Session 8: Cell Biology
Chair: Wolf B. Frommer, University of Tübingen,Germany
email:frommer@uni-tuebingen.de
The session on Plant Cell Biology highlighted recent developments in this increasingly important and exciting research area which establishes links between fields as diverse as development and physiology. The first talk by Richard Meagher from Athens, GA presented an excellent overview on the different isoforms of actin present in Arabidopsis. Using a variety of elegant approaches, including antisera specific for different isoforms, he demonstrated that the family of highly related actin genes contains divergent members which differ in patterns of expression and, presumably, functions. For example, two distinct subclasses (ACT2 & ACT8; ACT7) make up the vegetative class, whereas three distinct subclasses (ACT11; ACT1 & 3; ACT4 & 12) make up the reproductive class. Even within a class, the genes are differentially expressed. Data from the genome project together with knock-out mutants will soon provide us with an almost complete picture of the role and function of different actin genes.The second talk by Hui-Ju Wu from Michel Delseny´s lab in Perpignan described the cloning of an Arabidopsis gene complementing the yeast sac1 mutant which is affected in the organisation of the actin cytoskeleton. Although AtG5 restores growth of yeast sac1 mutants at the restrictive temperature, its function in plants remains to be investigated.
Gerd Jürgens from Tübingen discussed the role of the KNOLLE protein in cytokinesis. The KNOLLE gene was originally identified by mutations affecting embryo development and was found to encode a protein related to vesicle-docking proteins of the syntaxin family. Evidence from detailed biochemical and cell biological analyses suggest that KNOLLE is specifically involved in the fusion of membrane vesicles that form the cell plate during cell division. Since knolle null mutations impair but do not completely block cell plate formation, future studies are directed toward the identification of other components involved in cytokinesis.
In addition to its role in cell division, the cytoskeleton is involved in other functions, such as the transport of mRNAs and proteins within and between cells, e.g. through plasmodesmata. Addressing this topic, Patricia Zambryski from Berkeley showed that virus can move between cells through plasmodesmata and that viral movement proteins can enlarge the pores. She also demonstrated that the cytoskeleton is necessary for macromolecular trafficking of protein and virus. These findings suggest that a plant system for transporting proteins between cells is misused by the virus. Transport of endogenous proteins was discussed in the last two talks which addressed the accumulation of proteins in enucleate sieve elements of the phloem. Jennifer Gottwald from Mike Sussman´s group in Madison presented her approach of studying the function of P-proteins in the phloem. Database searches had revealed a P-protein ortholog in Arabidopsis. Using a PCR-based reverse genetics approach, Jennifer identified a T-DNA insertion in the single known PP1a gene of Arabidopsis. Further analysis of this mutant is in progress.
Finally, I reported on a sucrose transporter that is essential for long-distance sugar transport and is located together with its mRNA in enucleate sieve elements. These data indicate that plant proteins can traffic between cells by receptor-mediated processes. Although these results were not obtained in Arabidopsis our current research is addressing the problem of protein trafficking between cells in the model species Arabidopsis. It will be interesting to learn whether the cytoskeleton also plays a role in the transport of endogenous mRNAs and proteins within or between cells.
In summary, the increasing knowledge from genome projects together with gene expression data, mutant analyses and cell-biological tools all combine to open up new research perspectives and underline the outstanding scientific potential of Arabidopsis.