Strigolactones’ Effect on Root Growth and Root-Hair Elongation May Be Mediated by Auxin-Efflux Carriers

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.…”

“…As observed in lateral root development, this MAX2-dependent root hair length phenotype is concentration-dependent and at high concentrations even the max2 mutant responds to GR24. In contrast, it was found that GR24 inhibited root hair elongation in tomato; however, this only occurred at non-physiological concentrations and was not tested at lower concentrations (Koltai et al 2010). At these high concentrations they also found that auxin could not rescue the phenotype.…”

“…Another study (Dubrovsky et al 2009) found cortical cells in the primary root of max4 to be shorter than those of the wild type, but root length was not mentioned. Further support for a promoting role for strigolactones in primary root length comes from a study where GR24 was shown to increase root length in tomato (Koltai et al 2010) but only after root length inhibition by auxin and only significant at very high concentrations. 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.…”

“…At these high concentrations they also found that auxin could not rescue the phenotype. Instead the auxin transport inhibitor 1-N-naphthylphthalamic acid (NPA) was able to enhance the root hair length, suggesting that high levels of strigolactones enhance auxin efflux in tomato (Koltai et al 2010).…”

“…Opposing evidence for this theory is that the strigolactone mutants of pea do not necessarily have a higher auxin content than that of the wild type (Beveridge et al 1996;Morris et al 2001). Additional proof against strigolactone directly regulating auxin biosynthesis is found in root hair lengths, where exogenous treatments with auxins using the PIN efflux pathway were unable to restore the phenotype (Koltai et al 2010), suggesting a more direct interaction with the PIN pathway.…”