Dehydrin expression in seeds: an issue of maturation drying

THE ECOLOGICAL RELEVANCE OF MATURATION DRYING DETERMINES DEHYDRIN EXPRESSION IN SEEDS In areas exhibiting typical seasonal changes, the seeds of most plants are getting mature in summer and fall, respectively. In order to prevent the destruction of the young and sensitive seedlings by the frosty winter conditions, any germination in fall has to be hindered. Accordingly, efficient outlasting mechanisms have been evolved to avoid or at least to minimize such wastages by suppressing the germination of mature seeds in the fall and allow the seeds to overwinter without any loss of their viability (Bewley, 1997). Yet, such persistence requires strongly reduced metabolic activity, i.e., by the reduction of the water content. Accordingly, apart from the suppression of germination, desiccation tolerance is a precondition for the overwintering of the seeds (for review see Finch-Savage and Leubner-Metzger, 2006). In this manner, during the last phase of embryogenesis, the water content of the seeds is strongly reduced in a process denoted as maturation drying. As result the seeds become dormant. In the succeeding spring, i.e., after a prolonged cold period, dormancy is broken (vernalization) and seeds are considered as quiescent (Baskin and Baskin, 2004). As soon as sufficient water is available, the quiescent seeds imbibe and germinate. In contrast to such orthodox seeds, seeds of most tropical plants are able to germinate as soon as the fruits are mature (for review see Farnsworth, 2000). In tropical rainforests there are no unfavorable climatic conditions that have to be overcome, since there is no necessity for putative outlasting mechanisms. In contrast, any extension of the exposition of ungerminated seeds would enhance the risk of being eaten by herbivores or being infected by pathogens. Accordingly, the seeds of most tropical plants have not evolved outlasting mechanisms, and, most tropical seeds are recalcitrant: Dormancy is lacking (e.g., Jurado and Flores, 2005) and they do not reveal a maturation drying (Berjak et al., 1989). As a result, recalcitrant seeds cannot be stored for a long period of time without losing their viability. The most popular plants that exhibit recalcitrant seeds are: avocado, cocoa, mango, lychee, and the rubber tree. Indeed, these ecological cognitions have been well known for several decades. Yet, due to the fact that recalcitrant seeds are not storable, in many scientific articles—especially those focusing on breeding, seed production and seed storage—recalcitrance per se is considered a negative property (e.g., Farnsworth, 2000). In consequence, many efforts are made to overcome the apparent drawbacks of recalcitrance by improving the seed storability of plants used in agronomy.