Plant growth is achieved by the combined effect of two fundamental cellular activities, cell division and cell expansion. Cell expansion is complex, and involves a variety of processes including secretion of cell wall components, turgor pressure, changes in cell wall extensibility, and endocytosis [1, 2]. An extreme form of polarized cell expansion occurs in root hairs, pollen tubes, and the protonemal cells of ferns and mosses, which expand via a process know as “tip growth”. It is well established that this type of growth is dependent on a dynamic actin cytoskeleton [1, 3]; but the mechanisms that link the actin cytoskeleton to tip growth remain unknown. Because my research aims to elucidate the molecular mechanisms underlying tip growth, my first goal is to understand the role of the actin cytoskeleton in this process.
Tip growth has been extensively studied in pollen tubes and root hairs. In pollen tubes, I discovered that low concentrations of actin inhibitory drugs stop tip growth, but have no effect on cytoplasmic streaming, another known actin-dependent process [4]. These results came as a surprise because it had been thought the role of actin in growth relied on its ability to deliver vesicles to the apex via cytoplasmic streaming. The observation that growth becomes inhibited when streaming is still present indicates that the actin cytoskeleton plays a more direct role in tip growth. This also indicates that actin polymerization and actin filament turnover are necessary for tip growth. Hence, a key unanswered question is: how do actin dynamics drive tip growth? To answer this question I am using two complementary approaches: first, I am dissecting the architecture and molecular regulation of the actin cytoskeleton; second, I am testing whether the key cellular processes of exocytosis is dependent on actin dynamics.
The moss Physcomitrella patens is an ideal model organism to investigate these questions. Moss protonemal cells grow exclusively via tip growth and are easily propagated in culture. Due to their filamentous nature, protonemal cells are highly accessible for high resolution microscopy; this permits visualization of subcellular events in living cells. In addition, Physcomitrella is amenable to genetic manipulation, homologous recombination, and RNA interference, allowing fast analysis of gene function. Finally, the availability of a complete genomic sequence greatly simplifies gene targeting as well as the design of RNAi constructs.
References
1. Hepler, P.K., Vidali, L., and Cheung, A.Y. (2001) Polarized cell growth in higher plants. Annu Rev Cell Dev Biol. 17:159-87.
2. Harold, F.M. (2002) Force and compliance: rethinking morphogenesis in walled cells. Fungal Genet Biol. 37:271-82.
3. Vidali, L. and Hepler, P.K. (2001) Actin and pollen tube growth. Protoplasma. 215:64-76.
4. Vidali, L., McKenna, S.T., and Hepler, P.K. (2001) Actin polymerization is essential for pollen tube growth. Mol Biol Cell. 12:2534-45.