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

Four ramosus mutants with increased branching at basal andaerial nodes have been used to investigate the genetic regulation of budoutgrowth in Pisum sativum L. (garden pea). Studies oflong-distance signalling, xylem sap cytokinin concentrations, shootauxin level, auxin transport and auxin response are discussed. A modelof branching control is presented that encompasses twograft-transmissible signals in addition to auxin and cytokinin. Mutantsrms1 through rms4 are not deficient in indole-3-aceticacid (IAA) or in the basipetal transport of this hormone. Three of thefour mutants, rms1, rms3 and rms4, have veryreduced cytokinin concentrations in xylem sap from roots. This reductionin xylem sap cytokinin concentration appears to be caused by a propertyof the shoot and may be part of a feedback mechanism induced by anaspect of bud outgrowth. The shoot-to-root feedback signal is unlikelyto be auxin itself, as auxin levels and transport are not correlatedwith xylem sap cytokinin concentrations in various intact and graftedmutant and wild-type plants. Rms1 and Rms2 act inshoot and rootstock to regulate the level or transport ofgraft-transmissible signals. Various grafting studies and double mutantanalyses have associated Rms2 with the regulation of theshoot-to-root feedback signal. Rms1 is associated with a secondunknown graft-transmissible signal that is postulated to move in thedirection of root-to-shoot. Exogenous auxin appears to interact withboth of the signals regulated by Rms1 and Rms2 in theinhibition of branching after decapitation. The action of Rms3and Rms4 is less apparent at this stage, although both appearto act largely in the shoot.