Coating seeds in silk, bacteria and sugar could help plants to grow in salty soils, researchers have revealed.
Saline soil is a growing problem around the world, particularly in regions with poor quality water for irrigation, and is a serious cause for concern as many important food crops do not grow well in such conditions.
While one area of research is to develop plants that are better able to cope with such conditions, the latest research takes a different tack – coating seeds in bacteria that can provide important nutrients to the plant.
The authors say it is not the first time scientists have embraced such an approach. But previous attempts have met with difficulties, including bacteria not surviving seed storage conditions.
“The moment you extract them from the soil and you dry them, they die,” said Dr Benedetto Marelli, co-author of the research from the Massachusetts Institute of Technology in the US. At present, he added, farmers must dip and then swiftly sow the seeds to see benefits – something he says is both inefficient and cannot be done everywhere.
Now Marelli and colleagues say they have tackled the problem by developing a novel type of coating to keep helpful bacteria in suspended animation until they reach the soil.
Writing in the Proceedings of the National Academy of Sciences, the team report that they used microbes called rhizobacteria. These bacteria interact with legumes in a mutually beneficial arrangement: the bacteria trigger the growth of nodules on the plant’s roots, which they then colonise. The bacteria receive sugars from the plant, while turning nitrogen from the air into nutrients that the plant can use.
The team report that they chose a type of rhizobacteria that is known to tolerate hot and salty conditions. They also incorporated silk and a sugar called trehalose into the seed coating.
Marelli says these ingredients help protect and preserve the bacteria: the silk acts as a way to attach the bacteria to the seed while it also provides a robustness to the coating and a controlled release of the bacteria – since it is soluble in water. The trehalose, meanwhile, replaces water around the bacteria, helping them to survive dehydration, and could also provide an initial energy source to the microbes when in the soil, helping to “resuscitate” them.
To test their approach, the team used a common legume – French beans – planting seeds with and without the coating in both salty and non-salty soil. For each condition 12 seeds were planted.
The team found that in the non-salty soil 88% of coated seeds sprouted compared with 62% of non-coated seeds, while in the salty soil the figures were 71% for the coated seeds and 45% for the non-coated seeds. After two weeks in the salty soil the coated seeds were found to have significantly longer stems, and longer roots with more branches than those from seeds without the coating. Only seedlings from coated seeds had developed root nodules.
“[The coating] increases the amount of seeds that germinate, it makes them germinate quicker, and it provides sprouts that are in better health,” said Marelli, adding that recent, additional work by the team in Morocco has shown such benefits continue to be seen in established crops.
The approach has drawbacks, including that it would not work with crops that are not legumes, such as barley or wheat. But Marelli said researchers are already looking at ways to harness rhizobacteria for plants that do not form root nodules, including by engineering either the bacteria or plants to generate such a relationship..
Prof Gerhard Leubner, an expert in seed technology from Royal Holloway University of London who was not involved in the study, said seed coating was already big business, with many approaches being explored to solve difficulties in coating seeds with microbes – not least since certain chemicals including fungicides and insecticides have been banned in the EU.
However, he said the team’s approach using silk and trehalose was a positive step and appeared to boost germination and growth in stressful, saline conditions..
“The silk is something biodegradable and natural and can be used in many other applications,” he said. “I think it is a very good method.”
