AXR1 acts after lateral bud formation to inhibit lateral bud growth in Arabidopsis

Abstract

The AXR1 gene of Arabidopsis is required for many auxin responses. The highly branched shoot phenotype of mature axr1 mutant plants has been taken as genetic evidence for a role of auxin in the control of shoot branching. We compared the development of lateral shoots in wild-type Columbia and axr1-12 plants. In the wild type, the pattern of lateral shoot development depends on the developmental stage of the plant. During prolonged vegetative growth, axillary shoots arise and develop in a basal-apical sequence. After floral transition, axillary shoots arise rapidly along the primary shoot axis and grow out to form lateral inflorescences in an apical-basal sequence. For both patterns, the axr1 mutation does not affect the timing of axillary meristem formation; however, subsequent lateral shoot development proceeds more rapidly in axr1 plants. The outgrowth of lateral inflorescences from excised cauline nodes of wild-type plants is inhibited by apical auxin. axr1-12 nodes are resistant to this inhibition. These results provide evidence for common control of axillary growth in both patterns, and suggest a role for auxin during the late stages of axillary shoot development following the formation of the axillary bud and several axillary leaf primordia.

Figure 1

Morphometric analysis of lateral shoot development of wild-type and axr1-12 plants grown in long photoperiods. A, Time course of lateral shoot development in rosette leaf axils. For each genotype, five to eight plants were sampled every 3 d between d 3 and 18 after bolting of the primary inflorescence, and their rosette leaf axils were examined. Lateral shoot development was scored into three developmental stages (no visible bud [⋄]; visible bud [□]; and elongating inflorescence [○]), and the proportion of laterals at each stage calculated for each plant. The mean proportions ± se for each time point are plotted. B, Lateral shoot development at consecutive nodes along the primary shoot axis, 6 d after bolting. For one wild-type (upper row) and one axr1-12 plant (lower row), cotyledons and leaves were dissected from the primary shoot axis with their attached axillary shoots and laid out in the sequence in which they developed (left to right = base to apex). The uppermost rosette leaves are marked by arrows. Bar = 5 cm. C, Mean lateral inflorescence lengths ± se at consecutive nodes along the primary shoot axis 18 d after the primary inflorescence started bolting. Five plants were analyzed for each genotype (wild type [○]; axr 1–12 [●]). Lateral inflorescence lengths for axils without a visible bud or with a vegetative bud were scored as zero. As the mean number of cauline leaves was 3 for wild-type and 6 for axr1-12 plants, plots for wild type were started three node positions basal from those of axr1-12. Thus, node position 0 represents on average the uppermost rosette leaf for both genotypes.

Figure 2

Lateral shoot development at consecutive nodes along the primary shoot axis in short photoperiods 49 d after the end of cold treatment. For one wild-type (upper row) and one axr1-12 plant (lower row), the oldest 27 leaves (without the cotyledons that had senesced) were dissected from the primary shoot axis with their attached axillary shoots and laid out in the sequence in which they developed (left to right = base to apex). The undissected apical parts of the rosettes are shown on the right. Bar = 5 cm.

Figure 3

Longitudinal sections of wild-type (A–D) and axr1-12 (E–H) shoots grown in long photoperiods stained with toluidine blue. Plants in A and C and E to G were fixed on d 15, and plants in B, D, and H were fixed on d 20 after the end of cold treatment. Bar in A (for A and E) and in bar in B (for B–D and F–H) = 100 μm. A and E, Vegetative shoots. m, Vegetative shoot apical meristem. Arrowheads point to the axils of leaf primordia and more mature leaves sectioned in a median plane and lacking axillary meristems. B and F, Shoots at floral transition. Elongation of the primary inflorescence is initiated by cell divisions in the region underlying the inflorescence meristem (m). Arrowheads mark clusters of meristematic cells in the axils of leaf primordia. C and G, Shoots at floral transition. Arrowheads point to meristematic cells at the base of the petiole of more mature leaves. D and H, Shoots whose inflorescence meristems (m) have produced flower primordia (f). Axillary meristematic regions (marked by arrowheads) are of increased size or have started to bulge out.

Figure 4

Effect of the synthetic auxin 1-NAA on lateral inflorescence outgrowth from excised nodes of the wild type and axr1-12. The lowermost cauline node was excised from the elongating primary inflorescence of axenically grown plants and inserted between two separate agar slabs. Slabs contained either no 1-NAA or 1 or 10 μm 1-NAA in contact with the apical or with the basal part of the stem. Mean lateral shoot lengths ± se were determined each day until d 8 following excision. Measurements for some lateral shoots that wilted and ceased elongating after d 5 were included in the earlier time points but were excluded from the later time points. Node numbers tested for each treatment are: ○, 0 μm 1-NAA apical/0 μm 1-NAA basal, 5 to 8 wild type and 14 to 16 axr1-12; ▵, 0 μm 1-NAA apical/1 μm 1-NAA basal, 4 to 6 wild type and 5 axr1-12; □, 0 μm 1-NAA apical/10 μm 1-NAA basal, 4 wild type and 5 to 6 axr1-12; ▴, 1 μm 1-NAA apical/0 μm 1-NAA basal, 12 to 15 wild type and 10 axr1-12; ▪, 10 μm 1-NAA apical/0 μm 1-NAA basal, 8 wild type and 12 to 13 axr1-12.