Abstract
Light-induced chloroplast movement and attachment to the plasma membrane are dependent on actin filaments. In Arabidopsis thaliana, the short actin filaments on the chloroplast envelope, cp-actin filaments, are essential for chloroplast movement and positioning. Furthermore, cp-actin-filament-mediated chloroplast movement is necessary for the strong-light-induced nuclear avoidance response. The proteins CHLOROPLAST UNUSUAL POSITIONING 1 (CHUP1), KINESIN-LIKE PROTEIN FOR ACTIN-BASED CHLOROPLAST MOVEMENT 1 (KAC1) and KAC2 are required for the generation and/or maintenance of cp-actin filaments in Arabidopsis. In land plants, CHUP1 and KAC family proteins play pivotal roles in the proper movement of chloroplasts and their attachment to the plasma membrane. Here, we report similar but distinct phenotypes in chloroplast and nuclear photorelocation movements between chup1 and kac1kac2 mutants. Measurement of chloroplast photorelocation movement indicated that kac1kac2, but not chup1, exhibited a clear strong-light-induced increase in leaf transmittance changes. The chloroplast movement in kac1kac2 depended on phototropin 2, CHUP1 and two other regulators for cp-actin filaments, PLASTID MOVEMENT IMPAIRED 1 and THRUMIN 1. Furthermore, kac1kac2 retained a weak but significant nuclear avoidance response although chup1 displayed a severe defect in the nuclear avoidance response. The kac1kac2chup1 triple mutant was completely defective in both chloroplast and nuclear avoidance responses. These results indicate that CHUP1 and the KACs function somewhat independently, but interdependently mediate both chloroplast and nuclear photorelocation movements.
Highlights
Organelle movement is essential for many cellular activities and needs to be tightly regulated [1, 2]
To further examine the defective chloroplast photorelocation movements in chup1 and kac1kac2, we examined light-induced changes in leaf transmittance in these mutants (Fig 1A)
Weak blue light did not induce any changes in leaf transmittance, kac1kac2 exhibited a clear increase in leaf transmittance in response to strong blue light (Fig 1B and S1 Fig), indicating that the avoidance response-like chloroplast movements still occurs in the kac1kac2 double mutant
Summary
Organelle movement is essential for many cellular activities and needs to be tightly regulated [1, 2]. Because land plants are sessile organisms, the organelle movements should be appropriately regulated by environmental signals, such as light. Chloroplasts change their position in response to light (chloroplast photorelocation movement). Chloroplasts move towards weak light to capture light efficiently (the accumulation response). Chloroplasts escape from strong light and move to a position where light absorption is minimized (the avoidance response) [3, 4]. Phototropin (phot) is the blue light receptor for chloroplast photorelocation movement. In Arabidopsis thaliana, two phototropins, phot and phot, redundantly mediate the accumulation response, and phot primarily regulates the avoidance response [5,6,7]
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