Myosin XIK of Arabidopsis thaliana Accumulates at the Root Hair Tip and Is Required for Fast Root Hair Growth

Myosin motor proteins are thought to carry out important functions in the establishment and maintenance of cell polarity by moving cellular components such as organelles, vesicles, or protein complexes along the actin cytoskeleton. In Arabidopsis thaliana, disruption of the myosin XIK gene leads to reduced elongation of the highly polar root hairs, suggesting that the encoded motor protein is involved in this cell growth. Detailed live-cell observations in this study revealed that xik root hairs elongated more slowly and stopped growth sooner than those in wild type. Overall cellular organization including the actin cytoskeleton appeared normal, but actin filament dynamics were reduced in the mutant. Accumulation of RabA4b-containing vesicles, on the other hand, was not significantly different from wild type. A functional YFP-XIK fusion protein that could complement the mutant phenotype accumulated at the tip of growing root hairs in an actin-dependent manner. The distribution of YFP-XIK at the tip, however, did not match that of the ER or several tip-enriched markers including CFP-RabA4b. We conclude that the myosin XIK is required for normal actin dynamics and plays a role in the subapical region of growing root hairs to facilitate optimal growth.

肌球蛋白马达蛋白被认为可通过沿肌动蛋白细胞骨架转运细胞器、囊泡或蛋白质复合物等细胞组分，在细胞极性的建立与维持过程中发挥关键功能。在拟南芥（Arabidopsis thaliana）中，肌球蛋白XIK基因的功能缺失会导致高度极性化根毛的伸长量显著降低，提示该基因编码的马达蛋白参与了根毛的细胞生长调控。本研究通过细致的活细胞观测发现，相较于野生型根毛，xik突变体根毛的伸长速度更慢，且更早终止生长。包括肌动蛋白细胞骨架在内的整体细胞组织形态未见异常，但突变体中的肌动蛋白丝动力学活性明显降低。另一方面，携带RabA4b的囊泡的积累量与野生型相比无统计学显著差异。能够互补该突变体表型的功能性黄色荧光蛋白（YFP）-XIK融合蛋白，会以肌动蛋白依赖的方式在正在生长的根毛顶端积累。不过，YFP-XIK在根毛顶端的分布模式与内质网（ER）以及包括青色荧光蛋白（CFP）-RabA4b在内的多种顶端富集标记蛋白的分布模式并不匹配。综上，我们认为肌球蛋白XIK是维持正常肌动蛋白丝动力学所必需的，并在正在生长的根毛的亚顶端区域发挥作用，以促进根毛的最优生长。