GNOM regulates root hydrotropism and phototropism independently of PIN-mediated auxin transport

•gnomemb30-1 rescued ahydrotropic phenotype of gnommiz2, while gnomB4049 did not.•Requirement of GNOM function differs among the tropisms.•GNOM can regulate root phototropism independent of PIN2 polar targeting.•Auxin related defects of severe gnom mutants are fully recovered by gnommiz2.•A novel GNOM function regulates root hydrotropism and phototropism. Plant roots exhibit tropisms in response to gravity, unilateral light and moisture gradients. During gravitropism, an auxin gradient is established by PIN auxin transporters, leading to asymmetric growth. GNOM, a guanine nucleotide exchange factor of ARF GTPase (ARF-GEF), regulates PIN localization by regulating subcellular trafficking of PINs. Therefore, GNOM is important for gravitropism. We previously isolated mizu-kussei2 (miz2), which lacks hydrotropic responses; MIZ2 is allelic to GNOM. Since PIN proteins are not required for root hydrotropism in Arabidopsis, the role of GNOM in root hydrotropism should differ from that in gravitropism. To examine this possibility, we conducted genetic analysis of gnommiz2 and gnom trans-heterozygotes. The mutant gnommiz2, which lacks hydrotropic responses, was partially recovered by gnomemb30-1, which lacks GEF activity, but not by gnomB4049, which lacks heterotypic domain interactions. Furthermore, the phototropic response of gnom trans-heterozygotes differed from that of the pin2 mutant allele eir1-1. Moreover, defects in the polarities of PIN2 and auxin distribution in a severe gnom mutant were recovered by gnommiz2. Therefore, an unknown GNOM-mediated vesicle trafficking system may mediate root hydrotropism and phototropism independently of PIN trafficking.