When plants are flooded, the gas diffusion rate is impeded (Jackson 1985). Hence flooding not only leads to lower internal O2 concentrations (hypoxic conditions) but also higher CO2 and ethylene concentrations, as reviewed by Vriezen et al. (2003).
In Rumex palustris, one of the responses to submergence is a hyponastic movement of the leaves, caused by cell elongation at the abaxial side in the basal region of the petiole (Cox et al. 2004). The effect can be mimicked by adding ethylene to unsubmerged plants. Under submergence, ethylene concentrations in Rumex rise due to ethylene entrapment and not due to ethylene synthesis (Voesenek et al. 1993). However, ethylene is not the only signal that causes this response. Submerged plants treated with 1-MCP do show a hyponastic movement, although it is not as pronounced as that seen in untreated plants. Auxins are also necessary for hyponastic movements. Auxin deficiency cannot be rescued by ethylene or GA. GA promotes hyponastic movements, while ABA has inhibitory effects (Cox et al. 2004).
Ethylene biosynthesis of rice seedlings is enhanced by hypoxic conditions (Satler and Kende 1985; Van Der Straeten et al. 1992). As a result of the entrapment of ethylene, a positive feedback mechanism further enhances its synthesis (Chae et al. 2003). Growth upon submergence occurs in the youngest internode. An intercalary meristem is located just above the second node (Kende et al. 1998). Applying ethylene to rice plants that were older than 28 days stimulated the growth of internodes only in a deepwater rice variety (Metraux and Kende 1983). When AVG is present, this growth is inhibited.
As reviewed above (Sect. 1.2 Regulation of Synthesis), ethylene concentration can be enhanced by modulating ACS and ACO expression. In air-grown plants, OsACS5 is expressed at low levels in the shoot apex, meristems, leaves, adventitious root primordia, and in vascular tissues of unelongated stems and leaf sheaths. Upon submergence, OsACS5 expression is enhanced in vascular bundles of young stems and leaf sheaths. Furthermore, under hypoxic conditions, exogenously applied GA up-regulates OsACS5 expression, whereas ABA has the opposite effect (Van Der Straeten et al. 2001). The transcript levels of OsACO1 reached a maximum in the intercalary meristem, elongation zone and differentiation zone of the internodes after 15 hours of submergence (Mekhedov and Kende 1996).
In deepwater rice, submergence is accompanied by a decrease in ABA. This effect can be mimicked by applying exogenous ethylene (Hoffmann-Benning and Kende 1992; Azuma et al. 1995). The same effect was noted for a submergence-tolerant Rumex species (Rumex palustris). When R. palus-tris plants are flooded or treated with ethylene, a fast down-regulation of NCED (neoxanthine cis-epoxycarotenoid dioxygenase) and an increase in ABA breakdown is observed (Benschop et al. 2005). However, when 1-MCP
and fluridone (ABA biosynthesis inhibitor) are applied simultaneously, no elongation is observed. This indicates that a reduction in ABA must be accompanied by an increase in the ethylene concentration in order to induce stem elongation. Submergence-intolerant Rumex acetosa plants do not show a decrease in ABA levels (Benschop et al. 2005).
The submergence of deepwater rice enhances the concentrations of GA1 and GA2o (Hoffmann-Benning and Kende 1992; Van Der Straeten et al. 2001). Applying ABA to air-grown deepwater rice inhibited cell elongation, an effect that can be reversed by adding GA. It is thus reasonable to hypothesize that ethylene exerts its effect on GA through ABA (Hoffmann-Benning and Kende 1992). Both submergence and ethylene treatment increase the level of bioac-tive GA1 in R. palustris (Rijnders et al. 1997). Also, in submerged R. palustris plants, ABA inhibits the increase in the level of GA seen in submerged plants without exogenously applied ABA (Benschop et al. 2005).
Stem elongation in rice is an effect of both cell division and cell elongation (Vriezen et al. 2003). Cell elongation can be partially caused by increased expression of expansins. In rice, both submergence and GA treatment enhanced expansin A and B expression (Cho and Kende 1997). In Rumex, however, out of 13 studied expansins, only RpEXPAl is induced in petioles upon submergence or ethylene treatment. The addition of ABA or PAC to submerged plants does not inhibit this induction. Also, when GA is applied to air-grown plants, this effect can not be mimicked (Vreeburg et al. 2005). Submergence-induced acidification of the cell wall of Rumex is inhibited when petiole tissue is pretreated with 1-MCP. The addition of ABA to submerged plants had no effect on cell wall acidification (Vreeburg et al. 2005). Thus, in Rumex petioles, ethylene may be involved in two separate pathways. It is proposed to cause a decrease in ABA (and a subsequent increase in GA) and to enhance expansin expression and cell-wall acidification through a distinct mechanism (Vreeburg et al. 2005).
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