BRI1 activity in the root meristem involves post-transcriptional regulation of PIN auxin efflux carriers

Spatiotemporal coordination between multiple hormonal pathways is a key determinant of plant growth. This coordination can be mediated by distribution of the auxin network via the action of PIN auxin efflux carriers. We showed that brassinosteroids (BRs) promote cell proliferation and cell expansion of meristematic cells. Hence, roots with high epidermal expression of the BR receptor BRI1 have enlarged meristem whereas bri1 mutant has a reduced meristem size. Because the extent of mitotic activity and differentiation is tightly linked to auxin gradient we further asked how the BR pathway integrates with current proposed models for PIN regulation. We showed that the small meristem of BR deficient plants does not involve transcriptional modulation of PIN 1, 3 and 7 genes. Here, we found that PIN2 and PIN4 are under transcriptional regulation. However, their accumulation in the epidermis/cortex and columella respectively was also determined by BRs in a post-transcriptional manner. Thus, BRs impinge on auxin distribution through distinct regulatory modes and the self-organizing auxin system represents at least one mechanism that contributes to BR-mediated growth.