Elongation of Internodes
As with other stresses, there are also obvious avoidance reactions induced by hypoxia and anaerobiosis. Submerged plants usually produce strikingly long internodes. By such elongation growth, the apical parts of the shoots quickly reach the surface of the water and thus escape the hypoxic environment. This reaction has been particularly well studied in deep-water rice, where the submerged shoots elongate up to 25 cm/day.
A marked increase in the intercellular ethylene concentration is regarded as the phytohor-monal signal which stimulates elongation growth. However, the real initial signal is the low oxygen partial pressure in the submerged internode, leading to increased expression of the ACC-oxidase gene (ACC: aminocyclopropane carboxylic acid) and finally to increased ethylene synthesis (Box 1.4.2). The ACC produced by the hypoxic roots is transported by the xylem stream into the better aerated shoot since the ACC-oxidase requires molecular oxygen as substrate. The increased ethylene partial pressure triggers the decrease in the endogenous ABA level and thus in turn increases the effectiveness of the elongation hormone gibberellic acid. Thus, at least four phytohormones participate in the elongation growth of shoots and petioles triggered by hypoxia: ethylene, ABA, auxin and gibberellin.
Expansins are cell wall proteins with a molecular mass of 24.5 kDa (Box 1.4.3). In the shoots of deep-water rice, two such proteins occur in the intercalary meristem and in the adjacent extension zone (but not in the differentiation zone) mainly around the vascular bundles and in the tissue lining the inner epidermis, i.e. towards the pith channel (Cho and Kende 1997 a,b).
Submerged internodes accumulate considerably more expansins than those in air, and substantially expand their elongation zone. The molecular mechanism of expansins is still not well understood; it is assumed that they loosen the hydrogen bridges between the hemicellulose [(1 —> 3),(1 —> 4)-/?-d-glucans)] and the paracrys-talline segments of the cellulose microfibrils and thus increase the extensibility of the cell wall. Although this effect suggests a catalytic mechanism, expansins do not show any hydrolase activity.
Auxin also plays a role in shoot expansion: It stimulates acidification of the cell wall, probably by activating the ATP-dependent proton pumps in the plasma membrane. The effect of expansins can be mimicked by acidification of the cell wall. When rice grains were germinated under aerobic and anaerobic conditions, respectively, a
Ethylene biosynthesis from S-adenosylmethionine (AdoMet) and formation of the precursor
In addition to the biochemical pathways, the scheme shows internal and external factors that influence the synthesis or accumulation of ethylene. (After Taiz and Zeiger 2000)
Fruit ripening Senescence of flowers IAA
Wounding Frost stress Drought stress Flooding
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