The Motor Kinesin 4II Is Important for Growth and Chloroplast Light Avoidance in the Moss <i>Physcomitrella patens</i>

Abstract

Understanding how plant cells adapt dynamically to changes in the environment is a fundamental problem of plant biology. Under many conditions, plant cells respond to environmental changes by modifying their intracellular organization. A critical example of intracellular reorganization is chloroplast photo-relocation, which is required for optimal energy harvesting and avoiding photodamage. A key system responsible for the spatial organization of intracellular components is the microtubule cytoskeleton and its associated motor proteins, kinesins. Here we tested the hypothesis that members of the kinesin 4II subfamily are important for chloroplast photo-relocation in the moss Physcomitrella patens. Most land plants, including P. patens, use an actin cytoskeleton-dependent mechanism to transport chloroplasts in response to light. In addition to the actin-based system, P. patens can also transport chloroplasts via a microtubule-dependent mechanism, which is absent in flowering plants. Here, we used a P. patens line that contains an inducible RNAi system to silence all three kinesin 4-II genes present in this moss and evaluated their participation in the microtubule-dependent chloroplast light avoidance response. Because we found a significant effect on cell growth when kinesin 4IIs are silenced, we took advantage of the inducible system to establish a reproducible and quantitative assay to evaluate chloroplast photo-relocation in full-grown cells. Using a laser scanning confocal-based chloroplast light avoidance response assay, we found a reduction in chloroplast motility when kinesin 4IIs were silenced. Hence, in addition to identifying a role for kinesin 4II proteins in protonemal cell growth, our results strongly support the hypothesis that these kinesins play an important role in the chloroplast light avoidance response.