BREVIS RADIX is involved in cytokinin-mediated inhibition of lateral root initiation in Arabidopsis.

In contrast to auxin, relatively little is known about the molecular mechanism of cytokinin (CTK) inhibition of lateral root initiation. Previous studies demonstrated that BREVIS RADIX (BRX), a protein of unknown biochemical function, maintains a rate-limiting brassinosteroid biosynthesis enzyme expression to keep brassinosteroid biosynthesis above a critical threshold. Here, we show that the brx-2 mutant is insensitive to exogenous CTK-induced inhibition of lateral root initiation and that this can be restored by embryonic brassinosteroid treatment. However post-embryonic brassinosteroid treatment can not rescue brx-2 mutant phenotype in the presence of CTK. Meanwhile the brassinosteroid receptor defective mutant bri1-6 shows normal CTK-mediated inhibition on LR growth. These results suggest the CTK-mediated inhibition of LR initiation is not directly dependent on brassinosteroid level. Furthermore, compared with wild type, brx-2 exhibits altered auxin response in presumptive founder cells, lateral root primodia and primary root tip in the presence of exogenous CTK. We concluded that CTK inhibition on lateral root initiation depend on specific auxin response loss in presumptive founder cell. The aberrant primary root growth caused by the embryonic brassinosteroid shortage can indirectly result in the lateral root phenotype of brx-2 in presence of CTK.