Auxin and cytokinin have long been considered as the key hormones controlling branching in plants. This hypothesis is based on correlative data between bud outgrowth and levels of, or response to, both hormones. However, studies with the ramosus (rms) increased branching mutants of pea (Pisum sativum L.) have highlighted the role of novel signals. By utilising five non-allelic rms mutants, I will describe how the traditional hypothesis does not adequately explain branching in rms plants, and will build-on the proposed model of branching control in pea. Two other mutants, rms6 and rms7, are also introduced.
My initial studies provided a detailed description of rms5. Shoots of dwarf rms5 plants have extensive lateral growth at basal nodes and slightly increased bud outgrowth at aerial nodes. To link rms5 with previous rms studies, rms1 derived from the same wild-type (WT) progenitor as rms5 was generally used in comparative studies.
As observed with rms1, branching was inhibited in reciprocally grafted rms5 and WT plants, indicating that Rms5 controls the level or transport of a long-distance signal. In contrast, branching was not inhibited in reciprocally grafted rms1 and rms5 plants, indicating that Rms1 and Rms5 act closely at the biochemical or cellular level. By utilising a range of grafting techniques, it was deduced that, like Rms1, Rms5 controls an acropetally moving branching inhibitor.
The role of hormones in controlling bud outgrowth in rms plants was examined by quantifying auxin, abscisic acid (ABA) and cytokinins in rms1 and rms5 plants. Like the four previously characterised rms mutants, rms1 through rms4, auxin levels in the shoot tip and basal nodes of WT and rms5 were similar. ABA levels were also similar in the shoot tip of WT, rms1 and rms5, whereas rms1 and rms5 had reduced levels of ABA at basal nodes. Cytokinin quantification indicated that, like rms1, rms3 and rms4 plants, rms5 plants have reduced levels of root xylem sap cytokinins. Despite the reduced xylem sap cytokinin concentration of rms1 and rms5, WT and mutant plants had a similar level of cytokinins in the shoot. As previously described for rms1 through rms4, rms5 did not have impaired [3H]-indole-3-acetic acid ([3H]IAA) transport, with the rate of auxin transport almost 1 mm.h-1 faster than WT plants. Again like rms1, rms5 also causes a reduced response to auxin, with WT rootstocks able to partially restore this response in rms5 shoots.
As well as their similar phenotype, hormone levels and response, Rms1 and Rms5 acted similarly in reciprocal grafting studies with Rms2. Mutant rms2 rootstocks greatly reduced branching in rms1 and rms5 scions, whereas rms1 and rms5 rootstocks slightly increased branching in rms2 scions. The rms1 rms2 and rms2 rms5 double mutants have strongly transgressive phenotypes, further demonstrating a similarity in Rms1 and Rms5 action.
Previous studies have demonstrated that branching in pea is associated with low xylem sap cytokinin concentrations. Rms2 may control this feedback signal which down-regulates cytokinin export from the roots, as rms2 has a high xylem sap cytokinin concentration. The high cytokinin concentration of xylem sap in rms2 rms5 plants further supports the hypothesis that rms2 lacks this feedback signal.
The close association between branching in pea and low xylem sap cytokinin levels is further supported by the xylem sap cytokinin level in Y-grafted WT and rms5 plants, as plants with a branching shoot and a non-branching shoot of equal vigour have a low xylem sap cytokinin concentration. Therefore, the feedback signal is generated by the branching shoot rather than suppressed by the non-branching shoot. Furthermore, mutants with large and small laterals have a similar xylem sap cytokinin concentration, indicating that cytokinin levels are regulated by processes involved in the initiation of bud outgrowth rather than by bud outgrowth itself.
Previous studies have claimed that depleted auxin levels or transport cause decapitation-induced bud growth. I have demonstrated that in whole decapitated plants, visible bud growth occurred prior to depletion of stem IAA levels or a reduction in polar IAA transport. Furthermore, stem IAA levels may be independent of basipetal auxin transport, as decapitated plants with depleted IAA levels still transported [3H]IAA basipetally, 7 h after decapitation. These results question the fundamental role of auxin in axillary bud outgrowth.