The Actin Cytoskeleton Tracks or More 31 Actin Tracks and Baskets

The movement of plant organelles appears to be based predominantly on the acto-myosin system (for a review see Krzeszowiec and Gabrys 2011). Inhibition of chloroplast movements by the anti-actin drugs cytochalasins B and D provided the first evidence for the involvement of actin in the mechanism of these movements in seed plants (Izutani et al. 1990; Tlalka and Gabrys 1993; Malec et al. 1996). In several species, light treatments which activated the redistribution of chloroplasts were shown to reorganize concomitantly the actin network. The distinctive structures and arrangements of actin filaments which formed following weak or strong irradiation were first demonstrated in cryptogams (Blatt and Briggs 1980; Kadota and Wada 1992). The relationship between the light-activated spatial reorganization of the actin cytoskeleton and chloroplast redistribution has been investigated in detail in the epidermis of a water angiosperm Vallisneria gigantea (Dong et al. 1998). In this species, chloroplasts flow with the circulating cytoplasm and become trapped at the weakly illuminated cell walls. The striking light-dependent reorganization of the actin cytoskeleton entails the rearrangement of the loose network of thin bundles present in dark-adapted cells into a honeycomb array upon several hours of constant irradiation with weak red light. The honeycomb cavities become traps for chloroplasts at the most illuminated walls. The trapping can be reversed by strong blue light: following a few minutes of irradiation, the chloroplasts regain their motility and redistribute toward less illuminated walls (Takagi 2003; Sakurai et al. 2005). Concomitantly, a reorganization of actin filaments takes place, resulting in a more stretched network of thick bundles. Far-red light and/or DCMU [3-(3,4-dichlorophenyl)-1,1 -dimethylurea], an inhibitor of electron transport in photosynthesis, inhibit both movements and actin rearrangements. These facts have been interpreted in terms of a joint control of phytochrome and photosynthetic pigments over chloroplast redistribution via actin reorganization.

A structural association between actin cytoskeletal structures and chloro-plasts has been demonstrated in A. thaliana by means of immunocytochemistry

(Kandasamy and Meagher 1999). The authors hypothesized that some chloroplasts, encapsulated in actin structures and covered with motor proteins, might migrate along actin cables directly, while others might be pulled by fine filaments attached to the cables.

Inspired by the results obtained for Vallisneria, Kumatani et al. (2006) investigated the influence of light on the actin cytoskeleton in the pallisade cells of spinach. They reported the occurrence of prominent, long actin bundles specific to strong blue light irradiated cells. This evidence appears to be rather inconclusive because statistical evaluation is lacking.

Another attempt to find light-induced reorganization of the actin cytoskeleton analogous to that observed in cryptogams and water angiosperms was undertaken by Krzeszowiec et al. (2007) under conditions which activate chloroplast redistribution. No blue-light-induced changes in the actin architecture were demonstrated either in A. thaliana or in tobacco. However, although the plant material was fixed prior to actin labeling with Alexa fluor phalloidin, even small differences in the shape and distribution of F-actin formations were detectable. Noticeable changes were found in mesophyll cells of phot2 mutants exposed to strong red light as compared to those exposed to weak red light. As red light does not activate chloroplast responses, these changes cannot be associated with their movement. This line of investigation was continued using live mesophyll cells of transgenic tobacco labeled with GFP fused to a fragment of human plastin, an actin-bundling protein (Anielska-Mazur et al. 2009). Also in this model, which allows an examination of cytoskeleton reorganization in real time, no blue-light-specific actin rearrangement could be detected. Strong light of 40 prnol m~2 s_1 caused a reversible widening of actin bundles. This intensity of blue light resulted in an incomplete avoidance response (about 20% below saturation). The widening effect was neither directional nor blue light specific. In general, similar effects were observed both in blue light, which activates chloroplast movements in tobacco, and in inactive red light.

The chloroplasts were surrounded with dense cocoons built of thin actin filaments. These structures, which have been called baskets, were tightly bound together around adjacent chloroplasts and attached to cortical actin bundles. The baskets were remarkably stable and survived not only strong light but also EGTA treatment, the latter being much more damaging to cortical actin than to the fine filaments surrounding the chloroplast. On the basis of qualitative and quantitative analysis, the authors concluded that actin baskets and their interactions with the cortical actin cytoskeleton play a key role in chloroplast positioning in higher land plants. A more general conclusion drawn from this study is that the directionality of chloroplast responses is not based on specific blue-light-induced changes of F-actin.

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