A cell-type-specific defect in border cell formation in the Acacia mangium root cap developing an extraordinary sheath of sloughed-off cells.

BACKGROUND AND AIMS Root caps release border cells, which play central roles in microbe interaction and root protection against soil stresses. However, the number and connectivity of border cells differ widely among plant species. Better understanding of key border-cell phenotype across species will help define the total function of border cells and associated genes. METHODS The spatio-temporal detachment of border cells in the leguminous tree Acacia mangium was investigated by using light and fluorescent microscopy with fluorescein diacetate, and their number and structural connectivity compared with that in soybean (Glycine max). KEY RESULTS Border-like cells with a sheet structure peeled bilaterally from the lateral root cap of A. mangium. Hydroponic root elongation partially facilitated acropetal peeling of border-like cells, which accumulate as a sheath that covers the 0- to 4-mm tip within 1 week. Although root elongation under friction caused basipetal peeling, lateral root caps were minimally trimmed as compared with hydroponic roots. In the meantime, A. mangium columella caps simultaneously released single border cells with a number similar to those in soybean. CONCLUSIONS These results suggest that cell type-specific inhibitory factors induce a distinct defective phenotype in single border-cell formation in A. mangium lateral root caps.