Elongation growth of dark-grown azuki bean (Vigna angularis Ohwi et Ohashi cv. Takara) epicotyls was suppressed by hypergravity at 30 x g and above. Acceleration at 300 x g significantly decreased the mechanical extensibility of cell walls. The amounts of cell wall polysaccharides (pectin, hemicellulose-II and cellulose) per unit length of epicotyls increased under the hypergravity condition. Hypergravity also increased the amounts and the weight-average molecular mass of xyloglucans in the hemicellulose-II fraction, while decreasing the activity of xyloglucan-degrading enzymes extracted from epicotyl cell walls. These results suggest that hypergravity increases the amounts and the molecular mass of xyloglucans by decreasing xyloglucan-degrading activity. Modification of xyloglucan metabolism as well as the thickening of cell walls under hypergravity conditions seems to be involved in making the cell wall mechanically rigid, thereby inhibiting elongation growth of azuki bean epicotyls.

Seedlings of Arabidopsis thaliana (L.) Heynh. (ecotype Columbia and an ethylene-resistant mutant etr1-1) were cultivated for 68.5, 91.5 and 136 h on board during the Space Shuttle STS-95 mission, and changes in the elongation growth and the cell wall properties of hypocotyls were analyzed. Elongation growth of dark-grown hypocotyls of both Columbia and etr1-1 was stimulated under microgravity conditions in space. There were no clear differences in the degree of growth stimulation between Columbia and etr1-1, indicating that the ethylene level was not abnormally high in the cultural environment of this space experiment. Microgravity also increased the mechanical extensibility of cell walls in both cultivars, and such an increase was attributed to the increase in the apparent irreversible extensibility. The levels of cell wall polysaccharides per unit length of hypocotyls decreased in space. Microgravity also reduced the weight-average molecular mass of xyloglucans in the hemicellulose-II fraction. Also, the activity of xyloglucan-degrading enzymes extracted from hypocotyl cell walls increased under microgravity conditions. These results suggest that microgravity reduces the molecular mass of xyloglucans by increasing xyloglucan-degrading activity. Modifications of xyloglucan metabolism as well as the thickness of cell wall polysaccharides seem to be involved in an increase in the cell wall extensibility, leading to growth stimulation of Arabidopsis hypocotyls in space.

The effect of silicon on organ growth and its mechanisms of action were studied in rice ( Oryza sativa L. cv. Koshihikari), oat ( Avena sativa L. cv. Victory), and wheat ( Triticum aestivum L. cv. Daichino-Minori) seedlings grown in the dark. Applying silicon in the form of silicic acid to these seedlings via culture solution resulted in growth promotion of third (rice) or second (oat and wheat) leaves. The optimal concentration of silicon was 5-10 mM. No growth promotion was observed in early organs, such as coleoptiles or first leaves. In silicon-treated rice third leaves, the epidermal cell length increased, especially in the basal regions, without any effect on the number of cells, showing that silicon promoted cell elongation but not cell division. Silicon also increased the cell wall extensibility significantly in the basal regions of rice third leaves. These results indicate that silicon stimulates growth of rice and some other Poaceae leaves by increasing cell wall extensibility.

The relationship between the mechanical properties of cell walls and the levels of wall-bound ferulic (FA) and diferulic (DFA) acids was investigated in wheat (Triticum aestivum L.) coleoptiles grown under osmotic stress (60 mM polyethylene glycol [PEC] 4000) conditions. The cell walls of stressed coleoptiles remained extensible compared with those of the unstressed ones. l h e contents of wallbound FA and DFA increased under unstressed conditions, but the increase was substantially reduced by osmotic stress. In response to PEC removal, these contents increased and reached almost the same levels as those of the unstressed coleoptiles. A close correlation was observed between the contents of FA and DFA and the mechanical properties of cell walls. l h e activities of phenylalanine ammonia-lyase and tyrosine ammonia-lyase increased rapidly under unstressed conditions. Osmotic stress substantially reduced the increases in enzyme activities. When PEC was removed, however, the enzyme activities increased rapidly. lhere was a close correlation between the FA levels and enzyme activities. These results suggest that in osmotically stressed wheat coleoptiles, reduced rates of increase in phenylalanine ammonia-lyase and tyrosine ammonia-lyase activities suppress phenylpropanoid biosynthesis, resulting in the reduced level of wall-bound FA that, in turn, probably causes the reduced level of DFA and thereby maintains cell wall extensibility.Osmotic stress has been shown to affect the mechanical properties of cell walls. In light-grown maize leaves the cell wall extensibility was increased under osmotic stress (Acevedo et al., 1971). Sakurai et al. (1987b), Sakurai and Kuraishi (1988), and Kutschera (1989) also reported that the cell walls of dark-grown squash and mung bean hypocotyls remained extensible under PEG-induced osmotic stress conditions for several days. On the other hand, the application of PEG to the roots of light-grown maize seedlings induced cell wall hardening of leaves within several minutes (Chazen and Neumann, 1994). In dark-grown soybean hypocotyls the extensibility, particularly plasticity, of cell walls decreased when growth was inhibited under waterdeficit conditions (Nonami and Boyer, 1990). Thus, the effect of osmotic stress on cell wall extensibility is variable for plant species, organs, and 1 or growth conditions.Cell walls from growing plant tissues are mainly composed of cellulose and matrix polysaccharides. The amount and structure of wall polysaccharides have been considered to be factors that determine the mechanical properties * Corresponding author; e-mail wakabaBsci.osaka-cu.ac.jp; fax 81-6-605-2522, 967 of cell walls (Taiz, 1984; Sakurai, 1991). The primary cell wall of gramineous plants also contains a significant amount of hydroxycinnamic acids such as FA and coumaric acid, which are ester-linked to wall matrix polysaccharides such as arabinoxylans (Hartley, 1973; Harris and Hartley, 1976; Shibuya, 1984; Kato and Nevins, 1985; Nishitani and Nevins, 1988;Yamamoto et al., 1989). FA b...

We examined the changes in the orientation of cortical microtubules during the hypergravity-induced modification of growth anisotropy (inhibition of elongation growth and promotion of lateral growth) in azuki bean (Vigna angularis Ohwi et Ohashi) epicotyls. The percentage of cells with transverse microtubules was decreased, while that with longitudinal microtubules was increased, in proportion to the logarithm of the magnitude of gravity. The percentage of cells with longitudinal microtubules showed an increase within 0.5 h of transfer of the 1g-grown seedlings to a 300g-hypergravity condition. Lanthanum and gadolinium, blockers of calcium channels, nullified the modification of growth anisotropy and reorientation of microtubules by hypergravity. Horizontal and acropetal hypergravity modified growth anisotropy and reorientation of microtubules, as did basipetal hypergravity, and these changes were not seen in the presence of lanthanum or gadolinium. These results suggest that hypergravity changes activities of lanthanum- and gadolinium-sensitive calcium channels independently of its direction, which may lead to reorientation of cortical microtubules and modification of growth anisotropy in azuki bean epicotyls.