Auxin inhibition of decapitation-induced branching is dependent on graft-transmissible signals regulated by genes Rms1 and Rms2

Abstract

Decapitation-induced axillary bud outgrowth is a vital mechanism whereby shoots are able to continue normal growth and development. In many plants, including wild-type garden pea (Pisum sativum L.), this process can be inhibited by exogenous auxin. Using the ramosus (rms) increased branching mutants of pea, we present evidence that this response to auxin is dependent on graft-transmissible substance(s) regulated by the genes Rms1 and Rms2. The response to exogenous auxin is massively diminished in decapitated rms1 and rms2 mutant plants. However, basipetal auxin transport is not reduced in intact or decapitated mutants. Grafting rms1 or rms2 shoots onto wild-type rootstocks restored the auxin response, indicating that Rms1 and Rms2 gene action in the rootstock is sufficient to enable an auxin response in mutant shoots. We conclude that Rms1 and Rms2 act in the rootstock and shoot to control levels of mobile substance(s) that interact with exogenous auxin in the inhibition of bud outgrowth after decapitation. At least for rms1, the reduced auxin response is unlikely to be due to an inability of auxin to decrease xylem sap cytokinin content, as this is already low in intact rms1 plants. Consequently, we have genetic evidence that auxin action in decapitated plants depends on at least one novel long-distance signal.

Figure 1

Distribution of radioactivity in intact WT (cv Parvus), rms1-1, and rms2-2 shoots 18 h after supplying [³H]IAA (37 kBq per plant) to the apical bud. The diagram on the far right shows the numbering of stem segments. Each point is the value for a single segment and each line is a single plant. Data shown are from segments 2 to 12. Data are expressed as a percentage of the radioactivity recovered in the segment with the greatest ³H content within the basipetal wave. Absolute values are given in Table I; n = 5 or 6.

Figure 2

Distribution of radioactivity in decapitated auxin-treated WT (cv Parvus), rms1-1, and rms2-2 plants 19 h after supplying [³H]IAA (8 kBq per plant) to the cut stem surface. The diagram on the far right shows the numbering of stem segments. Unlabeled auxin (5,000 mg/L in lanolin) was applied to the stem stump before and after supplying the [³H]IAA. Each point is the value for a single segment and each line is a single plant. Data shown are from segments 3 to 12. Data are expressed as a percentage of the radioactivity recovered in the segment with the greatest ³H content within the basipetal wave. Absolute values are given in Table II; n = 5 or 6.

Figure 3

HPLC radio-histograms of pooled stem segments 2 to 8 from shoots of individual WT (cv Parvus), rms1-1, and rms2-2 plants fed with [³H]IAA, as described in Figure 1. The retention time of IAA is shown by the bar. Two additional replicates for each genotype yielded very similar results.

Figure 4

Effect of auxin application on decapitation-induced branching in WT (cv Parvus), rms1-1, and rms2-2 plants. Auxin was supplied to the cut stem stump as IAA in lanolin (approximately 20 μL per plant), and was replaced daily. n = 6 to 12. Data were collected 6 d after decapitation, and are plotted for each node as means ± se on a logarithmic scale.

Figure 5

Effect of auxin application on decapitation-induced branching in reciprocally grafted WT (cv Weitor) and rms1-2 plants. Immediately after decapitation, auxin was applied to the cut stem stump as IAA (2,000 mg/L) in lanolin (approximately 20 μL per plant), and was replaced daily. n = 10 to 13. Data were collected 7 d after decapitation, and are plotted for each node as means ± se. Notation is scion/rootstock.

Figure 6

Effect of auxin application on decapitation-induced branching in reciprocally grafted WT (cv Torsdag) and rms2-1 plants. Immediately after decapitation, auxin was applied to the cut stem stump as IAA (2,000 mg/L) in lanolin (approximately 20 μL per plant), and was replaced daily. n = 10. Data were collected 7 d after decapitation, and are plotted for each node as means ± se. Notation is scion/rootstock.