Arabidopsis Microtubule-Associated Protein WDL5 Is Involved In Ethylene Regulation Of Hypocotyl Elongation

Skotomorphogenesis occurs as buried seedlings fully elongate their hypocotyls upward in search of the soil surface. When elongated hypocotyls encounter mechanical obstacles during seedling extrusion from the soil, inhibition of rapid etiolated hypocotyl elongation is required to optimize the seedling’s ability to push through the soil without damaging its shoot meristem. Therefore, the negetive regulation of etiolated hypocotyl elongation is important.The phytohormone ethylene plays a crucial role in the negative regulation of hypocotyl elongation in the dark. When elongated hypocotyls encounter mechanical obstacles the ethylene production can be induced in Arabidopsis seedlings. Redundant transcription factors EIN3/EIL1 play central roles in ethylene regulation of etiolated hypocotyl elongation. Genetic evidence has shown that ethylene-overproduced or constitutive-ethylene-response mutants generally display defective etiolated hypocotyl cell growth phenotypes. The microtubule cytoskeleton also participates in hypocotyl cell elongation. Numerous studies have shown that microtubule-associated proteins (MAPs) mediate the hypocotyl cell growth through the regulation of microtubule orientation, dynamics and stability. Ethylene or its biosynthetic precursor 1-aminocyclopropane-l-carboxylic acid (ACC) has been reported to affect the organization of cortical microtubules in plant cells, however, it remains unclear if ethylene-mediated etiolated hypocotyl elongation involves the microtubule cytoskeleton.This study demonstrated that ethylene signaling participates in cortical microtubule reorientation from transverse to longitudinal in etiolated hypocotyls, which is essential for ethylene-mediated etiolated hypocotyl elongation. Moreover, regulation of microtubule stability was found to be important for ethylene-mediated cortical microtubule reorganization. Microtubule stability was significantly increased in ACC-treated wild-type cells, but not in ACC-treated ein2-5 cells. Further analysis found that the previously uncharacterized microtubule-associated protein WAVE-DAMPENED2-LIKE5 (WDL5) from WVD2/WDLs family, as a microtubule stabilizing protein, plays a role in ethylene-regulated microtubule reorientation and also etiolated hypocotyl cell elongation. Quantitative real-time PCR analyses showed that WDL5 expression was much higher in the etol-1 mutant, but lower in the ein2-5 mutant compared to the wild type, indicating ethylene upregulates WDL5 expression. ChIP-qPCR and EMSA assays demonstrated that WDL5 is an EIN3 target gene. Etiolated hypocotyls from a WDL5 loss-of-function mutant (wdl5-1) were more insensitive to ACC treatment than the wild type. Decreasing WDL5 expression partially rescued the shorter-etiolated-hypocotyl phenotype in the ethylene overproduction mutant etol-1. These observations demonstrate that WDL5 plays a positive role in ethylene-regulated etiolated hypocotyl elongation. Reorientation and stability of the cortical microtubules in the cells of etiolated hypocotyls from wdl5-1 mutant was less sensitive to ACC treatment. These findings indicate that WDL5 is an important participant in ethylene signaling regulation of microtubule reorientation and further inhibition of etiolated hypocotyl growth. This study reveals a mechanism involved in ethylene signaling regulation of microtubules through WDL5 to inhibit etiolated hypocotyl cell elongation.