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MAX1 and MAX2 control shoot lateral branching in Arabidopsis

Department of Biology, University of York, PO Box 373, York YO10 5YW, UK

View larger version (122K): [in a new window]   Fig. 1. Phenotypes of the max mutants. (A) Wild type (En-2, left) and max1-1 (V367, right) soon after floral transition. (B) Wild type (Col, left) and max1-1 (backcross 3 into Col, right) at maturity. (C) Wild-type (top) and max2-1 (bottom) light-grown seedlings.

View larger version (26K): [in a new window]   Fig. 2. Growth and lateral branching of wild-type (wt), max1-1, max2-1 and max1-1 max2-1 double mutant shoots. Plants were analysed near maturity, when flower production of the primary inflorescence had ceased (after 45-46 days of growth). (A) Length of the primary inflorescence. (B) Number of vegetative, leaf-bearing nodes on the primary shoot axis. (C) Number of first order lateral branches of at least 0.5 cm length, from nodes in the rosette and from cauline nodes on the primary inflorescence. (D) Fresh weight (FW) of the lateral shoot branches expressed as a proportion of the total shoot FW. (A-D) Means and 95% confidence intervals of the means are shown, n=17-18.

View larger version (62K): [in a new window]   Fig. 3. Lateral shoot development at consecutive node positions of wild-type (wt), max1-1 and max2-1 shoots. (A,B) Leaves and associated axillary shoots dissected from the shoot axis and laid out in the order of emergence, oldest leaf to the left. Scale bar: 5 cm. (A) Vegetative shoots after 52 days of growth in short photoperiods. The oldest 25 leaves and their axillary shoots have been dissected from the shoot axis. The remaining apical parts of the shoots are shown at the right. (B) Flowering shoots grown in long photoperiods, 9 days after the primary inflorescence started elongating. All the leaves and their axillary shoots were dissected from the shoot axis, the remaining primary inflorescences are shown at the top right. (C) Mean lateral inflorescence lengths and 95% confidence intervals of the means at consecutive node positions, conditions as in B. For nodes carrying a vegetative axillary shoot, lateral inflorescence length was scored as 0. Number of shoots analysed: wt n=12; max1-1, max2-1 n=7. The numbers of leaf-bearing nodes along the primary shoot axis ranged between 18 and 23 for wt, 19 and 23 for max1-1 and 19 and 27 for max2-1.

View larger version (81K): [in a new window]   Fig. 4. Timing of axillary shoot initiation in wild type, max1-1 and max2-1. Series of transverse sections of individual shoots grown in short photoperiods for 36 days were prepared and axillary shoot development at consecutive node positions was scored into three stages. (A-C) Sections from wild-type shoots, showing the developing axillary shoots (arrowheads) illustrating the stages scored. Scale bars: 100 µm. (A) Stage 1: axillary cell divisions at the base of a developing leaf, visible above the insertion point into the shoot axis. (B) Stage 2: axillary meristem bulging out from the base of the subtending leaf; the angle with the adaxial side of the leaf is at least 45°. (C) Stage 3: first axillary leaf primordium separated from the axillary shoot meristem by a cleft. (D) Node number from the shoot apex at which the three early stages of axillary shoot development first occurred in wild type (wt), max1-1 and max2-1. Circles represent the median, bars extend between the minimum and maximum observed node number of first occurrence of each stage. Number of shoots examined: wild type n=9; max1-1, max2-1, n=11.

View larger version (88K): [in a new window]   Fig. 5. Abnormal lateral branches observed in max1-1, max2-1 and max1-1 max2-1 double mutant shoots. (A) Lateral growth from one rosette leaf axil of wild type (left) and max2-1 (right). There is a single lateral inflorescence in wild type and two lateral inflorescences in the mutant. s, leaf subtending the lateral shoot; a, axillary leaf; i, stem of lateral inflorescence. (B-D) Fasciated lateral inflorescences of (B) max1-1, (C) max2-1, (D) max1-1 max2-1. (E-H) Transverse sections of (E) a wild-type lateral inflorescence and (F) max1-1, (G) max2-1 and (H) max1-1 max2-1 fasciated lateral inflorescences. Scale bars: 1 cm (A-D); 100 µm (E-H).

View larger version (176K): [in a new window]   Fig. 6. Scanning electron micrographs of developing axillary shoots at the base of the oldest pair of leaves of wild type (A-D), max1-1 (E-H) and max2-1 (I-L). Plants were fixed after 14-16 days of growth in long photoperiods. The figure shows normal wild-type buds and mutant buds that appeared abnormal. Scale bars: 100 µm. (A,E,I) Semicircular zone marks initiation of axillary shoot. The size increased in some mutant axils. (B,F,J) Axillary shoot meristem bulging out. The size increased in some mutant axils. (C,G,K) Formation of axillary leaf primordia. Two primordia form at opposite positions in the wild type, but the position can be random in the mutants. (D,H,L) Leaf bases with more than one axillary shoot meristem. One of the two axillary shoots is retarded in the wild type, but in the mutants both develop.

View larger version (19K): [in a new window]   Fig. 7. Size of axillary meristems in the axils of the oldest two leaves of wild-type (wt), max1-1 and max2-1 plants analysed by scanning electron microscopy. The area occupied by axillary meristematic zones or axillary meristems prior to axillary leaf primordium formation was measured on scanning electron micrographs of individual leaf axils. Relative frequency distribution is shown. Number of measurements: wild type n=37, max1-1 n=16, max2-1 n=33.

View larger version (21K): [in a new window]   Fig. 8. Hypocotyl length of wild-type (wt), max1-1 and max2-1 seedlings after 6 days of growth in the dark, or at two different light intensities in a 16-hour photoperiod. Means and 95% confidence intervals of the means are shown, n=20-21.

View larger version (35K): [in a new window]   Fig. 9. MAX2 encodes an F-box leucine-rich repeat protein. (A) Map-based cloning of the MAX2 gene. (Top) The markers flanking MAX2 that were used to screen for recombinants (m429, BIO2) and the closest flanking markers (F14N22-L, F7D19-H) that were located 57 kb apart on two overlapping BAC clones. The number of recombinant individuals, in a mapping population of 1300 plants, is given for each marker. (Bottom) The region between the closest flanking markers is enlarged to show the localisation of the PCR products for both markers (grey bars), the predicted gene structure (arrows), and the BAC subclones tested for mutant rescue (black and white bars). Mutant rescue by clones a and b identified F14N22.11 as the MAX2 gene. (B) The predicted MAX2 protein sequence contains an F-box motif (underlined) and imperfect leucine-rich repeats (LRR). Positions with similar amino acids in several repeats are shaded. Amino acids affected by the max2-1 and max2-2 mutations are boxed and the predicted changes are shown. (C) Alignment of the predicted MAX2 F-box motif with a translation of the corresponding region found in two partial Medicago truncatula ESTs homologous to MAX2 (AL369069, BE325112), and with the F-boxes of other Arabidopsis proteins. A general F-box consensus (Patton et al., 1998) is given above the MAX2 sequence and the residues of MAX2 that match this consensus are marked (*). The second column shows the classification of predicted Arabidopsis F-box proteins used by Xiao and Jang (Xiao and Jang, 2000).