A high concentration of nitrate causes temporal inhibition of lateral root growth by suppressing cell proliferation

Abstract

Root development is highly affected by nutrient supply under various environmental conditions. A high concentration of nitrate is known to inhibit lateral root growth after emergence from the parent root, implying that cell proliferation ceases at a specific stage in lateral root development. However, the mechanism by which external nitrate availability modulates the cell cycle remains to be elucidated. In this study, we analyzed cell cycle regulation during high-nitrate-mediated inhibition of lateral root growth in Arabidopsis. The expression of mitotic reporter genes, such as those for CDKB2;1 and CYCB1;1, was suppressed in emerged lateral root primordium of seedlings grown under high nitrate conditions, which temporally arrested the outgrowth of the primordium for the first 3 days. In contrast, the expression of CDKA;1, which encodes an ortholog of yeast Cdc2/Cdc28p, was not affected by external nitrate availability. These results indicate that the cell cycle in emerged lateral root primordia is possibly arrested at G1/S under high nitrate conditions but that the primordia retain the competence for cell division. The expression of an auxin response marker was reduced in stunted lateral root primordia under high nitrate conditions, but exogenous auxin application could not suppress growth inhibition. This suggests that reduced auxin accumulation and/or signaling are not the primary cause of high-nitrate-mediated inhibition of lateral root growth.