MDP25, A Novel Calcium Regulatory Protein, Mediates Hypocotyl Cell Elongation by Destabilizing Cortical Microtubules inArabidopsis     

Abstract

The regulation of hypocotyl elongation is important for plant growth. Microtubules play a crucial role during hypocotyl cell elongation. However, the molecular mechanism underlying this process is not well understood. In this study, we describe a novel Arabidopsis thaliana microtubule-destabilizing protein 25 (MDP25) as a negative regulator of hypocotyl cell elongation. We found that MDP25 directly bound to and destabilized microtubules to enhance microtubule depolymerization in vitro. The seedlings of mdp25 mutant Arabidopsis lines had longer etiolated hypocotyls. In addition, MDP25 overexpression resulted in significant overall shortening of hypocotyl cells, which exhibited destabilized cortical microtubules and abnormal cortical microtubule orientation, suggesting that MDP25 plays a crucial role in the negative regulation of hypocotyl cell elongation. Although MDP25 localized to the plasma membrane under normal conditions, increased calcium levels in cells caused MDP25 to partially dissociate from the plasma membrane and move into the cytosol. Cellular MDP25 bound to and destabilized cortical microtubules, resulting in their reorientation, and subsequently inhibited hypocotyl cell elongation. Our results suggest that MDP25 exerts its function on cortical microtubules by responding to cytoplasmic calcium levels to mediate hypocotyl cell elongation.

Arabidopsis MICROTUBULE DESTABILIZING PROTEIN40 Is Involved in Brassinosteroid Regulation of Hypocotyl Elongation

“…Many microtubule regulatory proteins, such as SPR1 and MDP25, are involved in regulating hypocotyl cell elongation by altering the stability and organization of cortical microtubules (Nakajima et al, 2004(Nakajima et al, , 2006Li et al, 2011a). This study showed that the stability of cortical microtubules is decreased in hypocotyl epidermal cells following treatment with BL, which likely explains the underlying mechanism regarding cortical microtubule reorientation in response to BR.…”

“…BZR1 functions in rapidly growing etiolated hypocotyl cells but not in cells that are in a stationary growth phase (Wang et al, 2002). Transverse patterns of cortical microtubules, which are associated with cells in a rapid growth phase, are detected in rapidly elongating etiolated hypocotyl epidermal cells (Le et al, 2005;Li et al, 2011a;Crowell et al, 2011). The orientation of cortical microtubules is interrelated with hypocotyl cell elongation, which has been tested using microtubule-disrupting drugs and the mutant and transgenic lines of microtubule regulatory proteins (Le et al, 2005;Paredez et al, 2006;Somerville, 2006;Buschmann and Lloyd, 2008).…”

“…The clockwise and counterclockwise rotations of cortical microtubules are dynamic features in growing hypocotyl cells as observed via longterm time-lapse imaging (Chan et al, 2007). In addition, the orientations of cortical microtubules, particularly on the inner face of the epidermis, are associated with the growth status of etiolated hypocotyls (Le et al, 2005;Li et al, 2011a;Crowell et al, 2011). For example, the parallel array of cortical microtubules is dominantly transversely oriented to the hypocotyl longitudinal growth axis in rapidly growing hypocotyl cells, while the microtubules are longitudinally oriented when cell elongation stops.…”

“…Disturbing cortical microtubules with the microtubule-disrupting drug propyzamide induces a stunted hypocotyl phenotype (Le et al, 2005). Mutation or overexpression of many microtubule regulatory proteins also results in abnormal hypocotyl cell elongation by altering the stability and organization of cortical microtubules, such as SPIRAL1 (SPR1), MAP18, and MDP25 (Nakajima et al, 2004(Nakajima et al, , 2006Wang et al, 2007;Li et al, 2011a). These studies demonstrate that regulation of the organization and dynamics of cortical microtubules is crucial for hypocotyl cell growth.…”

The Microtubule Plus-End Tracking Proteins SPR1 and EB1b Interact to Maintain Polar Cell Elongation and Directional Organ Growth in Arabidopsis

“…The fact that a range of mutants altered in MT stability exhibit defects in cell growth (Ishida et al, 2007a;Perrin et al, 2007;Wang et al, 2007Wang et al, , 2012Yao et al, 2008;Li et al, 2011a) suggests that proper MT dynamics and stability may be key to proper cell expansion. Given our data showing that the cortical MT arrays in spr1-6 eb1b-2 expanding root and etiolated hypocotyl cells have relatively normal organization (Figure 4), it seems unlikely that the severe anisotropic expansion defects of these cells is due to the orientation and ordering of cortical MTs and, by extension, oriented cellulose deposition.…”