An analysis of fasciated mutants of Arabidopsis thaliana and the role of cytokinin in this phenotype

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Micrografting techniques for testing long-distance signalling in Arabidopsis

Grafting in species other than Arabidopsis has generated persuasive evidence for long-distance signals involved in many plant processes, including regulation of flowering time and shoot branching. Hitherto, such approaches in Arabidopsis have been hampered by the lack of suitable grafting techniques. Here, a range of micrografting methods for young Arabidopsis seedlings are described. The simplest configuration was a single-hypocotyl graft, constructed with or without a supporting collar, allowing tests of root–shoot communication. More complex two-shoot grafts were also constructed, enabling tests of shoot–shoot communication. Integrity of grafts and absence of adventitious roots on scions were assessed using plants constitutively expressing a GUS gene as one graft partner. Using the max1 (more axillary growth) and max3 increased branching mutants, it was shown that a wild-type (WT) rootstock was able to inhibit rosette branching of mutant shoots. In two-shoot grafts with max1 and WT shoots on a max1 rootstock, the mutant shoot branched profusely, but the WT one did not. In two-shoot grafts with max1 and WT shoots on a WT rootstock, neither shoot exhibited increased branching. The results mirror those previously demonstrated in equivalent grafting experiments with the ramosus mutants in pea, and are consistent with the concept that a branching signal is capable of moving from root to shoot, but not from shoot to shoot. These grafting procedures will be valuable for revealing genes associated with many other long-distance signalling pathways, including flowering, systemic resistance and abiotic stress responses

MAX2 participates in an SCF complex which acts locally at the node to suppress shoot branching

The Arabidopsis gene ORE9/MAX2 encodes an F-box leucine-rich repeat protein. F-box proteins function as the substrate-recruiting subunit of SCF-type ubiquitin E3 ligases in protein ubiquitination. One of several phenotypes of max2 mutants, the highly branched shoot, is identical to mutants at three other MAX loci. Reciprocal grafting, double mutant analysis and gene cloning suggest that all MAX genes act in a common pathway, where branching suppression depends on MAX2 activity in the shoot, in response to an acropetally mobile signal that requires MAX3, MAX4 and MAX1 for its production. Here, we further investigate the site and mode of action of MAX2 in branching. Transcript analysis and a translational MAX2–GUS fusion indicate that MAX2 is expressed throughout the plant, most highly in developing vasculature, and is nuclear-localized in many cell types. Analysis of cell autonomy shows that MAX2 acts locally, either in the axillary bud, or in adjacent stem or petiole tissue. Expression of MAX2 from the CaMV 35S promoter complements the max2 mutant, does not affect branching in a wild-type background and partially rescues increased branching in the max1, max3 and max4 backgrounds. Expression of mutant MAX2, lacking the F-box domain, under the CaMV 35S promoter does not complement max2, and dominant-negatively affects branching in the wild-type background. Myc-epitope-tagged MAX2 interacts with the core SCF subunits ASK1 and AtCUL1 in planta. We conclude that axillary shoot growth is controlled locally, at the node, by an SCFMAX2, the action of which is enhanced by the mobile MAX signal

Flower Development

Flowers are the most complex structures of plants. Studies of Arabidopsis thaliana, which has typical eudicot flowers, have been fundamental in advancing the structural and molecular understanding of flower development. The main processes and stages of Arabidopsis flower development are summarized to provide a framework in which to interpret the detailed molecular genetic studies of genes assigned functions during flower development and is extended to recent genomics studies uncovering the key regulatory modules involved. Computational models have been used to study the concerted action and dynamics of the gene regulatory module that underlies patterning of the Arabidopsis inflorescence meristem and specification of the primordial cell types during early stages of flower development. This includes the gene combinations that specify sepal, petal, stamen and carpel identity, and genes that interact with them. As a dynamic gene regulatory network this module has been shown to converge to stable multigenic profiles that depend upon the overall network topology and are thus robust, which can explain the canalization of flower organ determination and the overall conservation of the basic flower plan among eudicots. Comparative and evolutionary approaches derived from Arabidopsis studies pave the way to studying the molecular basis of diverse floral morphologies