Tracing start of starch granules in wheat

New factors that control the way starch granules form in wheat have been discovered.

The findings could lead long-term to the production of improved cereal crops containing high-quality, healthier starches.

Starch is a major storage carbohydrate in leaves and non-photosynthetic organs of many plants such as grains and potatoes.

Much is known about how different types of starch are made in plants but less is known about how they form their insoluble granule structures.

A key question concerns how granules are initiated and what controls granule size and shape in the endosperm of cereals, the inner part of the grain used to make white flour and an important source of calories in human diets.

Between cereal grains there is a wide diversity in starch granule shapes – for example wheat shows two distinct populations of granules: large A-type granules and small B-type granules. In contrast, rice, forms compound granules, where a single storage structure contains lots of polyhedral granules. However, currently the factors determining these differences are unknown.

In research which appears in the journal New Phytologist, John Innes Centre researchers, in collaboration with researchers at the Quadram Institute, provide answers to some of the key questions around starch initiation.

They identified a factor, which can convert wheat that usually makes A and B-type granules into a crop that mostly makes compound granules. This was done by investigating the wheat version of SS4 (starch synthase 4), a known factor that has been studied in Arabidopsis and found to have an important role in granule initiation in this model plant.

The study shows that SS4 is necessary for normal granule initiation within wheat endosperm. Loss of SS4 did not affect the content, composition or structure of endosperm starch, however, did result in the formation of compound granules early in grain development. Therefore, in the wheat endosperm, SS4 is required to control the number of granule initiations.

Interestingly, the leaves of these plants had fewer starch granules, and were smaller and rounder compared to their usual flattened shape. This echoes the changes that happen in Arabidopsis when SS4 is lost.

First author Dr Erica Hawkins says: “Understanding granule initiation could identify promising targets for the manipulation of granule size, shape and number within the storage organs of crops. This in turn, may lead to altered physiochemical properties and novel functionalities of starch which could be exploited to tailor and produce high quality starches for specific end uses. This research identified that SS4 has a key role in granule shape, however there are still many additional factors involved in this process which are yet to be fully investigated.”

The next steps for this research are to investigate further factors in starch initiation which have been identified via protein-protein interactions. Additionally, researchers are looking at how natural variation amongst crops influences granule shape and size in the endosperm.

Corresponding author Dr David Seung, a Group Leader at the John Innes Centre, added: “Describing the role of SS4 is an important first step towards developing a full understanding of starch granule initiation in wheat and other cereals. We are not able to say whether it is SS4 itself that determines the natural variation in morphology amongst crops, but we have other projects in the lab exploring this possibility, as well as looking at the important contribution of other proteins.”

The research team used electron microscopy bioimaging to observe the differences in granule shapes and formation and worked alongside the informatics platform to develop ways to analyse the data.

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