Physiological Effects of the Synthetic Strigolactone Analog GR24 on Root System Architecture in Arabidopsis: Another Belowground Role for Strigolactones?      

Abstract

In this study, the role of the recently identified class of phytohormones, strigolactones, in shaping root architecture was addressed. Primary root lengths of strigolactone-deficient and -insensitive Arabidopsis (Arabidopsis thaliana) plants were shorter than those of wild-type plants. This was accompanied by a reduction in meristem cell number, which could be rescued by application of the synthetic strigolactone analog GR24 in all genotypes except in the strigolactone-insensitive mutant. Upon GR24 treatment, cells in the transition zone showed a gradual increase in cell length, resulting in a vague transition point and an increase in transition zone size. PIN1/3/7-green fluorescent protein intensities in provascular tissue of the primary root tip were decreased, whereas PIN3-green fluorescent protein intensity in the columella was not affected. During phosphatesufficient conditions, GR24 application to the roots suppressed lateral root primordial development and lateral root forming potential, leading to a reduction in lateral root density. Moreover, auxin levels in leaf tissue were reduced. When auxin levels were increased by exogenous application of naphthylacetic acid, GR24 application had a stimulatory effect on lateral root development instead. Similarly, under phosphate-limiting conditions, endogenous strigolactones present in wild-type plants stimulated a more rapid outgrowth of lateral root primordia when compared with strigolactone-deficient mutants. These results suggest that strigolactones are able to modulate local auxin levels and that the net result of strigolactone action is dependent on the auxin status of the plant. We postulate that the tightly balanced auxin-strigolactone interaction is the basis for the mechanism of the regulation of the plants' root-to-shoot ratio.Strigolactones, exuded from plants, have been known for a long time to act as germination stimulants for seeds of root parasitic plants such as Orobanche and Striga spp. (for review, see Bouwmeester et al., 2003Bouwmeester et al., , 2007. As root parasitic plants consume a large proportion of the host plants' solutes, they cause wilting and early plant death. Initially, the discovery that strigolactones are also involved in the symbiotic interaction with arbuscular mycorrhizal fungi (Akiyama et al., 2005) was believed to provide an explanation for why the host plants' capacity to produce strigolactones was not lost during evolution. Because arbuscular mycorrhizal fungi are potent providers of nutrients such as phosphate (Pi) and nitrogen to their host, the observation that Pi starvation induced strigolactone biosynthesis in host plants' roots was not surprising (Yoneyama et al., 2007;Ló pez-Ráez et al., 2008). The recent discovery that strigolactones, or closely related compounds, also act as phytohormones inside the host plants and are involved in the inhibition of axillary bud outgrowth (Gomez-Roldan et al., 2008;Umehara et al., 2008) is an additional explanation why plants continue to produce these fatal ger-

Strigolactones fine-tune the root system

“…This study also demonstrated that GR24 could increase cortical cell length in the presence of auxin, while CYCB1 expression (indicative of cell division rates) was inhibited by GR24. In contrast to the study by Ruyter-Spira et al (2011), the findings of Koltai et al (2010) suggest that the longer primary root results from enhanced cell elongation rather than cell division. These contrasting reports demonstrate the need for further research to understand how strigolactones regulate cell division or elongation.…”

“…2) (Arite et al 2012;Guan et al 2012;Kretzschmar et al 2012). In support of the strigolactone-promoting primary root growth, Ruyter-Spira et al (2011) demonstrated that when grown without sucrose, GR24 applications trigger root growth in a MAX2-dependent manner. They also measured the number of cells in the meristem and showed that GR24 increased the number of cells in the meristem, possibly explaining the increase in root length.…”

“…Strigolactones affect auxin fluxes (see above) (Bennett et al 2006;Brewer et al 2009;Crawford et al 2010;Ruyter-Spira et al 2011;Shinohara et al 2013), so it was expected that strigolactones may also have an effect on lateral root formation. Thus far, two reports describe a negative influence of strigolactones on lateral root density in a MAX2-dependent manner (Fig.…”

“…Strigolactones regulate developmental programs and modulate plant architecture through the fine-tuning of shoot branching and root growth (Stirnberg et al 2007;GomezRoldan et al 2008;Umehara et al 2008;Ruyter-Spira et al 2011). In addition, strigolactones and analogous chemicals (i.e., karrikins) regulate seed germination as part of a light-dependent process (Tsuchiya et al 2010;Nelson et al 2011).…”

A petunia ABC protein controls strigolactone-dependent symbiotic signalling and branching

“…Scale bars, 20 mm. a-d, *, P , 0.05; ***, P , 0.001. more tolerant to the deleterious effects of high strigolactone concentrations on root elongation 24 than did wild-type plants ( Fig. 3c and Supplementary Fig.…”