In vivo visualization of F-actin structures during the development of the moss Physcomitrella patens

The 'in planta' visualization of F-actin in all cells and in all developmental stages of a plant is a challenging problem. By using the soybean heat inducible Gmhsp17.3B promoter instead of a constitutive promoter, we have been able to label all cells in various developmental stages of the moss Physcomitrella patens, through a precise temperature tuning of the expression of green fluorescent protein (GFP)-talin. A short moderate heat treatment was sufficient to induce proper labeling of the actin cytoskeleton and to allow the visualization of time-dependent organization of F-actin structures without impairment of cell viability. In growing moss cells, dense converging arrays of F-actin structures were present at the growing tips of protonema cell, and at the localization of branching. Protonema and leaf cells contained a network of thick actin cables; during de-differentiation of leaf cells into new protonema filaments, the thick bundled actin network disappeared, and a new highly polarized F-actin network formed. The controlled expression of GFP-talin through an inducible promoter improves significantly the 'in planta' imaging of actin.