LAUSR

Significance CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) is an E3 ubiquitin ligase acting as a central repressor of seedling photomorphogenesis in plants. Many nuclear-localized COP1 substrates have been identified in the last two decades; however, whether COP1 targets cytoplasmic factors for ubiquitination and degradation remains largely unknown. In this study, we show that COP1 interacts with a microtubule-associated protein, WAVE-DAMPENED 2-LIKE 3 (WDL3), in a dark-dependent manner at cortical microtubules. Thus, COP1 targets WDL3 for 26S proteasome-mediated degradation to control hypocotyl elongation in etiolated Arabidopsis seedlings. Collectively, our study uncovers a cytoplasmic substrate of COP1 that functions as a microtubule-associated protein in mediating hypocotyl cell elongation. CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1), a well-known E3 ubiquitin ligase, functions as a central regulator of plant growth and photomorphogenic development in plants, including hypocotyl elongation. It has been well-established that, in darkness, COP1 targets many photomorphogenesis-promoting factors for ubiquitination and degradation in the nucleus. However, increasing evidence has shown that a proportion of COP1 is also localized outside the nucleus in dark-grown seedlings, but the physiological function of this localization remains largely unclear. In this study, we demonstrate that COP1 directly targets and mediates the degradation of WAVE-DAMPENED 2-LIKE 3 (WDL3) protein, a member of the microtubule-associated protein (MAP) WVD2/WDL family involved in regulating hypocotyl cell elongation of Arabidopsis seedlings. We show that COP1 interacts with WDL3 in vivo in a dark-dependent manner at cortical microtubules. Moreover, our data indicate that COP1 directly ubiquitinates WDL3 in vitro and that WDL3 protein is degraded in WT seedlings but is abundant in the cop1 mutant in the dark. Consistently, introduction of the wdl3 mutation weakened, whereas overexpression of WDL3 enhanced, the short-hypocotyl phenotype of cop1 mutant in darkness. Together, this study reveals a function of COP1 in regulating the protein turnover of a cytosol-localized MAP in etiolated hypocotyls, thus providing insights into COP1-mediated degradation of downstream factors to control seedling photomorphogenesis.