WHY DO PLANTS HAVE PHOSPHOINOSITIDES

Phosphoinositides are inositol-containing phospholipids whose hydrolysis is a key step in the rapid responses of animal cells to extracellular signals. Whether they play similar roles in plant cells has not been established, and some have suggested alternative roles as direct modulators of specific proteins. Nonetheless, evidence is accumulating that phosphoinositide hydrolysis mediates transduction of some signals in plants. The evidence is strongest for a role in triggering the shedding of flagella by the unicellular alga Chlamydomonas reinhardtii under acid stress. Rapid kinetic analysis indicates that phosphoinositide hydrolysis occurs within half a second and could trigger the rapid loss of flagella. Plant responses to pathogens and osmotic stress, as well as the regulation of turgor changes which underlie stomatal opening and closing and the movements of leaves and flower parts, may also be mediated by phosphoinositide hydrolysis. The evidence thus indicates that at least one reason plants have phosphoinositides is to mediate transduction of environmental signals.