The kinesin-like proteins,KAC1/2, regulate actin dynamics underlying chloroplast light-avoidance in Physcomitrella patens

In plants, light determines chloroplast position;these organelles show avoidance and accumulation resesponses in high and low fluence-rate light, respectively.Chloroplast motility in response to light is driven by cytoskeletal elements. The actin cytoskeleton mediates chloroplast photorelocation responses in Arabidopsis thaliana. In contrast, in the moss Physcomitrella patens, both, actin filaments and microtubules can transport chloroplasts.Because of the surprising evidence that two kinesin-like proteins(called KACs) are important for actin-dependent chloroplast photorelocation in vascular plants, we wanted to determine the cytoskeletal system responsible for the function of these proteins in moss. We performed genespecific silencing using RNA interference in P. patens. We confirmed existing reports using gene knockouts, that Pp KAC1 and Pp KAC2 are required for chloroplast dispersion under uniform white light conditions, and that the two proteins are functionally equivalent. To address the specificcytoskeletal elements responsible for motility, this loss-of function approach was combined with cytoskeleton-targeted drug studies. We found that, in P. patens, these KACs mediate the chloroplast light-avoidance response in an actin filamentdependent, rather than a microtubule-dependent manner.Using correlation-decay analysis of cytoskeletal dynamics, we found that Pp KAC stabilizes cortical actin filaments, but has no effect on microtubule dynamics.