The perennial problem with Irish ryegrass

The quality and nutritional value of Irish perennial ryegrass has an important bearing on the economy. Our extensified grass-based farming methods, which are responsible for the food industry’s green credentials, mean that the dairy and livestock farming sectors are critically dependent on grass yields – and especially on perennial ryegrass. It is the major forage grass in Ireland.

The problem with perennial ryegrass is that it is resistant to normal breeding methods, the kind that have resulted in dramatic yield gains for cereal and other crops over the years.

Instead of selecting just two parent lines and breeding for the selection of the best characteristics of both, the programmes used for ryegrasses have changed little over the past 70 years. Tens of thousands of seeds are grown together and the best hybrids taken and grown over three or four years to establish if a real genetic gain has been made.

This has resulted in a situation where the annual average genetic gain for perennial ryegrass is 0.2 percent to 0.5 percent. This would result in a gain of a maximum of 5 percent in a decade, whereas a gain of 16 percent or better is achievable for cereal crops over the same period.

Big progress has been made in addressing this issue thanks to two pieces of ground-breaking research work undertaken by Teagasc. The first looks at the genetic mechanism at work in the pollination of the plant; the second involves the sequencing of the ryegrass genome.

“Self-incompatibility” is a genetic mechanism used by plants to avoid self-fertilisation and promote outcrossing. The mechanism works by enabling plants to recognise their own pollen and reject it in favour of pollen from another plant. It effectively prevents undesirable inbreeding.

The S and the Z

In perennial ryegrass, this mechanism is controlled by at least two genetic regions, which are called S and Z. Knowing the genes at S and Z would help develop hybrid breeding systems to produce better varieties for Irish farmers. But despite intense research efforts over nearly 70 years, the genes underlying S and Z remain uncharacterised.

While a mechanism to prevent inbreeding would appear to be a desirable characteristic in any species, it can sometimes present a problem.

In this case, the problem arises when the breeder wants to maximise and amplify certain traits. But life gets difficult if the plant rejects pollination by a plant with those traits because of its self-incompatibility mechanism. Breeders are prevented from developing hybrid ryegrasses, which in turn denies grassland farmers the benefits of hybrid vigour that have been demonstrated in other crops and types of livestock.

A European consortium of researchers from Teagasc in Ireland, the UK, Denmark and Switzerland has now uncovered the gene at one of the two main regions conferring self-incompatibility in perennial ryegrass. This research work has been reported in the scientific journal Molecular Biology and Evolution.

"Knowing the genetic nature of one of the two main determinants will enable us to now work towards more directed hybrids making use of the S and Z self-incompatibility system", says Dr Susanne Barth of Teagasc's Grasslands Research Programme.

This strategic research, she says, has been supported by Teagasc over the past decade by enabling two postgraduate students under the Teagasc Walsh Fellowship scheme to complete their PhDs on self-incompatibility and characterisation of the S and Z regions in perennial ryegrass.

“It is an exciting outcome,” Dr Barth adds, “and it will pave the way to simplifying and accelerating breeding procedures in a plant, with huge importance to the Irish bio-economy.”

This is only the beginning. “It will require a lot of work, of course,” she says. “We are working to identify the genes involved in a whole range of reproductive characteristics, and we hope to assemble a toolbox which will allow hybrid breeding to become a reality.”

Further progress in this critically important area for Irish agriculture has come from research into the perennial ryegrass genome, carried out by Teagasc in conjunction with the James Hutton Institute in Scotland. A paper on this research has been provisionally accepted by the leading international scientific journal Annals of Botany.

According to Dr Barth, the availability of this genome and of a complementing genome researched by a Danish team will help track down further reproductive genetic loci. It will also help push progress in dealing with complex traits, such as biomass yield in traditional breeding programmes.

Discovered in time

“The genome has come along just in time,” says

Dan Milbourne

, a senior research officer with Teagasc. “Just as the sequencing of the human genome has been useful in tackling certain genetic-based diseases, we can use the same approaches with perennial ryegrass.

“The genome is nowhere near the level of completeness of the human genome,” he says. “It is fairly big and only slightly smaller than the human genome. In fact, the wheat genome is five times the size of the human genome. What we have done is capture it in fairly large chunks, and this represents a huge advance on where we were. It will be of major assistance with gene discovery.”

That gene discovery is the most important aspect. It will allow scientists to identify the specific gene or genes responsible for the traits they want to breed in or out. This will obviate much of the trial-and-error processes in breeding programmes and allow for much more targeted approaches.

And, when combined with the work on self-incompatibility, the discovery could pave the way for genetic gains on the same scale as cereals – with obvious benefits to Ireland’s dairy and livestock sectors.