It is known that H2O2 promotes germination in various species. H2O2 can be used in high concentrations to promote germination of seeds with hard seed coats by scarification, it also has a germination-promoting effect in lower concentrations. Germination studies on Zinnia elegans Jacq. seed showed a dose-dependent promotion of germination by H2O2 (Ogawa and Iwabuchi, 2001). Inhibition of catalase in lettuce seeds led to higher concentrations of H2O2 in the seeds and to faster germination (Hendricks and Taylorson, 1975).
Our interpretation of this promotion is based on the model presented in Fig. 30.1: H2O2 reacts withE O-• in the presence of cell wall peroxidases, leading to the formation of •OH (Chen and Schopfer, 1999). These •OH then act on the cell wall by causing polysaccharide cleavage resulting in endosperm weakening. This model and our hypothesis for endosperm weakening could also explain why tobacco (Nicotiana tabacum L.) seeds overexpressing a cell wall peroxidase germinate faster in the presence of osmotica than the corresponding wild type (Amaya et al., 1999). Peroxidase activity increases in the micropylar endosperm of tomato seeds prior to endosperm rupture (i.e. during endosperm weakening) (Morohashi, 2002).
In addition to its effects on seeds, H2O2 promotes elongation growth requiring cell wall loosening in other plant parts. Overexpression of horseradish-peroxidase under the control of the CaMV-35S-promotor leads to faster elongation growth of zucchini (Cucurbita pepo L.) hypocotyls (Dunand et al., 2003). In the roots of onion (Allium cepa L.), the highest peroxidase activity is found in elongating tissues (Cordoba-Pedregosa et al., 2003). These tissues also show the highest concentrations of H2O2. However, H2O2 can also have an adverse effect (i.e. growth reduction). For example, addition of ABA to the growth medium leads to a higher activity of peroxidases, a higher concentration of H2O2 and reduced growth in the roots of rice (Oryzasativa L.) seedlings (Lin and Kao, 2001), and ROS production is essential for lignification and cross-linking of cell wall polymers in vascular tissue (Ogawa et al., 1997).
We propose that cleavage of cell wall polymers by •OH not only takes place in the endosperm, but also plays a role in radicle elongation. Here, cell walls have to be loosened in order to allow cell elongation, caused by water uptake, which takes place when the water potential in the embryo is lower than that of the surrounding medium.
When working with ROS, it is important to differentiate between ROS signalling effects (Laloi et al., 2004) and direct action, such as the cleavage of cell wall polymers. The only way to prove the latter would be to experimentally show both the presence of ROS and the products of reactions caused by the ROS.
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