In model plant Arabidopsis there are myosin XI

In model plant Arabidopsis, there are 13 myosin XI isoforms (XI-1, XI-2, XI-A, XI-B, XI-C, XI-D, XI-E, XI-F, XI-G, XI-H, XI-I, XI-J, and XI-K) and 4 myosin VIII isoforms (ATM1, ATM2, VIII-A, VIII-B) [9,13]. Arabidopsis myosin XIs have the common domain composition: a motor domain that interacts with regulator of g protein signaling and hydrolyzes ATP, a neck domain with six IQ motifs that binds light chains, a α-helical coiled-coil domain supporting dimer formation, and a globular tail domain that binds different cargoes [9]. In contrast, myosin VIIIs have a shorter neck domain (consisting of three or four IQ motifs) and a shorter predicted coiled-coil domain [9]. It is well established that myosin XI plays a major role in the generation of motive force for cytoplasmic streaming. Studies on subcellular localization of the myosin XI-K revealed that a yellow fluorescent protein-tagged myosin XI-K (YFP-XI-K) in the Arabidopsis cells is associated with the membranous vesicle-like compartments moving along F-actin bundles at the velocity of cytoplasmic streaming. Accordingly, transport of the XI-K-associated membrane compartment contributes to cytoplasmic streaming [14]. In contrast, green fluorescent protein (GFP)-fused ATM1, a member of myosin VIII, localized to plasmodesmata, plastid, newly formed cell walls, and actin filaments in cell cortex [15]. The recombinant ATM1 consisting of a head and single IQ motif moved actin filaments in the in vitro motility assay at 0.2 μm s−1. The localization and the low velocity of ATM1 suggested that myosin VIII functions as a tension sensor/generator, but not as a generator of cytoplasmic streaming [15]. Previous studies have reported an increased number of myosin XI isoforms in higher plants. For example, Physcomitrella patens has only 2 myosin XI genes [16], whereas Arabidopsis thaliana, Oryza sativa, and Brachypodium distachyon have 13, 12, and 9, respectively [13,17,18]. The diversity of plant myosin XI genes is required for specific functions of diverse cells, tissues, and organs. In this review, we have summarized the roles of myosin XI in cytoplasmic streaming, actin organization and dynamics, and plant development. We have also reviewed recent evidence for the diverse functions of the actin–myosin XI cytoskeleton in Arabidopsis.
Conclusion Recent advances in the understanding of the model plant Arabidopsis have revealed diverse physiological functions of the actin–myosin XI cytoskeleton in various tissues of higher plants. Arabidopsis myosin XI isoforms exhibited diverse expression patterns in different organs (Fig. 1). Although several myosin XI isoforms showed partially redundant functions for cytoplasmic streaming, other myosin XI isoforms acquired specific functions in specific tissues (e.g., XI-C and XI-E in pollen [57] and XI-F in fiber cells of the stem [48]). We summarized the functions and features of Arabidopsis myosin XI and actin isoforms in individual plant tissues (Table 1). These findings indicate that the diversity of actin–myosin XI cytoskeleton acts as an intracellular control network to regulate various biological processes in higher plants. A recent phylogenetic analysis revealed that myosin XI gene families have already emerged in green algae, and show concurrent expansions in flowering plants [59]. Similar to myosin XI, the actin isoforms also exhibited diversity in Arabidopsis. Recently, Arabidopsis actin isoforms, including ACT2, ACT7, ACT1, and ACT11, have been shown to possess different biochemical properties, such as phalloidin binding, relative polymerization rate, relative phosphate release rate, and interaction with profilin [60]. Moreover, Slajcherová et al. reported that the number of actin family members partially reflected the complexity of the organism [61]. For example, algae have only 2–3 actin isoforms, whereas gymnosperms and angiosperms have over than 10 isoforms. Cytoplasmic streaming is an intracellular movement observed in a variety of plant species, suggesting cytoplasmic streaming generated by the actin-myosin XI is a primitive and fundamental system that conserved from algae to angiosperm. For algae which live in water, the simple actin-myosin XI transportation should have been sufficient for their survival because the environment stimuli are less than those on the ground. In contrast, land plants may have acquired diverse actin and myosin XI isoforms to respond to strong and various environmental stimuli (e.g., intense gravity, wind, and rain). Furthermore, the specific expression of many actin and myosin XI isoforms in reproductive tissues may reflect that their appearance has been necessary for the revolution of reproductive system from using the sperm to using the pollen tube in angiosperms. In this regard, it is interesting to investigate the involvement of pollen specific myosin XI isoforms on the pollen tube guidance which is the essential system for reproduction of angiosperms. Taken together, the actin–myosin XI cytoskeleton may have acquired diverse higher functions during the coevolution of myosin XI and actin isoforms in higher plants.