The Modification of Cell Wall Properties Is Involved in the Growth Inhibition of Rice Coleoptiles Induced by Lead 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.

Modification of Xyloglucan Metabolism during a Decrease in Cell Wall Extensibility in 1-Aminocyclopropane-1-Carboxylic Acid-Treated Azuki Bean Epicotyls.

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.

Phenotypic screening of Arabidopsis T-DNA insertion lines for cell wall mechanical properties revealed ANTHOCYANINLESS2, a cell wall-related gene.

We performed a phenotypic screening of confirmed homozygous T-DNA insertion lines in Arabidopsis for cell wall extensibility, in an attempt to identify genes involved in the regulation of cell wall mechanical properties. Seedlings of each line were cultivated and the cell wall extensibility of their hypocotyls was measured with a tensile tester. Hypocotyls of lines with known cell wall-related genes showed higher or lower extensibility than those of the wild-type at high frequency, indicating that the protocol used was effective. In the first round of screening of randomly selected T-DNA insertion lines, we identified ANTHOCYANINLESS2 (ANL2), a gene involved in the regulation of cell wall mechanical properties. In the anl2 mutant, the cell wall extensibility of hypocotyls was significantly lower than that of the wild-type. Levels of cell wall polysaccharides per hypocotyl, particularly cellulose, increased in anl2. Microarray analysis showed that in anl2, expression levels of the major peroxidase genes also increased. Moreover, the activity of ionically wall-bound peroxidases clearly increased in anl2. The activation of peroxidases as well as the accumulation of cell wall polysaccharides may be involved in decreased cell wall extensibility. The approach employed in the present study could contribute to our understanding of the mechanisms underlying the regulation of cell wall mechanical properties.

Leaf growth of hybrid poplar following exposure to elevated CO2.

Leaf extension was stimulated following exposure of three interamerican hybrid poplar clones (Populus trichocarpa P. deltoides); 'Unal', 'Boelare', and 'Beaupre' and a euramerican clone 'Primo' (Populus nigra×P. deltoides) to elevated CO2 , in controlled environment chambers. For all three interamerican clones the evidence suggests that this was the result of increased leaf cell expansion associated with enhanced cell wall extensibility (WEx), measured as tensiomerric increases in cell wall plasticity. For the interameriean clone 'Boelare', there was also a significant increase in cell wall elasticity following exposure to elevated CO2 (P⩽ 0.001). The effect of elevated CO2 in stimulating cell wall extensibility was confirmed in a detailed spatial analysis of extensibility made across the lamina of expanding leaves of the clone 'Boelare'. For two of the interamerican hybrids, 'Unal' and 'Beaupre', both leaf cell water potential Ψ and turgor pressure (P) were lower in elevated than in ambient CO2 . By contrast, no significant effects on the cell wall properties or leaf water relations for the euramerican hybrid 'Primo' were observed following exposure to elevated CO2 . suggesting that the mechanism for increased leaf extension in elevated CO2 , differed, depending on clone. The cumulative total length of leaves of 'Boelare' grown in elevated CO2 , was significantly increased (P≤ 0.05) and since leaf number was not significantly increased in any inter-american clone it is hypothesized that final leaf size was stimulated in elevated CO2 for these clones. By contrast, there was no significant effect of CO2 on cumulative total leaf length for the euramerican clone 'Primo', but leaf number was significantly increased by elevated CO2 . The measurements suggest that total tree leaf area was stimulated for a range of poplar hybrids exposed to elevated CO2 . Given the short rotation of a coppiced crop, it is likely that increased leaf areas will result in enhanced stemwood production when hybrid poplars are grown in the CO, concentrations predicted for the next century.

The epidermal surface of the maize root tip: III. Isolation of the surface and characterization of some of its structural and mechanical properties.

The surface of the young epidermal cells of maize roots is composed o) three layers; the inner layer (LI), which is the outer epidermal Wall, overlaid by a pellicle consisting of a thick, coherent inner layer (L2) and a very thin, loosely organized outer layer (L3). The entire surface can be removed intact to produce either narrow, circumferential strips or apical halftones, by gently prying it loose in the circumferential direction by hand with insect pins. Usually only short remnants of anticlinical walls of the epidermal cells remain attached. These isolated surface pieces always curl outward at the free circumferential edges in the longitudinal direction of the original intact root. When the strips are deliberated stretched alone their long axis (i.e., the original circumferential direction) they elongate irreversibly by us much us two thirds of their length, before showing some elastic deformation and breaking. Some plastic deformation may occur in the original longitudinal direction of the root during removal of the strips. The plastic deformation opens the helicoidal array of microfibrils in the L1 layer. Deformation also produces structural changes over the original radial walls and those transverse anticlinal walls that form boundaries of cell packets derived from single cells. In these positions the L1 layers over adjacent cells separate in the direction of the applied stress. This occurs by the separation of the L I layers of adjacent cells and the stretching of the inward projection of the amorphous L2 layer of the pellicle which lies; over these original anticlinal walls. There is much less or no separation of the L1 layers over anticlinal walls Of adjoining daughter cells in the epidermis. The pellicle always remains firmly attached to the outer epidermal wall during removal of the surface strips. On removal, these strips shorten in their original longitudinal direction in situ, indicating a release of tension imposed by underlying cells. Their outward curling suggests stress between the wall and pellicle of the outer epidermal surface in the intact root. These findings focus attention on structural differences between sites where anticlinal walls of different origin join the outer surface, and on possible differences in surface extensibility at each sites.