Ethylene promotes the induction by auxin of the cortical microtubule randomization required for low-pH-induced root hair initiation in lettuce (Lactuca sativa L.) seedlings

Transverse cortical microtubule (CMT) arrays in lettuce root epidermal cells randomize soon after a shift from pH 6.0 to pH 4.0, and this randomization is essential for root hair initiation. We investigated the hormonal regulation of CMT randomization. At pH 4.0, 1 muM of the auxin competitive inhibitor 2-(p-chlorophenoxy)-2-methylpropionic acid (PCIB), 0.1 muM of the ethylene biosynthesis inhibitor aminoethoxyvinylglycine (AVG) or 0.1 muM of the ethylene action inhibitor Ag+ suppressed CMT randomization and root hair initiation. At pH 6.0, addition of 0.1 muM indole-3-acetic acid (IAA) or 1 muM of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) induced CMT randomization and root hair initiation. Culturing with 0.1 muM IAA plus 0.1 muM AVG, or 1 muM ACC plus 1 muM PCIB also induced these phenomena. ACC (1 muM) plus 100 muM PCIB inhibited CMT randomization and root hair initiation, but 1 muM AVG with 0.1 muM Ag+ and 0.1 muM IAA induced them. These results suggest that auxin is essential for CMT randomization. As a higher concentration of PCIB was required to suppress CMT randomization when ACC was added, the greater amount of ethylene produced at pH 4.0 may promote the induction by auxin of CMT randomization in hair-forming cells.