Trends in Plant Science
Cell shape development in plants
Section snippets
Shape changes result from changing the growth focus of a cell
Differential growth is best studied in easily accessible cell types such as epidermal pavement cells and leaf trichomes 7, 8, stomatal guard cells [9], root-hair cells [10], elongating pollen tubes [11] and differentiating tracheids [12], which allow experimental manipulation and can be studied in pertinent morphological mutants. Although the final shapes of these model cells differ greatly, they do share a common growth mechanism [7]. Based on the area over which growth is spread during the
Cytoskeletal elements during differential growth
In higher plants, amongst all other cellular components, the two major cytoskeleton elements actin microfilaments and microtubules 11, 13 appear to play a decisive role in the shape-determining process. Treatment of plant cells with cytoskeleton-interacting drugs has been particularly helpful in developing this view and has allowed particular changes in cell morphology to be linked to altered activity of specific cytoskeletal proteins 13, 14, 15, 16, 17, 18. The following general observations
Cortical weakening can trigger the growth process
The previous roles attributed to actin and microtubules during cell morphogenesis need to be reassessed so that we can assimilate the observations made above into a coherent operational framework that addresses how growth can be targeted to a region of the cell to make it grow differently from other regions. The observation that all plant cells initially possess a regular shape suggests an even distribution of intracellular resources and the growth machinery at this developmental stage.
Reinforcement of weak cortical sites and fixation of growth directionality requires microtubules
A continuous stream of signals feeding into the actin-mesh-loosening mechanism can potentially extend cortical weakening to the entire cell surface. A general release of growth materials by vesicles in the expanding weak region would ultimately produce an isodiametric cell shape (Figure 3). Because this does not usually happen, the further weakening of the cortex must obviously be curtailed at some stage for growth to become localized. Activation of actin-bundling proteins might counteract the
Conclusions and perspectives
Molecular-genetic and cell-biological evidence of cell morphogenesis strongly suggests that localized growth occurs as a cellular response to combat regional weakening of the cortex. The weakening is created by a localized increase in cortical actin dynamics. Polar growth directionality becomes fixed once endoplasmic microtubules approach the weak actin mesh and both reinforce it and establish the concurrence of both cytoskeletal elements at the particular cortical location. All
Acknowledgements
I thank Alice Cheung, Nancy Dengler, Patrick Hussey, Thomas Berleth, Peter Hepler and Marie-Theres Hauser for their critical comments and suggestions on the ideas presented here, and Takashi Hashimoto for sharing prepublication details.
References (81)
GFP technology for live cell imaging
Curr. Opin. Plant Biol.
(2003)- et al.
The plant cytoskeleton: vacuoles and cell walls make the difference
Cell
(2002) Local, efflux-dependent auxin gradients as a common module for plant organ formation
Cell
(2003)Growing up green: cellular basis of plant development
Mech. Dev.
(2003)- et al.
Microtubules and microfilaments in cell morphogenesis in higher plants
Curr. Biol.
(2002) - et al.
Stomatal development: cross talk puts mouths in place
Trends Plant Sci.
(2003) Latrunculin B-induced plant dwarfism: plant cell elongation is F-actin-dependent
Dev. Biol.
(2001)lilliputian mutant of maize lacks cell elongation and shows defects in organization of actin cytoskeleton
Dev. Biol.
(2001)Functional analysis of the tubulin-folding cofactor C in Arabidopsis thaliana
Curr. Biol.
(2002)The plant microtubule-associated protein AtMAP65-3/PLE is essential for cytokinetic phragmoplast function
Curr. Biol.
(2004)