Polarized cell growth in higher plants

Pollen tubes and root hairs are highly elongated, cylindrically shaped cell s whose polarized growth permits them to explore the environment for the be nefit of the entire plant. Root hairs create an enormous surface area for t he uptake of water and nutrients, whereas pollen tubes deliver the sperm ce lls to the ovule for fertilization. These cells grow exclusively at the ape x and at prodigious rates (in excess of 200 nm/s for pollen tubes). Underly ing this rapid growth are polarized ion gradients and fluxes, turnover of c ytoskeletal elements (actin microfilaments), and exocytosis and endocytosis of membrane vesicles. Intracellular gradients of calcium and protons are s patially localized at the growing apex; inward fluxes of these ions are api cally directed. These gradients and fluxes oscillate with the same frequenc y as the oscillations in growth rate but not with the same phase. Actin mic rofilaments, which together with myosin generate reverse fountain streaming , undergo rapid turnover in the apical domain, possibly being regulated by key actin-binding proteins, e.g., profilin, villin, and ADF/cofilin, in con cert with the ion gradients. Exocytosis of vesicles at the apex, also depen dent on the ion gradients, provides precursor material for the continuously expanding cell wall of the growing cell. Elucidation of the interactions a nd of the dynamics of these different components is providing unique insigh t into the mechanisms of polarized growth.