Long-distance signalling and a mutational analysis of branching in pea

Four ramosus mutants with increased branching at basal and aerial nodes hav e been used to investigate the genetic regulation of bud outgrowth in Pisum sativum L. (garden pea). Studies of long-distance signalling, xylem sap cy tokinin concentrations, shoot auxin level, auxin transport and auxin respon se are discussed. A model of branching control is presented that encompasse s two graft-transmissible signals in addition to auxin and cytokinin. Mutan ts rms1 through rms4 are not deficient in indole-3-acetic acid (IAA) or in the basipetal transport of this hormone. Three of the four mutants, rms1, r ms3 and rms4, have very reduced cytokinin concentrations in xylem sap from roots. This reduction in xylem sap cytokinin concentration appears to be ca used by a property of the shoot and may be part of a feedback mechanism ind uced by an aspect of bud outgrowth. The shoot-to-root feedback signal is un likely to be auxin itself, as auxin levels and transport are not correlated with xylem sap cytokinin concentrations in various intact and grafted muta nt and wild-type plants. Rms1 and Rms2 act in shoot and rootstock to regula te the level or transport of graft-transmissible signals. Various grafting studies and double mutant analyses have associated Rms2 with the regulation of the shoot-to-root feedback signal. Rms1 is associated with a second unk nown graft-transmissible signal that is postulated to move in the direction of root-to-shoot. Exogenous auxin appears to interact with both of the sig nals regulated by Rms1 and Rms2 in the inhibition of branching after decapi tation. The action of Rms3 and Rms4 is less apparent at this stage, althoug h both appear to act largely in the shoot.