Physiological, molecular and environmental aspects of the control of seed dormancy in grain crops.

BENECH-ARNOLD, ROBERTO LUIS

Wageningen, Holanda

Conferencia; III International Symposium on Plant Dormancy.; 2004

International Society for Seed Science

While the adaptive significance of dormancy is quite evident for plants living in the “wild”, it has been seen always as a complication in seeds from plants that are grown as crops. Indeed, a persistent dormancy would prevent the utilization of a seed lot either for the generation of a new crop or for industrial purposes (i.e malting). But on the other hand, most crops that originally must have had dormancy, have been selected so heavily against dormancy throughout their domestication process, that seeds are germinable even prior to crop harvest; this frequently leads to preharvest sprouting.             Due to the paucity of our knowledge on the genetic, physiological and environmental control of dormancy, it is very difficult to adjust the timing of dormancy loss to a precise and narrow “time window” (i.e. neither as early as to expose the crop to the risk of preharvest sprouting, nor as late as to have a dormant seed lot at the time of the next sowing or industrial utilization). Among the cereals, malting barley is possibly the most problematic crop. The malting process itself requires grain germination, so a low dormancy level at harvest is a desirable characteristic because the grain can be malted immediately after crop harvest, thus avoiding costs and deterioration resulting from grain storage until dormancy is terminated. The possibility of solving the compromise between obtaining genotypes with low dormancy at harvest, but not with such an anticipated termination of dormancy that leads to sprouting risks, requires a thorough knowledge of the mechanisms determining dormancy release in the maturing grain. Moreover, it is essential to understand how those mechanisms are genetically and environmentally controlled.             In this paper we comment on some aspects of the environmental and hormonal control of dormancy in grain crops, using sorghum and barley as model systems. The relationship between temperature experienced by the crop at a particular “sensitvity window” within grain development, and velocity of dormancy release after physiological maturity, has been quantified and is discussed in terms of its predictive value for crop management. The antagonism ABA/GAs in the imposition and expression of dormancy is especially dealt with. Two features have been identified as instrumental for the expression of different grain dormancy levels: i) embryo responsiveness to ABA and, ii) embryo capacity to produce GA de novo synthesis upon imbibition. We investigated the nature of the differential sensitivity to ABA displayed by embryos from sorghum varieties with contrasting dormancy and concluded that a disruption in the ABA signalling pathway is most likely behind the reduced sensitivity to ABA displayed by embryos from varieties with low dormancy prior to harvest. In barley grains, in contrast , this low embryo sensitivity to ABA appears to be  related to a fast rate of metabolization/conjugation of ABA; in turn, this rate seems to be modulated by oxygen availability and temperature. GA de novo synthesis upon imbibition in grains with low dormancy has been shown to be regulated through the expression of the gene that codifies for the enzyme GA 20-oxidase, previously pointed out as a “site of regulation” in GA biosynthesis. However, other genes might be involved in this differential regulation of GA biosynthesis existing between grains with different dormancy level. Possible applications of these findings are discussed.