Tip growth is driven by turgor pressure and mediated by the polarized accumulation of cellular materials. How a single polarized growth site is established and maintained is unclear. Here, we analyzed the function of NIMA-related protein kinase 1 (MpNEK1) in the liverwort In the wild type, rhizoid cells differentiate from the ventral epidermis and elongate through tip growth to form hair-like protrusions. In Mp knockout mutants, rhizoids underwent frequent changes in growth direction, resulting in a twisted and/or spiral morphology. The functional MpNEK1-Citrine protein fusion localized to microtubule foci in the apical growing region of rhizoids. Mp knockouts exhibited increases in both microtubule density and bundling in the apical dome of rhizoids. Treatment with the microtubule-stabilizing drug taxol phenocopied the Mp knockout. These results suggest that MpNEK1 directs tip growth in rhizoids through microtubule organization. Furthermore, MpNEK1 expression rescued ectopic outgrowth of epidermal cells in the mutant, strongly supporting an evolutionarily conserved NEK-dependent mechanism of directional growth. It is possible that such a mechanism contributed to the evolution of the early rooting system in land plants.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 IntroductionThe growth and morphogenesis of plant cells relies on the orientation of cellulose microfibrils and cortical microtubules. Microtubules are cytoskeletal polymers composed of α-and β-tubulin heterodimers. Microtubules are polarized with a fast growing plus end and a slow growing minus end, and exhibit dynamic behaviors such as rapid growth and shrinkage both in vivo and in vitro (Mitchison and Kirschner 1984; Horio and Hotani 1986; Sammak and Borisy 1988; Shaw et al. 2003;Nakamura et al. 2004). Cortical microtubules are specifically found in plant cells during interphase and are localized close to the cell cortex (Ledbetter and Porter 1963). Cortical microtubules align perpendicularly to the growth direction and regulate anisotropic growth and morphogenesis of rapidly expanding cells (Green 1962; Shibaoka 1994;Wasteneys 2002; Fig. 1). Findings from genetic studies of Arabidopsis thaliana mutants strongly support the essential roles of cortical microtubule arrays on directional cell growth (Whittington et al. 2001; Thitamadee et al. 2002; Abe et al. 2004; Ishida et al. 2007a; Ishida et al. 2007b; Sedbrook and Kaloriti 2008;Wasteneys and Ambrose 2009). In addition, microtubules regulate cell division and chromosome segregation. In the mitotic phase, microtubules form a series of arrays; a preprophase band that determines the future cell division plane, mitotic spindle that segregate chromosomes, and a phragmoplast that constructs the new cell plate ( 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 suggesting that cortical microtubules could also regulate directional cell growth independently of cellulose microfibrils (Sugimoto et al. 2003). Fujita et al. (2011) have shown that cortical microtubule abundance affects cellulose crystallinity to promote directional cell growth.Microtubules might regulate the mobility and stability of cellulose synthase complexes to affect physical properties of cellulose microfibrils. Because it is beyond the scope of this review, Interested readers could consult the recent literature and references therein (Bringmann et al. 2012; Fujita et al. 2012; Lei et al. 2014). In this review, we will summarize recent findings on microtubule regulation with focus on phosphorylation-dependent regulatory mechanisms. Microtubule regulationMicrotubule-associated proteins (MAPs) play pivotal roles in the regulation of microtubule dynamics (Hamada 2014). MAPs affect microtubule assembly and bundling and regulate their geometry and organization. Because the function and regulation of MAPs have been well described in detail, we show here a few examples from a cellular and developmen...

Tip growth is a highly polarized growth process essential for plant growth and reproduction. However, its molecular mechanism and evolutional significance remain elusive. Here, we show that plant-specific armadillo-repeat containing kinesin, MpARK1, drives long-range organelle transport and microtubule convergence in tip growing rhizoids in a basal land plant Marchantia polymorpha. MpARK1 is required for anterograde transport of the nucleus and plastids to maintain their position during persistent tip growth. Furthermore, MpARK1 participates in microtubule focusing on the apex to stabilize growth direction. MpARK1-dependent rhizoid growth is essential for plant anchorage and soil holding capacity. Thus, ARK might be co-opted to serve as a long-range transporter and intracellular organizer in the primitive rooting structure (rhizoid) for the adaptation to terrestrial environments. Our findings suggest that ARK was comparable to animal anterograde kinesin, challenging the traditional view that actomyosin-based cytoplasmic streaming is a dominant transport mode in plant kingdom.

Articles you may be interested inDevelopment of flexible array eddy current probes for complex geometries and inspection of magnetic parts using magnetic sensors AIP Conf.ABSTRACT. We developed a signal evaluation system of flexible array eddy current testing (ECT) probes for inspecting complexly shaped metal surfaces. Our probes employ excitation and pickup coil pairs arrayed on a bendable substrate to measure curved targets. Since the developed probes produce signals due to the change in the relative position between the coil pair, an evaluation module for flaw signal identification is integrated into the system. The identification utilizes signal phase combination of the coil pairs. It is represented as a two-dimensional diagram constituting the phase values of two of the coil pairs with different positions. The phase of signals around flaws is significantly affected by their orientations. The diagram is therefore capable of separating flaw information from other information and of imaging flaw positions. We applied the system to complex geometric mock-ups. Flaw signals were successfully identified and the images obtained for identified positions could be used to efficiently measure the length of surface flaws by the 12 decibel down method.

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