Life behind cell walls: paradigm lost, paradigm regained

Abstract

This review of the living cell wall and its protein components is in two parts. The first is anecdotal. A personal account spanning over 40 years research may perhaps be an antidote to one stereotypical view of scientists as detached and humorless. The second part deals with the meaning of function, particularly as it applies to hydroxyproline-rich glycoproteins. Function is a difficult word to define objectively. However, with help from such luminaries as Humpty Dumpty: "A word means what I want it to mean, neither more nor less," and Wittgenstein: "Giving examples of usage ... is the only way to talk about meaning," it is possible to construct a ziggurat representing increasingly complex levels of organization from molecular structure to ecology. Forty years ago I suggested that hydroxyproline-rich structural proteins played a key role in cell wall functioning. But because the bulk of the wall is carbohydrate, there has been an understandable resistance to paradigm change. Expansins, paradoxically, contribute greatly to this resistance because their modus operandi as cell-wall-loosening proteins is based on the idea that they break hydrogen bonds between polysaccharide chains allowing slippage. However, this view is not consistent with the recent discovery [Grobe et al. (1999) Eur. J. Biochem 263: 33-40] that beta-expansins may be proteases, as it implies that the extensin network is not a straightjacket but a substrate for expansin in muro. Such a direct role for extensins in both negative and positive regulation of cell expansion and elongation may constitute a major morphogenetic mechanism operating at all levels of plant growth and development.

Response of the Leaf Cell Wall to Desiccation in the Resurrection Plant Myrothamnus flabellifolius

“…Recently, potato (Solanum tuberosum) tuber tissue genetically engineered to be deficient in arabinan and galactan side chains in cell wall pectin was shown to be significantly more brittle to imposed stress, highlighting the critical role that these side chains play in controlling the biophysical properties of the cell wall (Ulvskov et al, 2005). In addition, arabinogalactan polymers in the form of AGPs are believed to function in cell lubrication (Yates et al, 1996) and have been proposed to act as cell wall plasticizers (Lamport, 2001;Lee et al, 2005). A similar role has been proposed for the stress response protein Hsp 12 in yeast (Saccharomyces cerevisiae; Motshwene et al, 2004;Karreman et al, 2005).…”

Role of the Extensin Superfamily in Primary Cell Wall Architecture

“…Acidic Hyp-arabinogalactans (Lamport et al, 2006) cover the plasma membrane and could potentially chelate calcium , thus allowing AGPs to act as a calcium reservoir. We know that phospholipase C releases membrane-bound AGPs as soluble periplasmic AGPs that are then incorporated into the growing wall as putative pectic plasticizers (Lamport, 2001;Lamport et al, 2006). We also know that the b-D-glucosyl Yariv reagent inhibits expansion growth (Jauh and Lord, 1996) and that this reagent strongly associates with AGPs, potentially negating the role of AGPs as a plasticizer in muro and thus inhibiting expansion growth.…”

Arabinogalactan Proteins Are Required for Apical Cell Extension in the Moss Physcomitrella patens

“…The continuous tip-localized release of AGPs into the cell wall and then medium as a protonemal filament extends may be a requirement for growth. One possibility, supported by the suggestion that AGPs may act as plasticizers during cell wall extension (Lamport, 2001), is that movement of AGPs through the cell wall may modulate cell wall properties at the tip where considerable assembly and remodeling is taking place. Other possibilities include their involvement in the localized unloading of Golgi-derived vesicles at the tip or deposition of cell wall polymers.…”