Study On The Molecular Mechanisms Of Cytokinin Repression Of Lateral Root Initiation And Regulation Of Root System Development In Arabidopsis

Cytokinins (CTKs), as an important plant hormone, regulate plant growth and development, including the root development. In this research, we investigated the physiological and molecular mechanisms of exogenous CTK (6-BA) regulation of lateral root (LR) initiation and development in Arabidopsis seedlings, and analyzed the effects of the CTK on primary root elongation and root hairs development. The results are summarized as follows:1. The application of the exogenous CTK inhibits primary root elongation and LR initiation, but promotes LR elongation and root hairs growth.2. The exogenous CTK shortens the root length by reducing the cell divisions in root meristem and by decreasing the cortex cell size in root maturation region. A slight decrease in root meristem size after CTK treatment also has inhibitory effect on root elongation.3. The exogenous CTK repression of LR initiation is not by re-specifying the identity of pericycle founder cells, but through blocking the cell cycling at the G2-to-M transition. During LR initiation, the CTK reduced the transcription levels of A- and B-cyclins (CYC), and of the A-type and B-type cyclin-dependent kinases (CDK) which are all expressed specifically at G2 to M transition. The synchronous down-regulation in the expression of cyclins and CDKs probably inhibited the formation of CDK - CYC complex and the transition of the cell cycling from G2 to M phase. The exogenously applied CTK remarkably induced the expression of the G1 to S transition- and S phase-specific D-type cyclins during LRP development, supporting the point of view that CTK activates cell divisions by stimulating the G1 to S cell cycle transition.4. Exogenously applied auxin (IAA and NAA) at different concentrations could not rescue the CTK-mediated inhibition of LR initiation. In addition, no significant difference was observed in the inhibitory effect of CTK on LR formation between wild-type plants and four mutants defective in auxin receptor or transporter, including tir1, aux1, eir1 and axr4. CTK blocked the cell cycling in pericycle founder cells at the transition of G2 to M, which is a different checkpoint from the G1 to S transition mediated by auxin in the process of LR initiation. Our data suggests that CTK and auxin might control LR initiation through two separate signaling pathways in Arabidopsis.5. The CTK-mediated repression of LR initiation is transmitted through the two-component signal system and mediated by the receptor CRE1.6. CTK and auxin have an additive role in promoting root hairs development of Arabidopsis.