Abstract
The mechanisms through which plant cells control growth and shape are the result of the coordinated action of many events, notably cell wall stress relaxation and turgor-driven expansion. The scalar nature of turgor pressure would drive plant cells to assume spherical shapes; however, this is not the case, as plant cells show an amazing variety of morphologies. Plant cell walls are dynamic structures that can display alterations in matrix polysaccharide composition and concentration, which ultimately affect the wall deformation rate. The wide varieties of plant cell shapes, spanning from elongated cylinders (as pollen tubes) and jigsaw puzzle-like epidermal cells, to very long fibres and branched stellate leaf trichomes, can be understood if the underlying mechanisms regulating wall biosynthesis and cytoskeletal dynamics are addressed. This review aims at gathering the available knowledge on the fundamental mechanisms regulating expansion, growth and shape in plant cells by putting a special emphasis on the cell wall-cytoskeleton system continuum. In particular, we discuss from a molecular point of view the growth mechanisms characterizing cell types with strikingly different geometries and describe their relationship with primary walls. The purpose, here, is to provide the reader with a comprehensive overview of the multitude of events through which plant cells manage to expand and control their final shapes.
Highlights
The structural organization and functional specialization of plant tissues is achieved through metabolic specialization (an emblematic example is represented by plants with CAM) and morphogenetic programs, which depend on the capacity of cells to expand and change their shape
The concepts of turgor pressure and cell wall mechanical properties were for the first time described in the seminal work by Lockhart [11], who elaborated an equation explaining the mathematics of plant cell expansion
Symplastic growth is typical of shoot/root epidermal cells and marks the initial phase of fibre development [20,21]; intrusive growth is found in bast fibres which can reach considerable lengths and is typical of polarized structures, such as pollen tubes; protruding growth is a hallmark of trichome formation [20]
Summary
The structural organization and functional specialization of plant tissues is achieved through metabolic specialization (an emblematic example is represented by plants with CAM (crassulacean acid metabolism)) and morphogenetic programs, which depend on the capacity of cells to expand and change their shape. The concepts of turgor pressure and cell wall mechanical properties were for the first time described in the seminal work by Lockhart [11], who elaborated an equation explaining the mathematics of plant cell expansion. The turgor pressure and the physical and mechanical properties of the cell wall are factors that contribute to shape complex cellular geometries. In this respect, it is possible to understand the role of the cell wall during growth if one assumes that a plant cell regulates its volume by working as a hydrostat [12,13], where the cell wall acts as a sensor of the system. When the cell has expanded, a new equilibrium is reached, since the solutes are more diluted because of water inflow and the cell will strengthen the wall network to counteract the newly formed turgor pressure and to be able to initiate a new wave of expansion
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