Running head : Strigolactones and root system architecture Corresponding author :

In the present 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 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-GFP intensities in provascular tissue of the primary root tip were decreased, whereas PIN3-GFP intensity in the columella was not affected. During phosphate-sufficient 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 NAA, 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 act through the modulation of 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.