Abstract
Regulation of branching within perennial prostrate clonal herbs differs from the annual orthotropic species, Arabidopsis and pea, as the dominant signal transported from roots is a branching promoter, not an inhibitor. Trifolium repens, an exemplar of such prostrate species, was used to investigate the interaction between roots and branch development. This study tests whether or not current knowledge when synthesized into a predictive model is sufficient to simulate the branching pattern developing on the shoot distal to a basal root. The major concepts underpinning the model are: (i) bud outgrowth (activation) is stimulated in a dose-dependent manner by branching promoter signals from roots, (ii) the distribution of this net root stimulus (NRS) is uniform throughout the shoot system distal to the basal root but declines geometrically in intensity upon continued enlargement of this shoot system, and (iii) each bud has an outgrowth potential, equal to the activation level of the apical bud in which it forms, that moderates its response to NRS. The validity of these concepts was further tested by running simulations of the branching of a phylogenetically-distanced prostrate perennial monocotyledonous species, Tradescantia fluminensis. For both species the model reasonably accounted for the observed pattern of branching. The outgrowth potential of buds plays an important role in limiting the number of hierarchies of branching that can develop on a plant. In conclusion, for both species, the model accounted for the major factors involved in the correlative regulation of branching and is possibly also pertinent for all prostrate clonal species.
Highlights
A pattern of branching commonly observed on actively growing shoots of a wide range of plants is one in which early-formed nodes bear strong second order branches that are often themselves further branched to give several higher orders of branching
Using T. repens as an exemplar for this group, our work has shown that this stimulatory influence of roots is the dominant factor involved in the regulation of axillary bud outgrowth into branches (Thomas et al, 2002, 2003a, b; Thomas and Hay, 2007, 2008a, 2009)
Numbers of 3rd order branches predicted by the –outgrowth potential (OP) model tended to be greater than the observed values at the distal-most nodes on the primary stem (Fig. 4A), not significantly so as indicated by the conservative Kolmogorov– Smirnov test, whereas the number predicted by the +OP model is markedly lower and the same as on the observed plants
Summary
A pattern of branching commonly observed on actively growing shoots of a wide range of plants is one in which early-formed nodes bear strong second order branches that are often themselves further branched to give several higher orders of branching. With increasing distance from the base of the shoot, the branching vigour declines rapidly until, the latest formed nodes remain unbranched Such a pattern of decline is apparent in prostrate clonal herbs in which the number of branching orders is restricted to three or four as shown, but even in the largest woody plants, under the most favourable growing conditions, the number of branching orders rarely exceeds seven (Hallé et al, 1978; Barthélémy and Caraglio, 2007). Classical studies into the regulation of branching have used mainly erect annual species grown from seed
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