Plant mechanosensitive ion channels: an ocean of possibilities

Abstract

Mechanosensitive ion channels, transmembrane proteins that directly couple mechanical stimuli to ion flux, serve to sense and respond to changes in membrane tension in all branches of life. In plants, mechanosensitive channels have been implicated in the perception of important mechanical stimuli such as osmotic pressure, touch, gravity, and pathogenic invasion. Indeed, three established families of plant mechanosensitive ion channels play roles in cell and organelle osmoregulation and root mechanosensing - and it is likely that many other channels and functions await discovery. Inspired by recent discoveries in bacterial and animal systems, we are beginning to establish the conserved and the unique ways in which mechanosensitive channels function in plants.

Figure 1. Subcellular Localization and Ionic Preference for Known Plant Mechanosensitive Ion Channels

The subcellular localization of MS ion channel proteins so far identified in land plants is indicated [–23,32,58]. The outer membrane of the chloroplast is permeable to ions [59], and Voltage-dependent Anion Channels (VDACs) are thought to mediate flux across the outer mitochondrial membrane [60]. MSL, MscS-Like; TPK, Two-pore K⁺; MCA, Mid1-Complementing Activity. Note that only general ion permeability preferences are indicated; these channels are likely to be permeable to additional species.

Figure 2. Proposed Role of MSL8 in Controlling Turgor During Pollen Hydration, Germination, and Tube Growth

Wild-type pollen grains successfully survive hydration in distilled water, germinate effectively in germination media, produce intact pollen tubes, and are optimally fertile. Pollen grains from msl8-4 null mutants, or null mutants expressing the MSL8F^720L allele, display reduced viability upon hydration in distilled water, and we propose that this is due to an inability to relieve turgor pressure by releasing ions upon hypoosmotic shock. Excess turgor after hydration leads both to germination at a rate higher than the wild type, but also to frequent bursting, and an overall loss of fertility. When MSL8 is overexpressed from the pollen-specific, strong LAT52 promoter, pollen grains survive hydration but are unable to maintain the threshold turgor pressure required for pollen germination or tube elongation. Green arrows, optimal turgor; red arrows, excessive turgor; blue arrows, insufficient turgor.