Myosins form a large superfamily of molecular motors that move along actin filaments. The functions of myosins in plant cells are thought to be related to various processes: cell division, movement of mitochondria and chloroplasts, cytoplasmic streaming, rearrangement of transvacuolar strands, and statolith positioning. Class VIII and XI myosins are represented in the Arabidopsis thaliana genome by 4 and 13 potential genes, respectively. The roles of individual class XI myosins and their cellular targets in A. thaliana are still unclear. In this work we implemented a reverse genetic approach to analyse the loss-of-function mutants of XIK, a representative of class XI myosins in A. thaliana. Three different T-DNA insertion mutants in the myosin XIK gene showed similar phenotypes: impaired growth of root hair cells, twisted shape of stem trichomes, and irregular size, branch positioning, and branch expansion of leaf trichomes. Morphometric analysis of mutant seedlings showed that the average length of root hairs was reduced up to 50% in comparison with wild-type root hairs, suggesting an involvement of the class XI myosin XIK in tip growth. On leaves, the proportion of trichomes with short branches was doubleed in mutant plants, and the mutant trichomes possessed a mildly twisted shape. Therefore, we concluded that myosin XIK is involved also in the elongation of stalks and branches of trichomes.
BackgroundThe positioning and dynamics of vesicles and organelles, and thus the growth of plant cells, is mediated by the acto-myosin system. In Arabidopsis there are 13 class XI myosins which mediate vesicle and organelle transport in different cell types. So far the involvement of five class XI myosins in cell expansion during the shoot and root development has been shown, three of which, XI-1, XI-2, and XI-K, are essential for organelle transport.ResultsSimultaneous depletion of Arabidopsis class XI myosins XI-K, XI-1, and XI-2 in double and triple mutant plants affected the growth of several types of epidermal cells. The size and shape of trichomes, leaf pavement cells and the elongation of the stigmatic papillae of double and triple mutant plants were affected to different extent. Reduced cell size led to significant size reduction of shoot organs in the case of triple mutant, affecting bolt formation, flowering time and fertility. Phenotype analysis revealed that the reduced fertility of triple mutant plants was caused by delayed or insufficient development of pistils.ConclusionsWe conclude that the class XI myosins XI-K, XI-1 and XI-2 have partially redundant roles in the growth of shoot epidermis. Myosin XI-K plays more important role whereas myosins XI-1 and XI-2 have minor roles in the determination of size and shape of epidermal cells, because the absence of these two myosins is compensated by XI-K. Co-operation between myosins XI-K and XI-2 appears to play an important role in these processes.
Chloroplasts alter their distribution within plant cells depending on the external light conditions. Myosin inhibitors 2,3-butanedione monoxime (BDM), N-ethylmaleimide (NEM), and 1-(5-iodonaphthalene-1-sulfonyl)-1H-hexahydro-1,4-diazepine hydrochloride (ML-7) were used to study the possible role of myosins in chloroplast photorelocation in Arabidopsis thaliana mesophyll cells. None of these agents had an effect on the chloroplast high-fluence-rate avoidance movement but all of the three myosin inhibitors blocked the accumulation movement of chloroplasts after a high-fluence-rate irradiation of the leaves. The results suggest that myosins have a role in A. thaliana chloroplast photorelocation.
Neurotrophins NGF, BDNF, NT-3, and NT-4/5 are members of the neurotrophin family of proteins, which support the survival and induce differentiation of vertebrate neurons. We have studied the effects of neurotrophins on growth cones of embryonic sensory neurons. BDNF and NT-4/5 cause growth cone collapse and transient neurite growth inhibition in NGF-dependent or NT-3-dependent rat dorsal root ganglion (DRG) neurons but not in BDNF-dependent or NT-4/5-dependent neurons, whereas NGF and NT-3 do not produce growth-cone collapse in these neurons. All neurotrophins show a chemoattractive effect on growth cones of embryonic DRG neurons: NGF and NT-3 are chemoattractants for all DRG neurons, except NT-3-dependent and NT-4/5-dependent neurons; BDNF and NT-4/5 are chemoattractants only for BDNF-dependent DRG neurons. BDNF-induced and NT-4/5-induced growth cone collapse is quantitatively characterized as a 50% decrease in F-actin content, total protein content, and area of growth cones of NGF-dependent or NT-3-dependent neurons, and a reorganization of microfilaments. BDNF induces a rapid transient 3-fold to 4-fold increase of F-actin concentration at the central part of growth cones of NGF-dependent neurons. Our results suggest that different neurotrophins have chemoattractive or inhibitory effects on the same growth cone, and that they may act as specific growth cone guidance cues.
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