ROS GENERATION IN WILLOW SEEDS: A DELICATE EQUILIBRIUM BETWEEN SEED AGEING AND NORMAL SEEDLING GROWTH

CAUSIN HUMBERTO FABIO; ROQUEIRO GONZALO; MALDONADO SARA

FROM SEED GERMINATION TO YOUNG PLANTS: ECOLOGY, GROWTH AND ENVIRONMENTAL INFLUENCES

Nova Science Publishers, Inc.

Lugar: Hauppauge, N.Y.; Año: 2013; p. 1 - 18

Willow (Salix spp.) seeds are desiccation-tolerant, like orthodox seeds. However, they lose viability in a few weeks when stored at room temperature. They also differ with most orthodox seeds in that the chloroplasts of the embryo tissues conserve intact their chlorophyll and endomembrane system. The loss of viability is preceded by a decrease in normal germination (NG) percentage (i.e., the percentage of embryos having intact cotyledons and primary roots after germination), a phenomenon that correlates to an increase in the production of reactive oxygen species (ROS) and free radicals. Light and oxygen play an important role in free-radicals generation producing photooxydative damage in embryo tissues. Subcellular and biochemical analyses of experimentally aged seeds indicate that the thylakoid membranes of the chloroplasts are probably the first target of the ROS. During imbibition, an increase in the generation of superoxide anions (O2.-) in cotyledonary tissues is among the first symptoms associated to willow seed ageing. In agreement with this view, a short term treatment of aged seeds with different ROS scavengers may significantly increase their NG percent age. However, the continuous presence of O2.- (but not H2O2) scavengers in the germination media inhibits root growth, suggesting that an adequate generation of superoxide anions should be maintained for normal growth of this organ. In fact, normal seedlings show an intense production of O2.- in the root apical zone. The activity of NADPH-oxidase appears to be involved in this process, although the presence of endogenous O2.- sources cannot be discarded. The exogenous supply of either abscisic acid (ABA) or inhibitors of the gibberellic acid (GA) synthesis suppressed root growth without inhibiting O2.- production. This indicates that, even though O2.- generation in the root apex would not depend on changes in ABA or GA availability, an adequate endogenous ABA/GA ratio is necessary for the ROS stimulation of root growth. O2.- production is also very active at the sites of root hair budding. Nevertheless, root hair density and growth proved to be affected by changes in H2O2 rather than in O2.- levels. Possible mechanisms of action of the different ROS in seed ageing as well as root growth and development of willow seedlings are discussed.