CASE STUDY 9.1 Models for control of shoot branching: more than just auxin and cytokinin

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C.G.N. Turnbull

The conventional view of apical dominance control in plants is that auxin inhibits branching whereas cytokinin promotes it. The original paper by Sachs and Thimann (1967) examined responses to auxin applied to cut shoot stumps or cytokinin applied directly to buds. Radiolabelled auxin applied to a shoot stump is transported in a polar manner down the stem (Figure 9.3). These data were extrapolated to the assumption that endogenous hormones will behave similarly, that is, on decapitation (removal of the shoot tip and hence a main source of auxin) auxin supply down the stem will diminish and bud growth is permitted. However, endogenous IAA and cytokinin levels in buds both increase within a few hours of shoot decapitation (Gocal et al. 1991; Turnbull et al. 1997). Since the late 1980s, many transgenic plants have been created with altered auxin or cytokinin content. Superficially, the phenotype of these lines supports the auxin–cytokinin hypothesis: high-cytokinin ipt plants branch more, as do low-auxin iaaL plants (Medford et al. 1989; Romano et al. 1991). However, closer examination of the developmental sequences reveals that often branching is hardly affected at all in young plants, even though the constitutively expressed genes cause altered hormone content at all stages of the life cycle. Instead, branching is promoted only later in development, around the time of floral initiation when wild-type plants also branch. Auxin and cytokinin therefore appear to modify rate of bud growth but not always the timing of its onset. In addition, there are questions about whether roots are the main source of cytokinins for the shoot: in experiments with grafted transgenic plants expressing the ipt cytokinin gene only in the roots, no increase in shoot cytokinin was seen and plants did not show enhanced branching (Faiss et al. 1997).

There is also evidence from pea mutants that branching control in intact plants cannot be explained by auxin and cytokinin alone. The ramosus (Latin for ‘branched’, abbreviated to rms) mutants all exhibit greater than normal branching. From Sachs and Thimann, we predict either low auxin or high cytokinin or both, or altered sensitivity to these hormones. However, analysis of xylem sap (the main pipeline supplying solutes including cytokinins from root to shoot) reveals that in some mutants, xylem cytokinin content is actually much reduced, by as much as 40-fold in the case of rms4. In addition, none of the mutants has reduced auxin content in the shoot. One mutant, rms2, does have slightly elevated xylem sap cytokinin, but it has up to five times the normal auxin level. Clearly, auxin and cytokinin levels do not conform with Sachs–Thimann predictions in these plants, so another model for branching control needs to be developed.

Using reciprocal grafting experiments, Beveridge et al. (1997a, b) have established where in the plant some of the Rms genes are expressed. For example, expression of the normal Rms1 gene does not seem to be restricted to the shoot. This conclusion is based on inhibition of branching in both rms1/wild-type (scion on rootstock) and wild-type/rms1 grafts; that is, provided there is one part of the plant with normal Rms1 expression, then branching will be inhibited compared with the rms1 mutant. The Rms1 gene therefore may control a branching inhibitor that can move from root to shoot, but can also act directly in the shoot. Because rms1 plants have normal IAA transport and are not IAA deficient, we deduce that this inhibitory signal is almost certainly not auxin.

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Figure 1 Control of lateral branching is regulated by several genes and probably several mobile signals. In pea, the rms4 mutant is highly brached and has extremely low levels of cytokinins moving from root to shoot inthe xylem sap. The conventional theory of apical dominance regulation suggests that high cytokinin levels would be associated with increased branching. The evidence here from reciprocal grafts between rms4 and its wild type is that the extent of shoot branching governs the export of cytokinins from the root rather than vice versa. ZR = zeatin riboside.

(Based on Beveridge et al. 1997a; reproduced with permission of Blackwell Science)

On the other hand, rms4 shoots grafted onto wild-type roots still branch, but the wild-type roots now export rms4 levels (i.e. very low) of cytokinin. The converse graft of wild-type shoots onto rms4 mutant roots does not branch but the roots now export wild-type levels of cytokinin (Figure 1). The deduction is that the normal Rms4 gene is acting in the shoot only, and two consequences of the rms4 mutation are enhanced branching and downregulation of root cytokinin export. The latter must require a shoot-to-root signal, but auxin is again an unlikely candidate. We are therefore left with two intriguing conclusions:

1. Auxin and cytokinin alone do not explain the control of branching in intact plants.

2. There is evidence for at least two novel (i.e. not auxin or cytokinin) graft-transmissible branching signals, one moving from root to shoot (the Rms1 factor) and one moving from shoot to root (a signal relating to Rms2).

Plant architecture is closely tied to shoot branching and is an important character in many crop plants. For example, increased bushiness is desirable in ornamental pot plants, but a single trunk, non-branching phenotype is most valuable in plantation timber species. In the future, there may be potential for regulating branching through genes such as the Rms series, in addition to manipulation of auxin and cytokinin
status.

References

Beveridge, C.A., Murfet, I.C., Kerhoas, L., Sotta, B., Miginiac, E. and Rameau, C. (1997a). ‘The shoot controls zeatin riboside export from pea roots. Evidence from the branching mutant rms4’, Plant Journal, 11, 339–345.

Beveridge, C.A., Symons, G.M., Murfet, I.C., Ross, J.J. and Rameau, C. (1997b). ‘The rms1 mutant of pea has elevated indole-3-acetic acid levels and reduced root-sap zeatin riboside content but increased branching controlled by graft-transmissible signals’, Plant Physiology, 115, 1251–1258.

Faiss, M., Zalubilová, Strnad, M. and Schmülling, T. (1997). ‘Conditional transgenic expression of the ipt gene indicates a function for cytokinins in paracrine signaling in whole tobacco plants’, Plant Journal, 12, 401–415.

Gocal, G.F., Pharis, R.P., Yeung, E.C. and Pearce, D. (1991). ‘Changes after decapitation in concentrations of indole-3-acetic acid and abscisic acid in the larger axillary bud of Phaseolus vulgaris L. Cv Tender Green’, The Plant Physiology,
95, 344–350.

Medford, J.I., Horgan, R., El-Sawi, Z. and Klee, H. (1989). ‘Alterations of endogenous cytokinins in transgenic plants using a chimeric isopentenyl transferase gene’, The Plant Cell,
1, 403–413.

Romano, C.P., Hein, M.B. and Klee, H.J. (1991). ‘Inactivation of auxin in tobacco transformed with the indoleacetic acid–lysine synthetase gene of Pseudomonas savasanoi’, Genes and Development, 5, 438–446.

Sachs, T. and Thimann, K.V. (1967). ‘The role of auxins and cytokinins in the release of buds from dominance’, American Journal of Botany, 54, 136–144.

Turnbull, C.G.N., Raymond, M.A.A., Dodd, I.C. and Morris, S.E. (1997). ‘Rapid increases in cytokinin concentration in lateral buds of chickpea (Cicer arietinum L.) during release of apical dominance’, Planta, 202, 271–276.

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