Pin2 Alleles Suppress the Ahydrotropic Phenotype of Miz1 and Miz2¹

Root hydrotropism is the tendency of plant roots to grow towards higher water potential, avoiding areas with water deficiency. To date, only two genes, named MIZ1 and MIZ2 (GNOM), were found using forward genetic screens to be indispensable for hydrotropism. We sought to identify other genes associated with hydrotropism by isolating suppressors of the two known ahydrotropic miz1 and miz2 mutants, by mutagenizing their seeds, screening at the M2 generation for the appearance of normal hydrotropism phenotype, followed by gene mapping through Next Generation Sequencing (NGS). We have isolated three miz2 single-nucleotide suppressor mutants mapped to the AtPIN2 auxin efflux carrier gene. All three alleles partially rescue the ahydrotropic phenotype of the miz2 mutant. Mutants #350 and #580 exhibit an agravitropic phenotype, which resembles that of the known pin2 T-DNA insertion mutant (SALK_102987). However, the third allele (#499) partially suppressed the ahydrotroic phenotype of miz2 without compromising the gravitropic response. To corroborate the effect of pin2 mutations on hydrotropism, we crossed the miz1 T-DNA insertion mutant (Salk_76560) and the miz2 (GNOM) mutant, separately, with the pin2 T-DNA SALK_102987 mutant. Consistent with our suppressor screens, the pin2 T-DNA insertion mutation partially rescued both miz1 and miz2 ahydrotropic phenotypes, while exhibiting an agravitropic phenotype. These results support our past findings, showing the interplay between hydrotropic and gravitropic root responses. The mutation of the #499 suppresser mutant, which holds a normal gravitropic response, raises questions about the role of PIN2 in the interplay between gravitropism and hydrotropism, and is being further investigated.

¹Supported by the I-CORE Program of the Planning and Budgeting Committee and The Israel Science Foundation (grant no. 757/12).