Weight-induced radial growth in plant stems depends on PIN3

How multiple growth programs coordinate during development is a fundamental question in biology. During plant stem development, radial growth is continuously adjusted in response to longitudinal-growth-derived weight increase to guarantee stability.^1,2,3 Here, we demonstrate that weight-stimulated stem radial growth depends on the auxin efflux carrier PIN3, which, upon weight increase, expands its cellular localization from the lower to the lateral sides of xylem parenchyma, phloem, procambium, and starch sheath cells, imposing a radial auxin flux that results in radial growth. Using the protein synthesis inhibitor cycloheximide (CHX) or the fluorescent endocytic tracer FM4-64, we reveal that this expansion of the PIN3 cellular localization domain occurs because weight increase breaks the balance between PIN3 biosynthesis and removal, favoring PIN3 biosynthesis. Experimentation using brefeldin A (BFA) treatments or arg1 and arl2 mutants further supports this conclusion. Analyses of CRISPR-Cas9 lines for Populus PIN3 orthologs reveals that PIN3 dependence of weight-induced radial growth is conserved at least in these woody species. Altogether, our work sheds new light on how longitudinal and radial growth coordinate during stem development.