During plant growth and defense, cell cycle activity needs to be coordinated with cell wall integrity. Little is known about how this coordination is achieved. Here, we investigated coordination in Arabidopsis thaliana seedlings by studying the impact of cell wall damage (CWD, caused by cellulose biosynthesis inhibition) on cytokinin homeostasis, cell cycle gene expression and cell shape in root tips. CWD inhibited cell cycle gene expression and increased transition zone cell width in an osmosensitive manner. These results were correlated with CWD-induced, osmosensitive changes in cytokinin homeostasis. Expression of CYTOKININ OXIDASE/DEHYDROGENASE 2 and 3 (CKX2, CKX3), which encode cytokinin-degrading enzymes, was induced by CWD and reduced by osmoticum treatment. In nitrate reductase1 nitrate reductase2 (nia1 nia2) seedlings, CKX2 and CKX3 transcript levels were not increased and cell cycle gene expression was not repressed by CWD. Moreover, established CWD-induced responses, such as jasmonic acid, salicylic acid and lignin production, were also absent, implying a central role of NIA1/2-mediated processes in regulation of CWD responses. These results suggest that CWD enhances cytokinin degradation rates through a NIA1/2-mediated process, leading to attenuation of cell cycle gene expression.

Department of Biology Høgskoleringen 5 Realfagbygget Norwegian University of Science and Technology 7491 Trondheim Norway

Laboratory of Growth Regulators Centre of the Region Haná for Biotechnological and Agricultural Research Institute of Experimental Botany of the Czech Academy of Sciences and Faculty of Science of Palacký University Šlechtitelů 27 CZ 78371 Olomouc Czech Republic

Laboratory of Molecular Plant Physiology and Functional Genomics and Proteomics of Plants CEITEC Central European Institute of Technology Masaryk University Kamenice 5 CZ 625 00 Brno Czech Republic

School of Biosciences University of Melbourne Parkville VIC 3010 Australia

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