchestrated by a regulatory network, in which WUSCHEL (WUS)-CLAVATA3 (CLV3) feedback loop plays a pivotal role. Although the regulation of stem cell maintenance by internal developmental cues in the SAM has been elucidated in some detail, how the SAM senses and responds to external stress signals remains mostly elusive. In nature, plants frequently suffer from wounding stress, which is mainly caused by herbivores and abiotic factors. Jasmonate (JA) is the key phytohormone that regulates plant early response to wounding. However, how JA impinges on the activity of the SAM is unclear. In my thesis work, I investigated the activity of the SAM in response to systemic wounding in Arabidopsis thaliana. I found that long-term wounding treatment in leaves inhibits inflorescence stem growth and reduces cell number of the SAM and that this process is mediated by the JA pathway. At a molecular level, exogenous JA triggers the expansion of the WUS protein domain. In turn, WUS suppresses early JA responses in the SAM and thus protects the stem cell system from inhibitory effects of wounding. Consistently, WUS directly promotes the expression of the JA signaling repressor JASMONATE-ZIM DOMAIN 3 (JAZ3) and directly interacts with the JAZ3 protein. Similar to WUS, JAZ3 also inhibits the effect of wounding. In the second part of my study, I characterized the function of TIFY8, a protein related to JAZ3 in stem cell maintenance. TIFY8 also interacts with WUS, but the expression of TIFY8 is not affected by WUS. Genetic evidence reveals that TIFY8 acts as a positive regulator of cell proliferation in the SAM. On the other hand, TIFY8 interacts with WUSCHEL-RELATED HOMEOBOX 5 (WOX5) and potentially negatively regulates stem cell maintenance in the root apical meristem. Overall, my study uncovers a novel mechanism of JA-mediated systemic wounding signal perception and transduction in the SAM. Additionally, it characterizes the distinct roles of two TIFY proteins, namely JAZ3 and TIFY8, in the regulation of stem cell maintenance.