HOME :: CHAPTER 19 :: Essay 19.2
PREVIOUS :: NEXT
Strigolactones: The Unmasking of a New Branching Hormone
Christine Beveridge, School of Biological Sciences, The University of Queensland, Brisbane, Australia
Strigolactones: The Unmasking of a New Branching Hormone
Shoot branches grow from axillary buds that arise in leaf axils. Auxin and cytokinin have long been known to be important plant hormones involved in regulating the outgrowth of axillary buds into branches. Recently, thanks to the use of branching mutants in several species, we now know the identity of a third hormone group, strigolactones, involved in this process.
Branching Mutants Identify Roles of Elusive Signals: Strigolactones
For the most part, research has focused on increased branching mutants where the increased branching is not associated with changes in flowering or vegetative vigor. These branching mutants fall into two major classes: those that have altered levels of graft-transmissible signals, and those that do not. These are described here as synthesis and response mutants respectively. In the pea, the key mutants are named the ramosus mutants. The word ramosus is related to the word ramification and means “with many branches.” Branching in rms1 and rms5 shoots is inhibited by grafting to wild type rootstocks (reviewed in Beveridge et al. 2009). Analyses of xylem sap cytokinin levels and levels of IAA in the shoot in the pea led to the conclusion that the graft-transmissible signal in these synthesis mutants was novel. The identification and cloning of the orthologous genes (genes with similar DNA sequences and similar function) in Arabidopsis, MAX4 and MAX3, (for More AXillary branches), further suggested that these genes encoded enzymes that produced some type of carotenoid derivative (Sorefan et al. 2003).
The carotenoid origin of the elusive long-distance signal provided the essential clue as to its identification. Strigolactone had already been identified as a carotenoid signal in plants, although its function had been previously attributed only to the rhizosphere. Strigolactones stimulate the important symbiosis of plant roots with arbuscular mycorrhizal fungi, enhancing nutrient and water uptake. They were orginally discovered because devastating parasitic weed species such as Striga and Orobanche sp. co-opted strigolactones as a cue to identify the presence of roots and to stimulate seed germination in close proximity to roots and hence to enable rapid contact.
Two reports simultaneously showed that strigolactone levels were depleted in the enzymatic mutants of pea and rice discussed above, but not in the putative signal transduction response mutants (Gomez-Roldan et al. 2008; Umehara et al. 2008). Importantly, exogenous strigolactone restored branching inhibition in the enzymatic mutants, but not in the putative signal transduction mutants. Consequently, we now conclude that strigolactone, or some strigolactone-derived product, is the elusive branching signal. Current research includes identifying the bioactive strigolactone in shoot branching and in determining which strigolactone(s) or product(s) act as the mobile signal(s).
Mutants in the response category produce scions that continue to branch even when grafted to wild type rootstocks. It is thought that they may be involved in the response to signals involved in shoot branching (see Figure 1). The corresponding rms4, max2 and d3 mutants in pea, Arabidopsis and rice respectively, have lesions in an orthologous F-Box protein (see Chapter 14 for a discussion of F-box signalling). D14, an α/Β hydrolase characterised in rice, is another protein that may be involved in signal transduction for shoot branching inhibition. The supporting evidence for the function of the genes identified as response mutants is indirect: the mutants (i) do not respond to strigolactone, (ii) show a reduced response to signals in grafting studies, and (iii) have disrupted proteins commonly involved in signal transduction (reviewed in Beveridge and Kyozuka, 2010). Molecular confirmation must yet be obtained in order to confirm any function in strigolactone signal transduction.
|Figure 1 Strigolactones inhibit bud outgrowth. Their production requires the carotenoid cleavage dioxygenase-like enzymes CCD7 and CCD8. This step is regulated by auxin and an unknown long-distance feedback signal. (Click image to enlarge.)|
Interestingly, MAX2 has recently been shown as essential for the action of another phytohormone-like signal, karrikin (Nelson et al. 2011). Karrikins were recently discovered as one of the main bioactive signals in smoke that promote seed germination after fire and are structurally related to strigolactones. Although they may not be endogenous plant signals, karrikins induce a range of developmental responses, many of which do not overlap with those of strigolactone. Similarly, max2 has a number of phenotypes that differ from strigolactone synthesis mutants and several of these are karrikin related (Nelson et al. 2011). This indicates that MAX2 function is specific to each of these signals, strigolactone and karrikin. Moreover, it provides a clear message of caution in the use of max2 as a strigolactone specific signalling mutant.
In most species, auxin depletion substantially suppresses expression of strigolactone synthesis genes (Foo et al. 2005; Hayward et al. 2009) and stimulates expression of cytokinin synthesis genes (Shimizu-Sato et al. 2009). Moreover, strigolactone can inhibit bud outgrowth in auxin depleted decapitated plants of pea or in auxin signaling mutants of Arabidopsis (Brewer et al. 2009). This suggests that auxin depletion has a dual action of causing strigolactone depletion and cytokinin enhancement either of which could act directly in buds. As auxin and strigolactones are thought to move in opposite directions in shoots, this would provide a mechanism for the indirect action of auxin in shoot branching as auxin in the basipetal transport stream cannot move acropetally to inhibit axillary buds. In addition, strigolactones may directly or indirectly affect auxin flow from buds (reviewed in Brewer et al. 2009; Crawford et al. 2010).
Feedback regulation is common in biological systems because different parts of the system are often carefully balanced relative to other parts. All max branching mutants and four of five rms mutants show greatly suppressed root-xylem sap cytokinin content and greatly enhanced strigolactone biosynthesis gene expression (Foo et al. 2005, 2007; Hayward et al. 2009). Grafting studies have shown these effects are caused by a predominantly shoot-derived long-distance signal, often termed the “feedback signal” (reviewed by Beveridge et al. 2009; Dun et al. 2009). In grafted plants, rms3 or rms4 shoots cause reduced root xylem sap cytokinin content and elevated rootstock epicotyl RMS1 expression, even when grafted to wild type rootstocks, suggesting a role for shoot-to-root signalling. Auxin levels themselves are likely feedback upregulated in branching mutants, but auxin content alone is insufficient to explain the long-distance feedback effects (Hayward et al. 2009). Short and long-distance feedback signalling may be part of a homeostasis system which prevents excessive branching in shoots.
Photoperiod has a major influence on the developmental strategy adapted by a plant in its given environment (see Chapter 25). Environment, particularly photoperiod, affects the nodes at which bud outgrowth occurs in rms and max mutant plants (reviewed in Waldie et al. 2010). One explanation for this observation is that auxin and strigolactones may only affect the growth of buds that are in a particular responsive stage, as discussed below (Beveridge et al. 2009).
Physiology of Apical Dominance
Apical dominance is the term used to describe the role of the shoot tip in mediating the growth of axillary buds at nodes along the stem below. Auxin, transported from the shoot tip in a polar direction towards roots, is implicated in apical dominance. The classical apical dominance dogma states that indole acetic acid (IAA) produced in the shoot tip moves down the stem and inhibits axillary bud outgrowth (Video 1). As mentioned above, this effect of auxin is likely to act indirectly as auxin cannot move upwards into axillary buds.
|Click here to open video.|
|Video 1 Classical apical dominance experiment. The plants represented in the Video have three expanded leaves, one fully expanded true leaf, and two scale leaves|
As is the case with other hormone-controlled processes, regardless of the mechanism, the changes in hormone levels, in this case shoot-derived auxin, should correlate with the timing of the response. This has been investigated in garden pea using time-lapse video of plants at a developmental stage whereby there is a substantial spatial separation between the shoot tip and the buds (Morris et al. 2005). The dynamics of auxin depletion and movement along the stem after decapitation was followed along with the timing of bud outgrowth. Although auxin depletion at upper nodes occurs within the time frame of bud outgrowth at those nodes, bud outgrowth at lower nodes occurs well before the depletion of local auxin content in the stem (Video 2). Auxin application to the cut stump immediately after decapitation results in bud inhibition, but this inhibition occurs only after an initial period of outgrowth.
|Click here to open video.|
|Video 2 Simulation of auxin dynamics and bud outgrowth on a large spatial scale. Auxin level and bud outgrowth is simulated in a plant with five leaves, expanded as described in Video 1.|
Although this approach does not allow direct visualization of the molecular events that precede axillary bud outgrowth, they effectively demonstrate that the control mediated by the shoot tip on the outgrowth of axillary buds has an auxin-independent component. This is supported by an important experiment using an auxin transport inhibitor (N-1-napthylpthalamic acid, or NPA) supplied in a lanolin ring around the stem below the shoot tip of intact plants. This treatment, shown to cause depleted IAA levels in the stem, did not cause bud outgrowth in comparable plants to those used for the decapitation studies above (Morris et al. 2005).
In summary, decapitation is somehow perceived rapidly along the stem (Video 3) and induces an initial burst of axillary bud expansion that is apical auxin-independent. Continued outgrowth of these newly expanding buds is allowed only if auxin levels are depleted. This makes sense from an adaptation perspective as rapid regrowth after decapitation is absolutely essential for plants to remain competitive with their neighbors and to complete the life cycle after damage. On the other hand, excessive axillary shoot formation could be detrimental to plant fitness. Auxin is therefore thought to function as a mediator to provide communication among apical and axillary shoots and to regulate the number that finally proceeds to develop into a lateral branch.
|Click here to open video.|
|Video 3 Hypothesis showing the rapid decapitation-induced signal, relative to auxin. An intact plant (left) and a decapitated plant (right) are shown with a simulation of a hypothetical decapitation-induced signal, shown in yellow, in relation to the timing of auxin depletion and bud outgrowth. Bud outgrowth is correlated with the faster signal.|
How then might this auxin regulation work? Although they may appear phenotypically similar, not all small axillary buds are “equal” and may not respond to signals uniformly. Molecular analysis by Stafstrom and Sussex (1988) identified stages of bud outgrowth that may be simplified to describe a dormant stage, a transition or receptive stage, and a growing stage (Beveridge 2006; Figure 2). In the example outlined above, buds unable to respond to depleted auxin may be considered dormant, even though they may be metabolically active. After the plants are decapitated, they enter an auxin responsive transition stage in which auxin plays an inhibitory role. As in the strigolactone mutants described above, bud and whole plant developmental stage, photoperiod, and flowering genotype also influence the response of axillary buds to decapitation and/or pharmacological inhibition of auxin transport (reviewed in Waldie et al. 2010). Current research is investigating how decapitation, which also involves removal of a substantial sink, can rapidly induce the initial outgrowth of axillary buds at long distances.
Thanks to Jim Hanan and Micheal Renton for creating models used for videos.
Beveridge, C.A. (2006) Axillary bud outgrowth: sending a message. Curr. Opin. Plant Biol. 9: 35–40.
Beveridge, C.A., Dun, E.A., and Rameau, C. (2009) Pea has its tendrils in branching discoveries spanning a century from auxin to strigolactones. Plant Physiol. 151: 985–990.
Beveridge, C.A., and Kyozuka, J. (2010) New genes in the strigolactone related shoot branching pathway. Curr. Opin. Plant Biol. 13: 34–39.
Brewer, P.B., Dun, E.A., Ferguson, B.J., Rameau, C., and Beveridge, C.A. (2009) Strigolactone acts downstream of auxin to regulate bud outgrowth in pea and Arabidopsis. Plant Physiol. 150: 482–463.
Crawford, S., Shinohara, N., Sieberer, T., Williamson, L., George, G., Hepworth, J., Müller, D., Domagalska, M.A., and Leyser, O. (2010) Strigolactones enhance competition between shoot branches by dampening auxin transport. Development 137: 2905–2913.
Dun, E.A., Brewer, P.B., and Beveridge, C.A. (2009) Strigolactones: discovery of the elusive shoot branching hormone. Trends in Plant Science 14: 364–372.
Foo, E., Bullier, E., Goussot, M., Foucher, F., Rameau, C., and Beveridge C (2005) The branching gene RAMOSUS1 mediates interactions among two novel signals and auxin in pea. Plant Cell 17: 464–474.
Foo, E., Morris, S.E., Parmenter, K., Young, N., Wang, H., Jones, A., Rameau, C., Turnbull, C.G.N, and Beveridge, C.A. (2007) Feedback regulation of xylem cytokinin content is conserved in pea and Arabidopsis. Plant Physiol. 143: 1418–1428.
Gomez-Roldan, V., Fermas, S., Brewer, P.B., Puech-Pagès, V., Dun, E.A., Pillot, J-P., Letisse, F., Matusova, R., Danoun, S., Portais, J-C., Bouwmeester, H., Bécard, G., Beveridge, C.A., Rameau, C., and Rochange, S.F. (2008) Strigolactone inhibition of shoot branching. Nature 455: 189–194.
Hayward, A., Stirnberg, P., Beveridge, C.A., and Leyser, O. (2009) Interactions between auxin and strigolactone in shoot branching control. Plant Physiol. 151: 400–412.
Morris, S.E., Cox, M.C.H., Ross, J.J., Krisantini, S., and Beveridge, C.A. (2005) Auxin dynamics after decapitation are not correlated with the initial growth of axillary buds. Plant Physiol. 138: 1665–1672.
Nelson, D.C., Scaffidi, A., Dun, E.A., Waters, M., Flematti, G.R., Dixon, K.W., Beveridge, C.A., Ghisalberti, E.L., and Smith, S.M. (2011) The F-box protein MAX2 has dual roles in karrikin and strigolactone signaling in Arabidopsis thaliana. PNAS www.pnas.org/content/early/2011/05/03/1100987108.
Shimizu-Sato, S., Tanaka, M., and Mori, H. (2009) Auxin-cytokinin interactions in the control of shoot branching. Plant Molecular Biology 69:429–435.
Stafstrom, J.P., and Sussex, I.M. (1988) Patterns of protein synthesis in dormant and growing vegetative buds of pea. Planta 176: 497–505.
Sorefan, K., Booker, J., Haurogné , K., Goussot, M., Bainbridge, K., Foo, E., Chatfield, S., Ward, S., Beveridge, C., Rameau C., and Leyser, O. (2003) MAX4 and RMS1 are orthologous dioxygenase-like genes that regulate shoot branching in Arabidopsis and pea. Genes Dev. 17: 1469–1474.
Stirnberg, P., van de Sande, K., and Leyser H.M.O. (2002) MAX1 and MAX2 control shoot lateral branching in Arabidopsis. Development 129: 1131–1141.
Umehara, M., Hanada, A., Yoshida, S., Akiyama, K., Arite, T., Takeda-Kamiya, N., Magome, H., Kamiya, Y., Shirasu, K., Yoneyama, K., Kyozuka, J., and Yamaguchi, S. (2008) Inhibition of shoot branching by new terpenoid plant hormones. Nature 455: 195–200.
Waldie, T., Hayward, A., and Beveridge, C.A. (2010) Axillary bud outgrowth in herbaceous shoots: How do strigolactones fit into the picture? Plant Molecular Biology (Special Dormancy Issue) 73: 27–36.
HOME :: CHAPTER 19 :: Essay 19.2
PREVIOUS :: NEXT