Perception and response to mechanical stimuli are likely essential at the cellular and organismal levels. Elaborate and impressive touch responses of plants capture the imagination as such behaviors are unexpected in otherwise often quiescent creatures. Touch responses can turn plants into aggressors against animals, trapping and devouring them, and enable flowers to be active in ensuring crosspollination and shoots to climb to sunlit heights. Morphogenesis is also influenced by mechanical perturbations, including both dynamic environmental stimuli, such as wind, and constant forces, such as gravity. Even individual cells must sense turgor and wall integrity, and subcellular organelles can translocate in response to mechanical perturbations. Signaling molecules and hormones, including intracellular calcium, reactive oxygen species, octadecanoids and ethylene, have been implicated in touch responses. Remarkably, touch-induced gene expression is widespread; more than 2.5% of Arabidopsis genes are rapidly up-regulated in touch-stimulated plants. Many of these genes encode calcium-binding, cell wall modifying, defense, transcription factor and kinase proteins. With these genes as tools, molecular genetic methods may enable elucidation of mechanisms of touch perception, signal transduction and response regulation.

/pdf/in-touch-plant-responses-to-mechanical-stimuli-5csbhokg4d.pdf

In touch: plant responses to mechanical stimuli

▪ Abstract The nature of cell wall proteins is as varied as the many functions of plant cell walls. With the exception of glycine-rich proteins, all are glycosylated and contain hydroxyproline (Hyp). Again excepting glycine-rich proteins, they also contain highly repetitive sequences that can be shared between them. The majority of cell wall proteins are cross-linked into the wall and probably have structural functions, although they may also participate in morphogenesis. On the other hand, arabinogalactan proteins are readily soluble and possibly play a major role in cell-cell interactions during development. The interactions of these proteins between themselves and with other wall components is still unknown, as is how wall components are assembled. The possible functions of cell wall proteins are suggested based on repetitive sequence, localization in the plant body, and the general morphogenetic pattern in plants.

Plant cell wall proteins

In plants, the cell wall is a major determinant of cell morphogenesis. Cell enlargement depends on the tightly regulated expansion of the wall, which surrounds each cell. However, the qualitative and quantitative mechanisms controlling cell wall enlargement are still poorly understood. Here, we report the molecular and functional characterization of LRX1, a new Arabidopsis gene that encodes a chimeric leucine-rich repeat/extensin protein. LRX1 is expressed in root hair cells and the protein is specifically localized in the wall of the hair proper, where it becomes insolubilized during development. lrx1-null mutants, isolated by a reverse-genetic approach, develop root hairs that frequently abort, swell, or branch. Complementation and overexpression experiments using modified LRX1 proteins indicate that the interaction with the cell wall is important for LRX1 function. These results suggest that LRX1 is an extracellular component of a mechanism regulating root hair morphogenesis and elongation by controlling either polarized growth or cell wall formation and assembly.

/pdf/the-chimeric-leucine-rich-repeat-extensin-cell-wall-protein-2toxgj7f3o.pdf

The chimeric leucine-rich repeat/extensin cell wall protein LRX1 is required for root hair morphogenesis in Arabidopsis thaliana

Nearly two centuries of progress have established the major components of the plant cell wall, a composite that includes interpenetrating networks of cellulose (Payen, 1838; Schulze, 1891), microfibrils (Frey-Wyssling et al., 1948; Preston et al., 1948), pectin (Braconnot, 1825) and lignin (Payen, 1838). However, only over the last five decades has a relatively minor hydroxyproline-rich structural glycoprotein component emerged with essential roles in building and maintaining the growing primary cell wall. Here we highlight unique advances of each decade from the initial discovery of hydroxyproline (Hyp) in cell walls to the current definition of extensins as self-assembling amphiphiles that generate scaffolding networks, and where acid-base interaction - extensin pectate - may template assembly of the pectic matrix. Subsequent polymerization toughens up the wall as networks resisting both microbial and mechanical stress. At each stage we explore hypotheses arising from synthesis of emerging data with focus on structure. This review celebrates the 50th birthday of extensin.

/pdf/role-of-the-extensin-superfamily-in-primary-cell-wall-13vh6sv8bk.pdf

Role of the Extensin Superfamily in Primary Cell Wall Architecture

We sought to gain insight into functions potentially altered by mechanostimulation and investigate the relationship between touch and darkness responses. Microarrays and quantitative RT-PCR were conducted to identify genes and analyze behaviors of calmodulin-like (CML) and xyloglucan endotransglucosylase/hydrolase (XTH) genes. Strikingly, 589 genes had touch-inducible expression; 171 had reduced expression. Darkness increased expression of 461 genes and decreased expression of 72 genes. Over half of the touch-inducible genes resembled the TCH genes in that they were also up-regulated by darkness; 67% of those darkness-inducible were also touch inducible. Expression of 12 CMLs and four XTHs was elevated by touch; three XTHs had reduced expression. In darkness-treated plants, 10 CMLs and nine XTHs had increased expression and one XTH was repressed. Over 2.5% of total genes were touch-inducible. Many were also darkness up-regulated, consistent with the hypothesis that these stimuli have partially overlapping signal transduction pathways. Regulated gene identities suggest that calcium and kinase signaling, wall modification, disease resistance and downstream transcriptional responses may be altered in response to mechanostimulation or darkness.

Genome-wide identification of touch- and darkness-regulated Arabidopsis genes: a focus on calmodulin-like and XTH genes.

The expression of extA, an extensin gene from Brassica napus L. (oilseed rape) was examined in transgenic Nicotiana tabacum L. (tobacco) and untransformed Brassica juncea L. and B. napus tissues. Northern analysis showed that this gene maintained its normal pattern of expression when transferred to tobacco. In transgenic tobacco plants containing an extA promoter/β-glucuronidase coding sequence fusion, expression of extA was detected in the external and internal phloem of the main stem. High expression levels were seen in cortical parenchyma cells at the point where the axillary flowering branch joined the main stem. Expression was greatest in regions where the maximum tensile stress would seem to be exerted on the main stem by the weight of the axillary branch. It was confirmed that this expression pattern was due to tensile stress by using weights to induce expression of the fusion gene in axillary flowering stalks. In B. juncea pods, in-situ hybridisation studies showed that the extensin gene was strongly expressed in cells of the carpel walls within which considerable tensile stresses develop.

The Brassica napus extA extensin gene is expressed in regions of the plant subject to tensile stresses

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The Brassica napus extA extensin gene is expressed in regions of the plant subject to tensile stresses