A Kinesin-Like Protein, KAC, is Required for Light-Induced and Actin-Based Chloroplast Movement in Marchantia polymorpha.

Chloroplasts accumulate on the cell surface under weak light conditions to efficiently capture light but avoid strong light to minimize photodamage. The blue light receptor phototropin regulates the chloroplast movement in various plant species. In Arabidopsis thaliana, phototropin mediates the light-induced chloroplast movement and positioning via specialized actin filaments on the chloroplasts, chloroplast-actin filaments. KINESIN-LIKE PROTEIN FOR ACTIN-BASED CHLOROPLAST MOVEMENT (KAC) and CHLOROPLAST UNUSUAL POSITIONING 1 (CHUP1) are pivotal for actin-based chloroplast movement and positioning in land plants. However, the mechanisms by which KAC and CHUP1 regulate chloroplast movement and positioning remain unclear. In this study, we characterized KAC and CHUP1 orthologs in the liverwort Marchantia polymorpha, MpKAC and MpCHUP1, respectively. Their knockout mutants, Mpkacko and Mpchup1ko, impaired the light-induced chloroplast movement. Although Mpchup1ko showed mild chloroplast aggregation, Mpkacko displayed severe chloroplast aggregation, suggesting the greater contribution of MpKAC to the chloroplast anchorage to the plasma membrane. Analysis of the subcellular localization of the functional MpKAC-Citrine indicated that MpKAC-Citrine formed a punctate structure on the plasma membrane. Structure-function analysis of MpKAC revealed that the deletion of the conserved C-terminal domain abrogates its targeting to the plasma membrane and its function. The deletion of the N-terminal motor domain retains the plasma membrane targeting but abrogates the formation of punctate structure and shows a severe defect in the light-induced chloroplast movement. Our findings suggest that the formation of the punctate structure on the plasma membrane of MpKAC is essential for chloroplast movement.