A family of ROP proteins that suppress actin dynamics and are essential for polarized growth and cell adhesion

In plants, the small GTPase ROP has been implicated in the polarized growth of tip growing cells, such as root hairs and pollen tubes; however, most of the data derive from overexpressing ROP genes or constitutively active and dominant negative isoforms, while confirmation from loss-of-function studies has generally been lacking. Here, in the model moss Physcomitrella patens, we study ROP signaling during tip growth by using a loss-of-function approach to silence the entire moss ROP family with RNA interference. We find that plants with reduced ROP expression, in addition to failing to initiate tip growth, have perturbed cell wall staining, reduced cell adhesion, and have increased actin-filament dynamics. Although ROP RNAi plants also have reduced microtubule dynamics, this reduction is not specific to loss of ROP, as it occurs when actin function is compromised chemically or genetically. Our data suggest that ROP polarizes the actin cytoskeleton by suppressing actin filament dynamics leading to an increase in actin filaments at the site of polarized secretion.