If stresses occur in the first 15 days of germination, it can be enough to kill the whole seedling.
Photo by missyredboots/morguefile.com.
As climate change creates extreme temperature shifts, drought and flooding, what can seed scientists do to enable germinating seeds to survive such stresses and ensure healthy seedling establishment and crop stands? "Stress tolerability in plants is an age-dependent phenomenon," says Rajeev Arora, stress physiologist and horticulture professor at Iowa State University. "Plants have evolved to handle certain stresses, and we have been trying to understand how."
The destructive power of oxidative stress on the human body has been widely reported over the last decade and a half. Plants also suffer oxidative stress, or damage that occurs when free radicals attack plant tissue. Free radicals are produced in all plants under stress conditions, but can be removed by a complex system of antioxidants known as "scavengers." Recent research at Iowa State revealed one way to boost antioxidant power in seeds, thus creating stronger plants: osmopriming.
Osmopriming and plant health
At times, even moistening a seed can cause stress, which can result in the production of free radicals. When a dry seed is placed in a moist medium for germination, the imbibition process moistens the organelles and protoplasm within each cell of the seed, as well as the membranes around each cell. Membranes are made up of proteins and lipids. Under stress conditions, lipids produce free radicals, which cause oxidative damage by attaching to other molecules inside the cells.
Each seed also contains an antioxidant system that includes the enzymes ascorbate peroxidase (APX), catalase (CAT) and superoxide dismutase (SOD), along with nonenzyme molecules such as vitamin C and glutathione. The antioxidant system's role is to scavenge free radicals.
"Is the antioxidant machinery somehow bolstered in primed seeds? We discovered the answer is yes, and we are looking at this at the gene level," explains Arora, the lead researcher on Iowa State's project.
Arora, Dr. Keting Chen and their team knew that primed seeds outperform nonprimed seeds in optimal conditions. They wanted to learn whether primed seeds would also outperform nonprimed seeds in stress conditions, so they primed the seeds in their test lot, put them in the germination medium, and measured enzyme activity each day by extracting proteins from the seeds. In the first four days there was no change. Between five and 10 days of germination, the researchers found elevated levels of APX, CAT and SOD. Enzyme activity was higher in the primed seeds during those five days of germination than in the nonprimed seeds.
By day 15, the higher enzyme activity in primed seeds was starting to fade away. "The benefit of the priming on antioxidant activity was disappearing," reports Arora. The researchers did not examine the effects of priming on vitamin levels, but in the future Arora would like to explore the effects of osmopriming on vitamin C and glutathione content.
The researchers also discovered increased levels of plant proteins called dehydrins.
Dehydrins are produced in plant tissues when they sense desiccation stress. These hydrophilic proteins trap the water molecules around membranes and prevent damage from overdrying. During the study, Arora and Chen observed dehydrin activity in the first two days of an eight-day osmopriming treatment. By the second day, the dehydrins reached peak accumulation. By day four, the dehydrins disappeared.
The researchers planted their primed seeds with nonprimed seeds and allowed them to germinate under both optimal conditions and stressful conditions. What they found was surprising: Although the dehydrin proteins had disappeared by day four of the priming process, those proteins accumulated once again when these seeds were exposed to the stressful conditions during germination. The dehydrin protein levels were higher in primed seeds than in nonprimed seeds.
Arora and Chen now believe that during priming, the seeds develop a memory that they were primed. Later, when it is needed under stressful conditions, the memory is reawakened to help the seeds grow and thrive.
Taking it to the field
Arora and Chen's study occurred in a very controlled environment and a "not totally natural" situation, so they don't know how it would replicate in the field. Still, Arora sees the findings as significant. "Help from any of these stress-protective mechanisms, whether antioxidant pathway machinery, or dehydrins, or something else we have not yet explored, are often most critical in the early stages of seedling development because of abiotic stresses (chilling, heat, etc.). Those stresses are more damaging to seedlings than established plants. Established plants can actually handle abiotic stresses better because they produce their own defense mechanisms."
If stresses occur in the first 15 days of germination, it can be enough to kill the whole seedling. Arora anticipates that this work will ultimately benefit growers of horticultural crops, like high-value vegetables and flowers, who need to predict uniform germination and uniform production.
For Arora, this study opened a new avenue of horticultural research. For the past few decades, his focus has been on the effects of abiotic stresses on established plants. Once he acquires funding, he'll turn his focus to the long-term effects of priming. The seed's physiology will only tolerate a certain time in storage. There is a limit to how long one can store primed seed before the promotive effects of priming disappear. Arora wonders how storage conditions will affect the efficacy. "How does the promotive effect of priming drop in each storing condition? Does it drop precipitously? If you change something about the storing condition, can you delay the reduction in benefit?"
Arora will be looking for answers to these questions over the next several years.
The author is a freelance writer based in Massachusetts and a monthly contributor to Growing.