Because of the large number of myosin XI and ROP genes found in many angiosperms, it has been difficult to determine their precise role with respect to tip growth. In contrast, there are only two myosin XI genes in four ROP genes in the moss Physcomitrella patens. To determine their role in tip growth using a loss-of-function approach, I used RNA interference (RNAi) and found that both of these proteins are essential for tip growth. Consistent with a role in tip growth, I show that a functional, full-length fusion of mEGFP to myosin XI accumulates at a subcortical, apical region of actively growing protonemal cells. Myosin XI RNAi plants also appear to have decreased cellulose in the cell wall, suggesting a role in secretion of cellulose synthases. I found that silencing ROP increases cortical actin dynamics but does not appear to have a specific affect on the microtubule cytoskeleton. Further investigation found that ROP recruits class II formins to the cell cortex where they actively nucleate and elongate actin filaments. Loss of ROP also causes a decrease in intracellular adhesion. Unlike myosin XI RNAi plants, examination of the crystalline cellulose content of the cell wall shows that the deposition of the cell wall is not inhibited in the absence of ROP. Taken together my findings suggest that ROP defines a membrane region where myosin XI delivers secretory vesicles containing cellulose synthase and other materials needed to build new cell wall during tip growth
