Abstract
Vivipary, germination of seeds on the maternal plant, is observed in nature and provides ecological advantages in certain wild species, such as mangroves. However, precocious seed germination in agricultural species, such as preharvest sprouting (PHS) in cereals, is a serious issue for food security. PHS reduces grain quality and causes economical losses to farmers. PHS can be prevented by translating the basic knowledge of hormone biology in seeds into technologies. Biosynthesis of abscisic acid (ABA), which is an essential hormone for seed dormancy, can be engineered to enhance dormancy and prevent PHS. Enhancing nine-cis-epoxycarotenoid dioxygenase (NCED), a rate-limiting enzyme of ABA biosynthesis, through a chemically induced gene expression system, has successfully been used to suppress germination of Arabidopsis seeds. The more advanced system NCED positive-feedback system, which amplifies ABA biosynthesis in a seed-specific manner without chemical induction, has also been developed. The proofs of concept established in the model species are now ready to be applied to crops. A potential problem is recovery of germination from hyperdormant crop grains. Hyperdormancy induced by the NCED systems can be reversed by inducing counteracting genes, such as NCED RNA interference or gibberellin (GA) biosynthesis genes. Alternatively, seed sensitivity to ABA can be modified to rescue germination using the knowledge of chemical biology. ABA antagonists, which were developed recently, have great potential to recover germination from the hyperdormant seeds. Combination of the dormancy-imposing and -releasing approaches will establish a comprehensive technology for PHS prevention and germination recovery.
Highlights
Seed development and germination are not necessarily separate developmental programs in terms of timing (before or after seed dispersal) or location (on the maternal plant or soil)
Seed development and germination are not necessarily separate developmental programs in terms of timing or location
In Sorghum bicolor, GA2ox, a gibberellin deactivation gene, plays a critical role in preharvest sprouting (PHS), this gene is regulated by the abscisic acid (ABA) pathway through sorghum ABI4 and ABI5 (Cantoro et al, 2013). These findings demonstrate the robustness of hormone metabolism in the regulation of seed dormancy and germination, which makes engineering of the hormone metabolism pathways in seeds as a logical target of modification in technology development for PHS prevention
Summary
Seed development and germination are not necessarily separate developmental programs in terms of timing (before or after seed dispersal) or location (on the maternal plant or soil). There is PHS Prevention through Hormone Engineering a more active mechanism, which suspends germination of developing seeds, that is, seed dormancy. The potential of MFT, which was originally identified and characterized in the model plant Arabidopsis, for PHS prevention in wheat, suggests that the knowledge of ABA signal transduction obtained from the model plant can directly be translated into cereal crops.
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