Keep them moving!

Abstract

When introducing the cytoskeleton to undergraduates, teachers refer to paradigmatic examples. Mobile cells such as amoeba exemplify cytoskeleton-driven locomotion. In immobile plant cells, the significance of the cytoskeleton for the structuring of the cell interior becomes tangible owing to easily observed chloroplast movements. Generations of students have been taught that light-induced chloroplast movements serve to optimize photosynthetic efficiency while preventing photodamage. However, hard experimental evidence linking chloroplast movement to photosynthetic performance in multicellular plants has been lacking. Jeong et al. [1xA large population of small chloroplasts in tobacco leaf cells allows more effective chloroplast movements than a few enlarged chloroplasts. Jeong, W.J. et al. Plant Physiol. 2002; 129: 112–121Crossref | PubMed | Scopus (44)See all References][1] have now narrowed this gap.Leaf cells of transgenic tobacco overexpressing FtsZ1-2, a gene involved in chloroplast division, contain only one to three large chloroplasts instead of the many small ones present in wild-type cells. Growth of transgenic and wild-type plants was indistinguishable at normal illumination. At high and at low light intensities, growth of the transgenics was inhibited. Jeong et al. tested whether the photosynthetic efficiency of transgenic cells was lower by quantifying the cellular content of photosynthetic pigments, photosystem proteins and the key photosynthetic enzyme Rubisco – no differences were found. Likewise, chloroplast ultrastructure and the capacity for carbon assimilation were similar in wild-type and transgenic plants. The only differences were a greater susceptibility to photodamage of photosystem II reaction centers at high light intensity and the absence of chloroplast movements in transgenic plants. Applying ‘Occam's razor’, the authors concluded that reduced growth in transgenic plants at low and high light was caused by their inability to move their huge chloroplasts across their cells. These plants seemed unable to exploit weak light and avoid harmful irradiation by controlling the surface area and position of their illuminated chloroplasts.However, when mobility of chloroplasts is needed, a great number of small plastids is not necessarily better than a few big ones. In the classroom, some green algae such as Mougeotia, which contain just one huge chloroplast per cell, are among the most reliable objects for the demonstration of light-induced movements. Apparently, the optimal number of chloroplasts for a given cell type depends on cell geometry and the spatial organization of the cytoskeleton.