For a Struggling Seed, the First 48 Hours Can be Tough
Imagine the seed of a soybean, tucked below the topsoil. Before tofu, before soy milk, and even before the first leaves emerge from the ground, work must be done.
Be it soy, sassafras or salvia, the embryos of many plants have just 48 hours to germinate. New research reported in the journal Current Biology has revealed how the seeds of plants funnel essential proteins into chloroplasts, which are the organelles responsible for photosynthesis. Scientists from the University of Geneva and the University of Neuchâtel, both in Switzerland, found that a key growth hormone, called gibberellic acid, catalyzes this transformation.
“Photosynthesis is the basis of essentially all life on the planet, and in plants, that depends on the development of chloroplasts,” said Paul Jarvis, a plant biologist at the University of Oxford in England, who was not affiliated with the study.
Before a germinating seed can sprout into a seedling, it must prepare its cells for photosynthesis through a process called chloroplast biogenesis. Similar to the way stem cells morph into specialized cells, organelles within the plant cells known as proplastids transform into chloroplasts, the photosynthesizing organelles that produce sugar and give the plant its bright green color. Once the development of these first chloroplasts happens, the young plant can begin its journey to the surface.
But this process needs to happen quickly before the seed’s internal sugar reserves run out, which usually happens within the first 48 hours. This is where growth hormones come in. In particular, a growth hormone called gibberellic acid jump-starts the embryo’s process that creates chloroplasts.
"This is one of the most important hormones to agriculture,” said Joshua Gendron, a plant molecular geneticist at Yale University in New Haven, Connecticut. The tiny hormones have a hefty set of responsibilities: They help the seed germinate by turning starches into sugars, influence the sex of the plant, and assist with expansion of the plant’s cells. They also impact how tall the plant will grow.
Though scientists didn’t know it at the time, gibberellic acid helped spark the “Green Revolution” that earned plant biologist Norman Borlaug a Nobel Peace Prize in 1970 and nearly doubled crop yields in Mexico, Pakistan and India. Through selective breeding, researchers found that smaller, stockier plants were able to produce and hold more seeds. Unbeknownst to them, lower levels of gibberellic acid contributed to the plants’ shorter stature.
Now, said Gendron, “being able to understand the molecular steps that are between [gibberellic acid] and the production of chloroplasts is really important.”
Understanding the mechanisms behind chloroplast biogenesis can help to rescue struggling seeds. When faced with crippling heat, for example, plant cells produce less gibberellic acid and the process slows down.
When gibberellic acid levels are low, destructive proteins thrive in the cell. These unruly proteins, called DELLA proteins, weaken the cell’s ability to transport photosynthetic proteins to the chloroplast by damaging the receptors that grab them.
These receptors are the key that photosynthetic proteins need to unlock the pathway to the chloroplast; without them, they can’t pass through. If the chloroplasts can't photosynthesize within the first 48 hours, the seedling can grow into an albino plant that is unable to photosynthesize, or wither and die.
"We are doing very basic research, but we can translate it to agricultural systems," said Shanmugabalaji Venkatasalam, a plant biologist at the University of Geneva and the paper’s first author.
Knowing when to plant the seed can make all the difference. Venkatasalam hopes the work will help the agricultural community understand how rising temperatures will affect crops, such as rice, wheat and soy. In an increasingly warming climate, farmers can safeguard their seeds by strategically planting them during pockets of cooler weather, when they’ll be less vulnerable during the first two days in the ground.
“Plants can adapt -- but at such a level of fast changes?" said Venkatasalam. "I think it's very difficult."