Lupins are high-protein and high-fiber legume that is low in carbohydrates and has a low glycemic index. They are also easy to grow in a variety of climates. However, some varieties of lupins contain high levels of unpleasantly bitter alkaloids, which can make them unpalatable for both humans and animals. In new research, an international team of researchers has identified the “sweetness gene” responsible for low alkaloid levels in lupins. This discovery may make it easier to reliably produce more palatable plants, which could lead to increased consumption of lupins by humans and animals.

The history of sweet lupins

Around 100 years ago, plant breeders in Germany found natural mutations that produced “sweet lupins” with far lower levels of bitter alkaloids. They produced sweet varieties of white lupin (Lupinus albus), narrow-leafed lupin (Lupinus angustifolius, the main type grown in Australia), and the less common yellow lupin (Lupinus luteus).

Over the past 50 years or so, lupins have become more common as food for farm animals. Sweet lupins are good for this, as they don’t require extensive washing to be usable. They are also increasingly eaten by humans – and we are very sensitive to bitterness.

Identifying the “sweetness gene”

To find the genetic basis for “sweet” lupins, the researchers used a few approaches. Colleagues in Denmark studied the biochemistry of the different alkaloids in both bitter and sweet varieties. By looking at the changes in the composition of the alkaloids, they could get an idea of the genes involved. The researchers also analyzed 227 varieties of white lupin and tested their alkaloid levels. Then, with colleagues in France, they looked at markers across the lupin genome and tried to associate high and low alkaloid levels with genetics.

The researchers had clues about where they thought the gene would be, in a certain region of a few dozen genes. There was one they thought looked the most promising, so they designed a lot of DNA markers to work out what sequence varied in that gene. Eventually, they found a very strong link between a change in alkaloid levels and a variation of a single sequence in their gene.

Confirming the “sweetness gene”

The final test was to find out whether a variation in this gene would also produce sweetness in other types of lupin. In some other plants, genetic modification tools could be used to do this, but for various reasons, this is difficult in lupins. Instead, the researchers went to a company called Traitomic, which screened a huge number of seeds of narrow-leafed lupin until they found one that naturally had exactly the mutation they were looking for. And when they tested that plant, it had low alkaloids – confirming they really had found the “sweetness gene”.

Implications for plant Breeders

In practice, growing sweet white lupin can be a bit tricky. There are several different strains that have different low alkaloid genes, and if these strains cross-pollinate, the result can be bitter lupin plants once again. However, the research gives a reliable genetic marker for plant breeders to know what strains they are dealing with. This means it will be much easier to consistently grow sweet white lupin.

At the moment, most of what is grown in Australia is narrow-leafed lupin, in part because the industry had a hard time keeping the white lupin sweet (and in part because white lupin was plagued by a fungal disease called lupin anthracnose). So perhaps in the future, we’ll see white lupin make a comeback. The researchers’ vision is more cultivation of the high-protein, hardy lupins for consumption by humans.

Conclusion

Lupins are nutritious and versatile legumes that could play an important role in feeding both humans and animals. However, their bitter taste has limited their appeal. The recent discovery of the “sweetness gene” responsible for low alkaloid levels in lupins could make them more palatable and increase their consumption. The research provides a reliable genetic marker for plant breeders to consistently grow sweet white lupin, which could lead to increased cultivation of this high-protein, hardy legume for human consumption.