Local brassinosteroid biosynthesis enables optimal root growth

Brassinosteroid hormones are indispensable for root growth and they control both cell division
and cell elongation through the establishment of an increasing signalling gradient along the
longitudinal root axis. Because of their limited mobility, the importance of brassinosteroid
distribution for achieving the signalling maximum is largely overlooked. Expression pattern
analysis of all known brassinosteroid biosynthetic enzymes revealed that not all cells in the
Arabidopsis thaliana root possess full biosynthetic machinery and completion of biosynthesis
relies on cell-to-cell movement of the hormone precursors. We demonstrate that brassinosteroid
biosynthesis is largely restricted to the root elongation zone where it overlaps with
brassinosteroid signalling maxima. Moreover, optimal root growth requires hormone
concentrations, low in the meristem and high in the root elongation zone attributable to an
increased biosynthesis. Our finding that spatiotemporal regulation of hormone synthesis results
in a local hormone accumulation provides a paradigm for hormone-driven organ growth in the
absence of long-distance hormone transport in plants.