Turning On the “Off Switch”
Nonogaki explains that two key hormones are involved in a seed’s cycle of dormancy and germination — Abscisic acid (also called ABA) and gibberellin. Abscisic acid inhibits growth while gibberellin stimulates it. “In terms of germination, gibberellin is the good guy, and ABA is a bad guy, an inhibitor of germination,” he says.
But for the prevention of pre-harvest sprouting, the bad guy is the hero.
Nonogaki uses the plant’s own internal signaling system to enhance a positive feedback loop in the seed; producing ABA as the seed begins to mature amplifies the production of even more ABA. Higher levels of ABA induce dormancy naturally, he explains.
“We call it a spontaneous seed dormancy system,” Nonogaki shares. Downstream genes are activated by the presence of hormones, switching on or off as hormone levels change throughout the seed development process.
In the early stages of seed growth, the embryonic plant creates the roots of future plant organs, including roots and leaves. Dormancy usually sets in toward the end, before the seed begins to dry out.
Nonogaki’s team focuses on a seed-specific gene promoter region that gets turned on at this maturation stage. They introduce the promoter gene using agrobacterium, a species of bacteria that’s a common tool in biotechnology. The gene causes an ABA cascade, kicking in just as the plant is beginning to produce ABA on its own. It sounds paradoxical, but the ABA-producing gene is regulated by the production of ABA.
Nonogaki is also marking the seeds with green fluorescent proteins, which enables them to be sorted easily. The seeds are in a totally dormant state, so they won’t germinate even if there’s plenty of water and the seeds are at the right temperature. But by tweaking which gene-promoter region is targeted, Nonogaki is confident the same process could work the opposite way to promote germination.
“Our current plans are to create spontaneous dormancy in the field, which doesn’t require chemicals,” he says. Then when we need to recover germination, we can use a gene switch to trigger a positive change. We really think we need both approaches, a suppression and recover approach.”
Even as they make progress in understanding hormonal regulation of plant growth, biologists still have much to learn about how these processes change among generations.