The Class VIII myosin ATM1 is integral for root apical meristem function

Myosins are evolutionarily conserved motor proteins that interact with actin filaments to regulate organelle transport, cytoplasmic streaming and cell growth. Plant specific Class XI myosin proteins direct cell division and root organogenesis. However, the roles of Class VIII myosin proteins in plant growth and development are less understood. Here, we investigated the function of an auxin-regulated Class VIII myosin, Arabidopsis thaliana Myosin 1 (ATM1), using genetics and live cell microscopy. ATM1 is a plasmodesmata-localized protein that is enriched in actively dividing cells in the root apical meristem (RAM). Loss of ATM1 function results in impaired primary root growth due to decreased RAM size and reduced cell proliferation in a sugar dependent manner. Examination of stem cell marker line expression in atm1-1 indicated impaired division of the lateral root cap and columella cells and a diminished auxin response. Transcriptome analysis of atm1-1 linked these growth defects to dysregulation of cell cycle and auxin pathway genes. Complementation of ATM1 loss-of- function mutant restored root growth and cell cycle progression in the root meristem. Collectively, these results provide novel evidence that ATM1 functions to influence cell proliferation and differentiation in primary roots in response to auxin and sugar cues. Overall design: Col-0 (control) and atm1-1 (mutant) seedlings were grown on media supplemented with or without (mock) 15mM glucose or sucrose for 5 days (5DO). Three biological replicates were collected for each condition.