[Show abstract][Hide abstract] ABSTRACT: Cortical microtubule arrays are critical in determining the growth axis of diffusely growing plant cells, and various environmental and physiological factors are known to affect the array organization. Microtubule organization is partly disrupted in the spiral1 mutant of Arabidopsis thaliana, which displays a right-handed helical growth phenotype in rapidly elongating epidermal cells. We show here that mutations in the plasma membrane Na(+)/H(+) antiporter SOS1 and its regulatory kinase SOS2 efficiently suppressed both microtubule disruption and helical growth phenotypes of spiral1, and that sos1 and sos2 roots in the absence of salt stress exhibited altered helical growth response to microtubule-interacting drugs at low doses. Salt stress also altered root growth response to the drugs in wild-type roots. Suppression of helical growth appeared to be specific to spiral1 since other helical growth mutants were not rescued. The effects of sos1 in suppressing spiral1 defects and in causing abnormal drug responses were nullified in the presence of the hkt1 Na(+) influx carrier mutation in roots but not in hypocotyls. These results suggest that cytoplasmic salt imbalance caused by insufficient SOS1 activity compromises cortical microtubule functions in which microtubule-localized SPIRAL1 is specifically involved.