Brassinosteroid transport.

Brassinosteroids (BRs) are steroidal plant hormones that are important regulators of plant growth. These compounds are widely distributed throughout reproductive and vegetative plant tissues. This raises the question of whether or not BRs are transported over long distances between these tissues. Several lines of evidence indicate that this is not the case. Exogenous BRs move only slowly, if at all, after application to leaves; grafting BR-deficient mutants to wild-type plants has no phenotypic effect; removal of the apical bud or mature leaves does not reduce BR levels in the remaining internodes; and, in tomato, wild-type sectors do not substantially alter the growth of BR-deficient sectors when the two types are together in a variegated leaf. Although BRs do not undergo long-distance transport they may influence long-distance signalling by altering auxin transport. At the cellular level, BRs do appear to be transported. The enzymes for BR biosynthesis appear to be located within the cell, and to be associated with the endoplasmic reticulum, in particular. BR reception, on the other hand, is thought to occur on the exterior cell surface. Therefore, BRs must move from the interior of the cell to the exterior, where they are perceived by the same cell or by neighbouring cells. The existence of a feedback system, whereby bioactive BRs negatively regulate their own biosynthesis, provides further evidence that individual cells are able to both perceive and synthesize BRs.