What makes plants different? Principles of extracellular matrix function in 'soft' plant tissues.

An overview of the biomechanic and morphogenetic function of the plant extracellular matrix (ECM) in its primary state is given. ECMs can play a pivotal role in cellular osmo- and volume-regulation, if they enclose the cell hermetically and constrain hydrostatic pressure evoked by osmotic gradients between the cell and its environment. From an engineering viewpoint, such cell walls turn cells into hydraulic machines, which establishes a crucial functional differences between cell walls and other cellular surface structures. Examples of such hydraulic machineries are discussed. The function of cell walls in the control of pressure, volume, and shape establishes constructional evolutionary constraints, which can explain aspects commonly considered typical of plants (sessility, autotrophy). In plants, 'cell division' by insertion of a new cell wall is a process of internal cytoplasmic differentiation. As such it differs fundamentally from cell separation during cytokinesis in animals, by leaving the coherence of the dividing protoplast basically intact. The resulting symplastic coherence appears more important for plant morphogenesis than histological structure; similar morphologies are realized on the basis of distinct tissue architectures in different plant taxa. The shape of a plant cell is determined by the shape its cell wall attains under multiaxial tensile stress. Consequently, the development of form in plants is achieved by a differential plastic deformation of the complex ECM in response to this multiaxial force (hydrostatic pressure). Current concepts of the regulation of these deformation processes are briefly evaluated.

Carbohydrates in individual cells of epidermis, mesophyll, and bundle sheath in barley leaves with changed export or photosynthetic rate

Carbohydrate metabolism of barley (Hordeum vulgare) leaves induced to accumulate sucrose (Suc) and fructans was investigated at the single-cell level using single-cell sampling and analysis. Cooling of the root and shoot apical meristem of barley plants led to the accumulation of Suc and fructan in leaf tissue. Suc and fructan accumulated in both mesophyll and parenchymatous bundle-sheath (PBS) cells because of the reduced export of sugars from leaves under cooling and to increased photosynthesis under high photon fluence rates. The general trends of Suc and fructan accumulation were similar for mesophyll and PBS cells. The fructan-to-Suc ratio was higher for PBS cells than for mesophyll cells, suggesting that the threshold Suc concentration needed for the initiation of fructan synthesis was lower for PBS cells. Epidermal cells contained very low concentrations of sugar throughout the cooling experiment. The difference in Suc concentration between control and treated plants was much less if compared at the single-cell level rather than the whole-tissue level, suggesting that the vascular tissue contains a significant proportion of total leaf Suc. We discuss the importance of analyzing complex tissues at the resolution of individual cells to assign molecular mechanisms to phenomena observed at the whole-plant level.