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The exogenous application of ethylene or 1-aminocyclopropane-1-carboxylic acid (ACC), the biosynthetic precursor for ethylene, to plants decreases the capacity of the cell wall to extend, thereby inhibiting stem elongation. In this study, the mechanism by which the extensibility of cell walls decreases in ACC-treated azuki bean epicotyls was studied. ACC decreased the total extensibility of cell walls, and such a decrease was due to the decrease in irreversible extensibility. ACC increased the molecular mass of xyloglucans but decreased the activity of xyloglucan-degrading enzymes. The expression of VaXTHS4, which only exhibits hydrolase activity toward xyloglucans, was downregulated by ACC treatment, whereas that of VaXTH1 or VaXTH2, which exhibits only transglucosylase activity toward xyloglucans, was not affected by ACC treatment. The suppression of xyloglucan-degrading activity by downregulating VaXTHS4 expression may be responsible for the increase in the molecular mass of xyloglucan. Our results suggest that the modification of xyloglucan metabolism is necessary to decrease cell wall extensibility, thereby inhibiting the elongation growth of epicotyls in ACC-treated azuki bean seedlings.

How is the extensibility of growing plant cell walls regulated? In the past, most studies have focused on the role of the cellulose/xyloglucan network and the enigmatic wall-loosening agents expansins. Here we review first how in the closest relatives of the land plants, the Charophycean algae, cell wall synthesis is coupled to cell wall extensibility by a chemical Ca(2+)-exchange mechanism between Ca(2+)-pectate complexes. We next discuss evidence for the existence in terrestrial plants of a similar \"primitive\" Ca(2+)-pectate-based growth control mechanism in parallel to the more recent, land plant-specific, expansin-dependent process.

Background and aims Aluminum (Al) stress is a global problem that inhibits root growth and crop production in acidic soils. The inhibitive effect is greatly attributed to the reduction of cell wall elasticity. The present study aimed to investigate the function of a polygalacturonase gene MsPG4 in regulating cell wall extensibility and alfalfa resistance to Al stress. Methods The transgenic alfalfa plants of overexpression ( MsPG4 -OE) and knockdown ( MsPG4 -RNAi) of MsPG4 were treated with or without 100 µM AlCl 3 . The methods of field emission scanning electron microscope, transmission electron microscope and atomic force microscopy were used in physiological and histological analysis. Results The relative root elongation was higher in MsPG4 -OE lines and lower in MsPG4 -RNAi lines than wild type (WT) plants under Al stress. Al contents in root tips and cell wall decreased in MsPG4 -OE lines but increased in MsPG4 -RNAi lines, which positively related to their pectin contents. The contents of water soluble pectin (WSP) and chelator soluble pectin (CSP) decreased in MsPG4 -OE lines and increased in MsPG4 -RNAi lines in the absence or presence of Al compared to WT plants. Consequently, the dimensions of the two pectin’s molecules reduced, the porosity and extensibility of cell wall increased in MsPG4 -OE lines comparted to WT plants under Al stress. Conclusions MsPG4 effectively increases the cell wall extensibility and Al resistance of alfalfa via hydrolysis of pectins in the cell wall.

Carbon nanoparticles attract attention of plant researchers as a possible means of improving crop yield and its quality. There are grounds to believe that the beneficial influence of polyhydroxy fullerene (PHF) on plants is due to its antioxidant activity, but the mechanism of its action on their growth and development remains unclear. Our study shows that PHF added to the nutrient medium accelerates barley roots elongation owing to the increase of their longitudinal extensibility in the growth zone. The impact of PHF on root growth was much more pronounced under the action of stressors inducing the accumulation of reactive oxygen species, such as UV-B radiation, salt stress and the excess of salicylic acid. Dichlorofluorescein assay showed that PHF prevented oxidative stress development and subapical root swelling after UV-B irradiation of roots. The conclusion is drawn that the important reason of root growth acceleration in the presence of PHF is its ability to serve as a scavenger of free radicals. That’s why it may be especially useful for the improvement of plant growth under environmental stress.

Lead (Pb) is a widespread heavy metal pollutant that interferes with plant growth. In this study, we investigated the effects of Pb on the mechanical and chemical properties of cell walls and on the growth of coleoptiles of rice (Oryza sativa L.) seedlings grown in the air (on moistened filter paper) and underwater (submerged condition). Coleoptile growth of air-grown seedlings was reduced by 40% by the 3 mM Pb treatment, while that of water-grown ones was reduced by 50% by the 0.5 mM Pb. Although the effective concentration of Pb for growth inhibition of air-grown coleoptiles was much higher than that of water-grown ones, Pb treatment significantly decreased the mechanical extensibility of the cell wall in air- and water-grown coleoptiles, when it inhibited their growth. Among the chemical components of coleoptile cell walls, the amounts of cell wall polysaccharides per unit fresh weight and unit length of coleoptile, which represent the thickness of the cell wall, were significantly increased in response to the Pb treatment (3 mM and 0.5 mM Pb for air- and water-grown seedlings, respectively), while the levels of cell wall-bound diferulic acids (DFAs) and ferulic acids (FAs) slightly decreased. These results indicate that Pb treatment increased the thickness of the cell wall but not the phenolic acid-mediated cross-linking structures within the cell wall in air- and water-grown coleoptiles. The Pb-induced cell wall thickening probably causes the mechanical stiffening of the cell wall and thus decreases cell wall extensibility. Such modifications of cell wall properties may be associated with the inhibition of coleoptile growth. The results of this study provide a new finding that Pb-induced cell wall remodeling contributes to the regulation of plant growth under Pb stress conditions via the modification of the mechanical property of the cell wall.

Growth of turgid cells, defined as an irreversible increase in cell volume and surface area, can be regarded as a physical process governed by the mechanical properties of the cell wall and the osmotic properties of the protoplast. Irreversible cell expansion is produced by creating a driving force for water uptake by decreasing the turgor through stress relaxation in the cell wall. This mechano-hydraulic process thus depends on and can be controlled by the mechanical properties of the wall, which in turn are subject to modification by wall loosening and wall stiffening reactions. The biochemical mechanisms of these changes in mechanical wall properties and their regulation by internal signals (e.g., hormones) or external signals (e.g., light, drought stress) are at present incompletely understood and subject to intensive research. These signals act on walls that have the properties of composite materials in which the molecular structure and spatial organization of polymers rather than the distribution of mechanical stresses dictate the allometry of cell and organ growth and thus cell and organ shape. The significance of cell wall architecture for allometric growth can be demonstrated by disturbing the oriented deposition of wall polymers with microtubule-interfering drugs such as colchicine. Elongating organs (e.g., cylindrical stems or coleoptiles) composed of different tissues with different mechanical properties exhibit longitudinal tissue tensions resulting in the transfer of wall stress from inner to peripheral cell layers that adopt control over organ growth. For physically analyzing the growth process leading to seed germination, the same mechanical and hydraulic parameters as in normal growth are principally appropriate. However, for covering the influences of the tissues that restrain embryo expansion (seed coat, endosperm), an additional force and a water permeability term must be considered.

Progress in understanding the network of mechanisms involved in maize primary root growth maintenance under water deficits is reviewed. These include the adjustment of growth zone dimensions, turgor maintenance by osmotic adjustment, and enhanced cell wall loosening. The role of the hormone abscisic acid (ABA) in maintaining root growth under water deficits is also addressed. The research has taken advantage of kinematic analysis, i.e. characterization of spatial and temporal patterns of cell expansion within the root growth zone. This approach revealed different growth responses to water deficits and ABA deficiency in distinct regions of the root tip. In the apical 3 mm region, elongation is maintained at well-watered rates under severe water deficit, although only in ABA-sufficient roots, whereas the region from 3–7 mm from the apex exhibits maximum elongation in well-watered roots, but progressive inhibition of elongation in roots under water deficit. This knowledge has greatly facilitated discovery of the mechanisms involved in regulating the responses. The spatial resolution with which this system has been characterized and the physiological knowledge gained to date provide a unique and powerful underpinning for functional genomics studies. Characterization of water deficit-induced changes in transcript populations and cell wall protein profiles within the growth zone of the maize primary root is in progress. Initial results from EST and unigene analyses in the tips of well-watered and water-stressed roots highlight the strength of the kinematic approach to transcript profiling.

Turgor pressure provides the force needed to stress and deform the cell walls of plants, algae, and fungi during expansive growth. However, turgor pressure plays another subtle but equally important role in expansive growth of walled cells: it connects the two biophysical processes of water uptake and wall deformation to ensure that the volumetric rates of water uptake and enlargement of the cell wall chamber are equal. In this study, the role of turgor pressure as a ‘connector’ is investigated analytically by employing validated and established biophysical equations. The objective is to determine the effect of ‘wall loosening’ on the magnitude of turgor pressure. It is known that an increase or decrease in turgor pressure and/or wall loosening rate increases or decreases the expansive growth rate, respectively. Interestingly, it is shown that an increase in the wall loosening rate decreases the turgor pressure slightly, thus reducing the effect of wall loosening on increasing the expansive growth rate. Other analyses reveal that reducing the rate of water uptake results in a larger decrease in turgor pressure with the same increase in wall loosening rate, which further reduces the effect of wall loosening on increasing the expansive growth rate.

Fruit cracking caused by rainfall prior to harvest, a major problem in sweet cherry production, is being exacerbated by climate change. Currently, pre-harvest spraying with calcium salt solutions is the prevalent technique to reduce fruit cracking in cherry orchards not covered by plastic roofs. This study evaluated the effectiveness of canopy-applied silicon in the reduction of sweet cherry cracking under different field conditions. Four field trials were conducted on mature trees of the cultivars Van, New Star, and Emperor Francis. Treatments included water (control), calcium chloride, and sodium silicate. Multiple sprays (three) were applied weekly from fruit onset of color to approximately 1 week before harvest. The results showed that under conditions conducive to cracking, sodium silicate reduced the percentage of cracked fruits to a similar or larger extent than calcium chloride. This study highlights how canopy-applied silicon sources may effectively contribute to reducing cherry cracking, acting as an alternative technique to other preventive methods.

The role of cellulose microfibril orientation in determining cell wall mechanical anisotropy and in the control of the wall plastic versus elastic properties was studied in the adaxial epidermis of onion bulb scales using the constant-load (creep) test. The mean or net cellulose orientation in the outer periclinal wall of the epidermis was parallel to the long axis of the cells. In vitro cell wall extensibility was 30-90% higher in the direction perpendicular to the net microfibril orientation than parallel to it. This was the case for the size of the initial deformation occurring just after the load application and for the rate of time-dependent creep. Loading/unloading experiments confirmed the presence of a real irreversible component in cell wall extension. The plastic component of the time-dependent deformation was higher perpendicular to the net cellulose orientation than parallel to it. An acid buffer (pH 4.5) increased the creep rate by 25-30% but this response was not related to cellulose orientation. The present data provide direct evidence that the net orientation of cellulose microfibrils confers mechanical anisotropy to the walls of seed plants, a characteristic that may be relevant to understanding anisotropic cell growth.