Dehydrin expression in seeds: an issue of maturation drying.

Abstract

In areas exhibiting typical seasonalchanges, the seeds of most plants aregettingmatureinsummerandfall,respec-tively. In order to prevent the destructionof the young and sensitive seedlings by thefrosty winter conditions, any germinationin fall has to be hindered. Accordingly,efficient outlasting mechanisms have beenevolved to avoid or at least to minimizesuch wastages by suppressing the germi-nation of mature seeds in the fall andallow the seeds to overwinter without anyloss of their viability (Bewley, 1997). Yet,such persistence requires strongly reducedmetabolic activity, i.e., by the reductionof the water content. Accordingly, apartfrom the suppression of germination, des-iccation tolerance is a precondition forthe overwintering of the seeds (for reviewsee Finch-Savage and Leubner-Metzger,2006). In this manner, during the lastphase of embryogenesis, the water con-tent of the seeds is strongly reduced ina process denoted as maturation drying.As result the seeds become dormant. Inthe succeeding spring, i.e., after a pro-longed cold period, dormancy is broken(vernalization) and seeds are consideredas quiescent (Baskin and Baskin, 2004).As soon as sufficient water is available, thequiescent seeds imbibe and germinate. Incontrast to such orthodox seeds, seeds ofmost tropical plants are able to germinateassoonasthefruitsaremature(forreviewsee Farnsworth, 2000). In tropical rain-forests there are no unfavorable climaticconditions that have to be overcome, sincethereisnonecessityforputativeoutlastingmechanisms. In contrast, any extensionof the exposition of ungerminated seedswould enhance the risk of being eaten byherbivores or being infected by pathogens.Accordingly, the seeds of most tropi-cal plants have not evolved outlastingmechanisms, and, most tropical seeds arerecalcitrant: Dormancy is lacking (e.g.,Jurado and Flores, 2005) and they do notreveal a maturation drying (Berjak et al.,1989).Asaresult,recalcitrantseedscannotbestoredforalongperiodoftimewithoutlosing their viability. The most popularplants that exhibit recalcitrant seeds are:avocado, cocoa, mango, lychee, and therubber tree.Indeed, these ecological cognitionshave been well known for several decades.Yet, due to the fact that recalcitrantseeds are not storable, in many scien-tific articles—especially those focusingon breeding, seed production and seedstorage—recalcitrance