The D-type cyclin CYCD4;1 modulates lateral root density in Arabidopsis by affecting the basal meristem region.

Root cell division occurs primarily in the apical meristem, from which cells are displaced into the basal meristem, where division decreases and cell length increases before the final differentiation zone. The organization of the root in concentric files implies coordinated division and differentiation of cell types, including the xylem pole pericycle cells, which uniquely can resume division to initiate lateral roots (LR). Here, we show that D-type cyclin CYCD4;1 is expressed in meristematic pericycle protoxylem poles and is required for normal LR density. Cycd4;1 mutants also show a displacement of the apical/basal meristem boundary in the pericycle and longer pericycle basal meristem cells, whereas other cell layers and overall meristem size and root growth are unaffected. Auxin is proposed to separately prepattern and stimulate LR initiation. Stimulation is unimpaired in cycd4;1, suggesting CYCD4;1 requirement for normal spacing but not initiation. Both pericycle cell length and LR density phenotypes of cycd4;1 are rescued by low concentrations of applied auxin, suggesting that the basal meristem has a role in determining LR density. We further show CYCD4;1 is rate-limiting for sucrose-dependent LR formation, since CYCD4;1 expression is sucrose-dependent and wild-type roots fully phenocopy cycd4;1 in sucrose absence. We conclude that CYCD4;1 links meristem pericycle cell behavior to LR density consistent with a basal meristem prepatterning model and that D-type cyclins can confer division potential of defined cell types through cell-specific expression patterns.