Auxin-mediated cell cycle activation during early lateral root initiation.

Lateral root formation can be divided into two major phases: pericycle activation and meristem establishment. In Arabidopsis, the first lateral root initiation event is spatially and temporally asynchronous and involves a limited number of cells in the xylem pericycle. To study the molecular regulation during pericycle activation, we developed a lateral root-inducible system. Successive treatments with an auxin transport inhibitor and exogenous auxin were used to prevent the first formative divisions and then to activate the entire pericycle. Our morphological and molecular data show that, in this inducible system, xylem pericycle activation was synchronized and enhanced to cover the entire length of the root. The results also indicate that the inducible system can be considered a novel in planta system for the study of synchronized cell cycle reactivation. In addition, the expression patterns of Kip-Related Protein2 (KRP2) in the pericycle and its ectopic expression data revealed that the cyclin-dependent kinase inhibitor plays a significant role in the regulation of lateral root initiation. KRP2 appears to regulate early lateral root initiation by blocking the G1-to-S transition and to be regulated transcriptionally by auxin.