Arabidopsis Myosins XI1, XI2, and XIK Are Crucial for Gravity-Induced Bending of Inflorescence Stems

Abstract

Myosins and actin filaments in the actomyosin system act in concert in regulating cell structure and dynamics and are also assumed to contribute to plant gravitropic response. To investigate the role of the actomyosin system in the inflorescence stem gravitropism, we used single and multiple mutants affecting each of the 17 Arabidopsis myosins of class VIII and XI. We show that class XI but not class VIII myosins are required for stem gravitropism. Simultaneous loss of function of myosins XI1, XI2, and XIK leads to impaired gravitropic bending that is correlated with altered growth, stiffness, and insufficient sedimentation of gravity sensing amyloplasts in stem endodermal cells. The gravitropic defect of the corresponding triple mutant xi1 xi2 xik could be rescued by stable expression of the functional XIK:YFP in the mutant background, indicating a role of class XI myosins in this process. Altogether, our results emphasize the critical contributions of myosins XI in stem gravitropism of Arabidopsis.

Update on Myosin Motors: Molecular Mechanisms and Physiological Functions

“…While it has been shown that myosin VIII also localizes to the cortical division site prior to mitosis (Wu and Bezanilla, 2014) and to the cell plate region during cytokinesis (Van Damme et al, 2004), no clear defect in plant growth was detected in quadruple knockouts in Arabidopsis that had lost all their myosin VIII genes (Talts et al, 2016). This lack of an apparent mutant phenotype contrasts with the clear morphological defects resulting from global myosin VIII loss in P. patens (Wu et al, 2011;Wu and Bezanilla, 2014).…”

“…Specifically, the loss of Myo11B2, Myo11E, and Myo11F resulted in plants that had reduced stem growth rates, and these stems were stiffer and bent more slowly during gravistimulation (Talts et al, 2016). Interestingly, however, the sedimentation of amyloplasts was slower in the triple mutants (Talts et al, 2016), suggesting that their reduced gravitropism cannot be ascribed solely to slower growth.…”

“…Specifically, the loss of Myo11B2, Myo11E, and Myo11F resulted in plants that had reduced stem growth rates, and these stems were stiffer and bent more slowly during gravistimulation (Talts et al, 2016). Interestingly, however, the sedimentation of amyloplasts was slower in the triple mutants (Talts et al, 2016), suggesting that their reduced gravitropism cannot be ascribed solely to slower growth. Previous research has established that amyloplasts are remarkably dynamic in endodermis cells (Saito et al, 2005), and some of their movements are reminiscent of myosin-driven organelle movement.…”

“…Given that myosin motor activity is required for growth, it may not be astonishing that a growth response like gravitropism is affected in higher order myosin XI mutants (Talts et al, 2016). Specifically, the loss of Myo11B2, Myo11E, and Myo11F resulted in plants that had reduced stem growth rates, and these stems were stiffer and bent more slowly during gravistimulation (Talts et al, 2016).…”

Myosin-driven transport network in plants is functionally robust and distinctive

“…In this model, the myosin XI-MyoB compartments represent a specialized transport system that drives streaming, rather than directly engaging individual organelles and secretory vesicles, which in turn delivers organelles and other types of vesicles to their destinations throughout the cell (14). Furthermore, existing genetic evidence indicates that cytoplasmic streaming is required for both polarized and diffuse cell growth because: (i) inactivation of myosins alone results in reduced cell elongation and expansion and affects plant growth and morphogenesis (15)(16)(17)(18)(19)(20); (ii) simultaneous inactivation of myosins and MyoBs results in synergistic phenotypes (13,14); and (iii) reduction in streaming velocity reduces plant growth, whereas increased velocity boosts that growth (21).…”