Relationships among shoot sinks for resources exported from nodal roots regulate branch development of distal non‐rooted portions of Trifolium repens L.

2010

Evol Ecol

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The influence of nodal rooting on branching was studied in three evolutionarily and morphologically diverse species of prostrate clonal herbs: Tradescantia fluminensis (a monocotyledonous extreme 'phalanx' species), Calystegia silvatica (a dicotyledonous extreme 'guerrilla' species) and Trifolium repens (a dicotyledonous intermediate species). In all three, branch development from axillary buds is regulated by a positive signal produced by roots together with inhibitory influences from both pre-existing branches and shoot apical buds (apical dominance). Responses to nodal roots are cumulative and increased root activity leads to more vigorous bud outgrowth. In the absence of nodal roots, a single basal root system is unable to maintain continued extension growth of the shoot. We suggest that as individual nodal roots and stem internodes are both short-lived in these nodally-rooting clonal species, the plants' investment in them is minimal. Thus, in contrast to perennial species lacking nodal roots, individual root systems in prostrate clonal herbs are small and stems have little secondary thickening and development of long-distance transport tissues. Hence the decline in extension growth of the shoot in the absence of nodal roots could be linked to the weak development of long-distance transport tissues in their relatively thin horizontal stems and to resource sharing between primary stems and lateral branches (as suggested by the greater retardation of primary stem growth in the more profusely branched 'phalanx' species (Trifolium and Tradescantia) than in the weakly branched 'guerrilla' species, Calystegia). These findings are consistent with the view that the long-term persistence of genotypes of nodally-rooting prostrate species is dependent upon them encountering the moist conditions required to facilitate the continual development of new young nodal root systems.

Section: Basal Root Vigoursupporting

confidence: 78%

Section: Study Speciesmentioning

confidence: 99%

The role of nodal roots in prostrate clonal herbs: ‘phalanx’ versus ‘guerrilla’

2010

Evol Ecol

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“…exploitation of the resources of a niche. Our observation that RSR regulates both axillary bud outgrowth and stem elongation in T. repens (Thomas et al, 2003b) suggests it is likely that these inter-specific differences are mediated by differences in RSR supply and/or allocation.…”

Section: Discussionmentioning

confidence: 88%

Evidence suggests plagiotropic clonal species have evolved a branching physiology emphasizing regulation by nodal roots

2004

Evol Ecol

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In the plagiotropic nodally rooting clonal herb, Trifolium repens, the development of branches on stems is primarily controlled by the presence of nodal roots, and apical dominance is of secondary importance; only six to ten branches form distal to the youngest nodal root on a horizontal stem. We assessed the hypothesis that this phenomenon is general for clonal herbs with prostrate nodally rooting stems, and that they all have the same physiological system regulating branching, by testing a selection of species from diverse angiosperm families that exhibit either 'phalanx' (Leptinella (Asteraceae), Hydrocotyle (Apiaceae), Acaena (Rosaceae)) or 'guerilla' (Vinca (Apocynaceae), Glechoma and Lamiastrum (Lamiaceae)) growth strategies. In all these species the establishment of a single nodal root on a prostrate stem, otherwise prevented from nodally rooting, induced the outgrowth of a limited number of axillary buds (the number of which was species specific) at the nodes immediately distal to the newly established root, thereby indicating a phenotypic response similar to that in T. repens. Furthermore, their branching responses to manipulative treatments were also similar to those of T. repens, indicating that their regulatory physiology of axillary bud outgrowth from their prostrate stems is similar. We conclude that, for the group of prostrate nodally rooting clonal herbs as a whole, the apical dominance phenotype arises predominantly from variation in the supply of resources from nodal roots rather than from repression of axillary buds by apical tissues (apical dominance). We suggest that evolution of such a physiological mechanism enhances the exploration for patchily distributed favourable nodal rooting sites by regulating shoot development so as to efficiently utilise the diminishing intra-plant availability of root-supplied resources. For the species examined, inter-specific variation in intensity of branching response to a nodal root is shown to be linked to a trade off in foraging strategy, with the allocation of resources primarily to explorative growth (long internodes, few branches) in 'guerilla' species or to exploitive growth (short internodes, many branches) in 'phalanx' species.

“…Under these circumstances, following an initial phase in which axillary buds grow out vigorously into branches, a very predictable restriction in the branching pattern develops in all ten species that have been examined (Lötscher and Nösberger, 1996; Thomas et al ,2002, 2003a; Thomas and Hay, 2004, 2008 b , 2010), as shown for Trifolium repens L. (white clover) in Fig. 1.…”

Section: Introductionmentioning

confidence: 91%

“…The pattern of branching decline at successively formed unrooted nodes then repeats precisely that occurring in response to the basal root system (Thomas et al , 2002). 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, 2003 a , b ; Thomas and Hay, 2007, 2008 a , 2009). In the erect-stemmed model plant systems described above, however, it has clearly been demonstrated that there is a network of shoot and root feedback and interacting signals that collectively operate to regulate branching (Beveridge, 2006; Aguilar-Martínez et al , 2007; Simons et al , 2007; Ongaro and Leyser, 2008; Ferguson and Beveridge, 2009; Shimizu-Sato et al , 2009; Dun et al , 2009 a , b ; Leyser, 2009; Domagalska and Leyser, 2011).…”

Section: Introductionmentioning

confidence: 99%

Differential bud activation by a net positive root signal explains branching phenotype in prostrate clonal herbs: a model

2014

EXBOTJ

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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.