Phytochrome Mediates Light Signal for Cortical Microtubule Randomization that Enables Root Hair Formation in Lettuce Seedlings

Abstract

When hydroponically cultured lettuce seedlings are transferred from a medium at pH 6.0 to another one at pH 4.0, the formation of root hairs takes place. Previous studies have revealed that this low-pH-induced root hair formation requires a phytochrome-mediated light signal and randomization of the existing transverse cortical microtubule (CMT) arrays in the future hair-forming cells, which occurs before the bulge formation. Here, we investigated the relationship between these two requisites, i.e., whether phytochrome is involved in CMT randomization. To test this, seedlings were cultured throughout in the dark except the various periods and timings of red (R) or far-red (FR) light irradiation. In seedlings that had already been transferred to pH 4.0 medium, R light irradiation induced CMT randomization, whereas neither FR light irradiation nor continuous dark culturing instead of R light irradiation did. R light irradiation during the pre-culture at pH 6.0 also caused CMT randomization later when the seedlings were transferred to pH 4.0 medium. Irradiation of FR light immediately after the R light irradiation suppressed the R light-induced CMT randomization, regardless of whether R/FR irradiation was carried out before or after the transfer of seedlings to pH 4.0 medium. The efficacy of R light and R/FR photo-reversibility in the induction of CMT randomization proves that the light signal needed for CMT randomization was indeed mediated by phytochrome, during lettuce root hair formation.