Quantitative analysis of organelle distribution and dynamics in Physcomitrella patens protonemal cells

We used fluorescent protein fusions to visualize and track Golgi dictyosomes, mitochondria, and peroxisomes in live protonemal cells. We also visualized and tracked chloroplasts based on chlorophyll auto-fluorescence. We showed that in protonemata all four organelles are distributed in a gradient from the tip of the apical cell to the base of the sub-apical cell. For example, the density of Golgi dictyosomes is 4.7 and 3.4 times higher at the tip than at the base in caulonemata and chloronemata respectively. While Golgi stacks are concentrated at the extreme tip of the caulonemata, chloroplasts and peroxisomes are totally excluded. Interestingly, caulonemata, which grow faster than chloronemata, also contain significantly more Golgi dictyosomes and fewer chloroplasts than chloronemata. Moreover, the motility analysis revealed that organelles in protonemata move with low persistency and average instantaneous speeds ranging from 29 to 75？nm/s, which are at least three orders of magnitude slower than those of pollen tube or root hair organelles.To our knowledge, this study reports the first quantitative analysis of organelles in Physcomitrella patens and will make possible comparisons of the distribution and dynamics of organelles from different tip growing plant cells, thus enhancing our understanding of the mechanisms of plant polarized cell growth.Plant cells display a large variety of shapes and morphologies, such as the puzzle-like leaf epidermal cells and the filament-like elongated root hairs. Such diversity is made possible by the mechanism of differential cell growth, in which cell wall and plasma membrane extension occur at a confined region, and which requires a dynamic coordination of the cytoskeleton and the endomembrane system [1-3]. For example, in pollen tubes and root hairs which undergo polarized cell growth, expansion is restricted to the tip of the elongating cells. The moss Physcomitrella patens is a particularly suitable plant model system to inve