Double coating nurtures seeds

Extreme climate events mean crops are increasingly stressed by drought—especially during the crucial stages of seed germination. Inspired by chia seeds’ natural coating, which swells into a gel when wet, researchers have developed a new seed coating that helps seeds retain moisture while capturing nutrients from the soil during germination (Nat. Food 2021, DOI: 10.1038/s43016-021-00315-8).

Seeds are commonly coated with nutrients or fertilizers. For semiarid regions, which make up about 15% of global land, farmers and seed producers can apply superabsorbent polymers to seeds or soil to protect against drought. These practices can contaminate the soil with excess fertilizer and polymer residue.

Benedetto Marelli, a biomedical engineer at the Massachusetts Institute of Technology who led the research, wanted to create a more environmentally friendly coating. He started with silk, which is an emerging coating for drug delivery because its unique proteins stabilize biomolecules and because it degrades over time. Instead of incorporating a drug, he embedded the silk with rhizobacteria to create the first layer of seed coating. Rhizobacteria promote plant growth by binding to the roots and converting nitrogen gas trapped in the soil into ammonia, which the plant can use. Rhizobacteria also help the plants combat drought stress.

Marelli’s team made the outer seed coating from pectin and carboxymethylcellulose, which are both readily available, biodegradable, food-safe polymers that swell into a water-retaining gel when exposed to moisture. This gel helps the rhizobacteria colonize the plant’s roots early in germination and keeps the seed moist.

Marelli’s team applied this double coating to common bean seeds and germinated them under drought conditions. Compared with uncoated seeds, coated seeds grew significantly longer roots, and their seedlings’ leaves had a significantly higher tolerance to water stress.

David Britt, a bioengineer at Utah State University who was not involved with the research, is “very intrigued” and excited by the potential of this customizable, bioinspired design.