Round or Wrinkled; Mendel's Peas Explained

Belinda Clarke

Pea plants growing in a 19^th century monastery garden may seem a long way from the advanced genetic tools used by scientists today, yet those plants were to inspire Gregor Mendel to lay the foundations of modern day genetics. His plant breeding experiments revolutionised our understanding of inheritance, but it wasn’t until 130 years later that the precise gene responsible for Mendel’s favourite pea trait was found by scientists at the John Innes Centre (JIC) in the UK.

In 1866, Mendel reported a " difference in the form of the ripe seeds" of pea plants, having noticed that some pods contained round peas, while peas in other pods were wrinkled. Depending on which parent plants he used, a single pod in an offspring plant might contain a mixture of round and wrinkled peas. Mendel realised that the proportion of round to wrinkled peas could be predicted according to the characters of the parent plants, an observation that led him to propose his famous laws of inheritance. Importantly, round-seededness and wrinkled-seededness were two facets of the same character; suggesting to modern plant scientists that a single gene determines whether seeds are round or wrinkled.

Plant breeders realised that wrinkled peas have a sweeter taste than the round ones, and so commercial pea varieties were developed carrying the wrinkled trait. The sweet flavour is caused by higher levels of sugar in wrinkled peas compared with round ones. This suggests that the gene responsible for the difference between the two types might have something to do with sugar or carbohydrate production in the pea.

In fact, the wrinkled trait is due to a disruption in the biochemical process responsible for making starch in these peas. During development of normal round peas, sugar is converted into starch, which is stored by the mature seed until needed for germination. Wrinkled peas convert much less of their sugar into starch. The sugar level builds up inside the pea and so they taste sweeter than round ones.

Having established that the wrinkled trait was due to a disruption in starch synthesis, scientists at the JIC started looking for the exact part of the process that is defective in wrinkled peas. They discovered that wrinkled peas don’t produce one crucial enzyme needed for normal starch production. They concluded that the loss of this enzyme, known as starch branching enzyme, explains the block in the transformation of sugar into starch

So the JIC scientists set about discovering why normal, round peas have the starch branching enzyme but wrinkled peas do not. They found that the gene for starch branching enzyme in wrinkled peas contains an extra piece of DNA that stops the gene from working, so the enzyme that it encodes is not produced.

But why are the peas wrinkled? This is due to a process known as osmosis – the natural tendency of water to move from a more dilute to a more concentrated solution. The high sugar content of wrinkled peas means that water moves in by osmosis, causing the pea to expand inside its seed coat. Once it is mature, it loses the water as it dries and shrivels. But having been stretched, the seed coat then wrinkles as the pea inside shrinks. As round peas contain less sugar than wrinkled ones, they take up less water and the seed coat doesn’t stretch. As round peas dry, the seed coat shrinks to leave a smooth surface.

So, fifteen decades after Mendel’s original experiments, biochemists have finally solved the riddle of round and wrinkled peas. But closing this chapter in the story has now opened many others….such as finding out more about starch production in peas and other plants. This work on wrinkled peas has already provided insights into how other species such as maize and potatoes make this important carbohydrate. Understanding more about starch production in plants will pave the way for new opportunities to improve the quantity and quality of starch for both food and non-food uses.

Research at John Innes Centre

Many different varieties of pea exist. Image supplied by the John Innes Centre Historical Collection.

Gregor Mendel, an Austrian monk who studied inheritance in round and wrinkled peas. Image supplied by the John Innes Centre Historical Collection.