Actin and Myosin VIII in Plant Cell-Cell Channels
Jozef Samaj, Nigel Chaffey, Uday Tirlapur, Jan Jasik, Andrej Hlavacka, Zhan Feng Cui, Dieter Volkmann, Diedrik Menzel and Frantisek Baluska
Plasmodesmata (PD) are cell-cell channels interconnecting all the cells of the plant body into a huge syncytium, which makes plants ‘supracellular’ organisms. Recent studies have clearly revealed that both the targeting and gating of PD is highly regulated. Importantly, it is known that molecules below the size exclusion limits, such as auxin and calcium, cannot pass freely through plasmodesmata, strongly implicating a very effective sieve-like structure within PD. Even though the first PD-resident proteins emerge from recent molecular studies, the majority of these elusive proteins remain to be unveiled. Convincing evidence suggests that F-actin meshworks, supported by myosins of class VIII and perhaps also by ARP2/3 proteins and other actin-binding proteins, are components of gateable PD and potentially involved in sieve-like nature of these cell-cell channels. Interestingly, there are several structural and functional similarities between PD and nuclear pores. For example, homeodomain transcription factors are able to pass through both nuclear pores as well as PD via similar mechanisms. Some cells are also symplasmically isolated such as stomata and trichoblasts initiating root hairs, and root cap statocytes. The root cap statocytes and their PD are depleted in both F-actin and myosin VIII, implying that these cytoskeletal molecules are essential for transport through PD. Gravistimulation triggers the opening of a special subset of PD in the root cap statocytes suggesting that a physical force, such as gravity, is capable of targeting molecules/processes which regulate the gating of PD.