GM crops: new directions

It’s 2014, and grain farmers in South Australia, Tasmania and the ACT are still forbidden to plant genetically modified, herbicide-tolerant (GMHT) canola.

Australia’s GM crop revolution has stalled, after an auspicious debut in 1989 when South Australia became the first state in the world to release a genetically modified organism into the environment.

A quarter of a century later, only two GM crops are grown commercially in Australia: herbicide-tolerant canola and pest-resistant cotton. With such a modest record of uptake, whither GM crops in Australia?

Roger Hellens, Professor of Agricultural Biotechnology at the Queensland University of Technology, is an invited speaker at the Agriculture and Food Technology Symposium that forms part of this month’s (29-31 October) AusBiotech 2014 Life Science Conference at the Gold Coast Convention and Exhibition Centre. He has titled his talk ‘New emerging technologies - benefits and challenges’.

Hellens believes GM crops still have a bright future in Australia, but the next generation of GM crops will focus on traits providing health benefits to consumers, rather than agronomic benefits to farmers.

And they will be created with new molecular tools that permit molecular breeders to achieve big changes from minimal modifications to plant DNA, without disrupting crop genomes.

Hellens says the new tools and techniques are narrowing the gap between conventional and molecular plant breeding, so in theory, they should be an easier sell to consumers.

But he suggests it would be naive to believe the anti-GM movement will retreat from its implacable opposition to all GM crops, irrespective of how they are produced, or the efforts made to make very subtle changes to crop genomes, that merely replicate gene variants - alleles - that already occur in nature.

“They just don’t like the technology,” he said. “We can address most of their concerns, but they have an unfathomable opposition to all GM crops,” he said.

Hellens says governments responded to the anti-GM paranoia of the past 25 years by creating complex regulatory schemes to reassure consumers that GM crops were no threat to human health or the environment.

But some new molecular breeding techniques veer so close to conventional plant breeding that any attempt to bring them within current regulatory nets could end up entangling advanced conventional breeding techniques that have been in use for decades.

They could impede long-established breeding practices such as the use of ‘wide crosses’ to import germ plasm from distant wild relatives, or from natural field mutants that have arisen under selection pressure from herbicides or pathogens.

Hellens says that in some jurisdictions, existing regulatory regimes may not extend to novel  techniques like in situ editing of genes to create useful new alleles.

Rapid, inexpensive next-generation sequencing has made these new approaches feasible, allowing researchers to zero in on single-nucleotide polymorphisms that give rise to useful developmental or metabolic traits.

Hellens said the use of Arabidopsis as a model for identifying genes that could be transferred in to crop plants was laborious. The simple Arabidopsis phenome - the sum total of all the species’ phenotypic traits - was too small to represent the more complex phenomes of crop species, with their suites of genes.

“Next-generation sequencing makes it easy to relate traits to variations in sequence data from the transcriptomes of crop species,” Hellens said. “It allows you to focus on traits of importance to particular crops.

“You can then look for production traits or consumer traits in a much more targeted way. It’s been a real change, being able to offer traits that industry is more interested in.

“In the past, it was researchers trying to tell companies which traits they should commercialise.”

Hellens says plant biotechnology is a “fascinating” field to work in. “What people care about is the product, but they tend to be hung up about the way the product is made.

“In just about every country except Canada, it’s the technology that is regulated, not the trait. Most technologies can be used to advantage.

“The irony about regulating the trait is that the technology is generic.

“People are more comfortable with modern breeding techniques than they are with the GM approach, even though modern breeding is quite interventional.

“With the GM approach, there have been important developments in our ability to edit genomes in ways that were just not possible a few years ago. We can now go in and make precise changes to the DNA of a crop species, without affecting other parts of the genome.

“These changes are informed by new knowledge of the allelic diversity in already familiar germplasm.

“We now have the ability to change the DNA of a crop plant to replicate a desirable allele identified in a wild relative. With conventional breeding, it can take years of hybridisation and back-crossing to import an allele of interest, and it usually comes along with linked haplotypes that can be detrimental to what you’re trying to achieve.”

Hellens says the acetolactate synthase (ALS) gene from Arabidopsis is a case in point - a single point mutation in the ALS gene confers resistance to the herbicide chlorsulfuron urea. The gene is highly conserved across dicots and monocots, including rice.

‘Tweaking’ the wild-type gene in highly productive cultivars such as wheat or rice, to re-create the spontaneous mutation in Arabidopsis, would result in elite cultivars that could be sprayed with low concentrations of chlorsufuron urea to eliminate weeds.

“If an advantageous allele for a trait exists in nature, modern molecular breeding techniques like oligo-directed mutagenesis can now be used to replicate it in commercial cultivars of crop species,” Hellens said.

Hellens’ own research in recent times has focused on the biosynthesis pathways in fruit that produce flavonoid compounds and vitamin C, both potent antioxidants with potential health benefits for humans.

But his team’s work to increase vitamin C concentrations in fruit has a broader goal: dietary vitamin C is crucial to the body’s ability to absorb iron. “Increasing vitamin C in plants doesn’t affect the concentration of iron in the plant, but it does enhance the body’s ability to absorb iron,” Hellens said.

“Around 1.6 billion people around the globe are chronically anaemic because they don’t absorb enough iron from their diet. Elevating vitamin C concentrations in the diet could help correct iron-deficiency anaemia.”

Hellens says his team’s research has determined not only how vitamin C is made in fruit, but how its production is regulated.

Hellens says his team’s studies of kiwifruit, which has 50 times the vitamin C of oranges, and Australia’s Kakadu plum, Terminalia ferdinandiana, which has the highest concentration of vitamin C of any fruit in the world - 200 times more than oranges - have revealed that a single nucleotide substitution in the regulatory region of an enzyme involved in vitamin C biosynthesis changes one peptide in the enzyme, resulting in a very large increase in vitamin C concentration.

Vitamin C from fruits is more readily absorbed by the human gut than vitamin C in the form of ascorbic acid tablets, sold by chemists.

“Although we’re only at the preliminary stages of the project, because we now understand the mechanism involved, we have the tantalising prospect of elevating vitamin C concentrations in other plants in the human diet,” Hellens said.

“We could prevent anaemia and keep people alive and healthy by editing the genomes of important commodity crops to replicate naturally occurring alleles in high-vitamin C species like kiwifruit, Kakadu plum, Amazonian fruits like acerola and camu camu, or the Indian gooseberry,” Hellens said.

Targeted methylation or demethylation of genes, and homologous recombination, are among future tools for manipulating the activity of genes in situ in crop plants.

Hellens says it is already possible to demethylate genes, but selectively silencing genes by target methylation is not yet feasible - however, it may be possible, by demethylating repressor genes, to activate dormant downstream genes, to achieve gain-of-function traits, Hellens said.

Modifying genes in situ by homologous recombination has been used for years to create transgenic animals, but researchers have yet to find a way of making it work reliably in plants.

“I once drew an analogy between modern molecular genetics and toolmaking: all primitive tools were developed to join things, or separate them - the same is true of most of the power tools  in a modern garage,” Hellens said.

“Most of the things we do involve breaking or cutting DNA, and sticking it together again. Some of the emerging tools of molecular genetics are very good at making precise cuts, and excising DNA sequences with precision.

“But we’re not particularly good at sticking things back together again. It’s particularly true for plant DNA.

“I believe that, in the future, we will develop a better understanding of the mechanisms involved in homologous recombination.

“When conventional plant breeders introduce a gene into a crop, and back-cross, they are doing basically the same thing - double-strand breaks are required for the recombination events that lead to the hybrid progeny, so you’re further blurring the differences between molecular genetics and conventional breeding.”

Hellens says that for virtually every trait of importance in crop plants, there will be a pool of genetic diversity within the crop and its wild relatives to improve it. Rather than install the allele of interest, the preferred approach will be to use homologous recombination to ‘overwrite’ the gene in situ in the crop plant - the trait might affect yield, disease resistance, drought tolerance, dwarfing - “We know that lots of alleles exist in the wild, and we can use them,” he said.

“Many traits are multigenic, but I don’t believe any trait is so complex that it is not amenable to molecular dissection.

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And the first shall be last

In 1989, Australia became the first nation to release a genetically modified organism (GMO) into the environment with the historic release of a modified strain of the crown gall bacterium Agrobacterium tumifaciens.

Professor Alan Kerr’s modified strainprev, marketed around the world as NoGall, suppresses pathogenic strains of the same microbe infecting young stone and pome fruit trees. It was released at a ceremony at the Waite Agricultural Research Institute in South Australia.

South Australia led the world into the age of GM agriculture, yet today is the only mainland state that maintains a moratorium on genetically modified canola, and all other GM crops - Tasmania and the ACT also maintain their bans on GM canola.

SA’s moratorium has the support of all major and minor political parties in the state, and the current ALP government has no plans to lift it before 2019.

NSW and Victoria ended their four-year moratoria in 2008; WA followed suit, belatedly, after a change of government in 2010. GMHT canola is still banned in Tasmania and the ACT.

The only other GM crop commercialised in Australia is pest-resistant cotton, which is grown in NSW and southern Queensland, and accounts for 95% of Australia’s cotton production.

Cotton was the first GM crop to be commercialised in Australia, in 1996. Today’s varieties carry two transgenes for insecticidal toxins from the soil-dwelling bacterium Bacillus thuringiensis, which are highly lethal to destructive caterpillars of Helicoverpa moths. Some cotton varieties carry an additional gene that confers tolerance to the broad-spectrum herbicide glyphosate.

Many other GM crops or pasture species are in development, in field trials or are awaiting commercial release. They include the grain crops wheat, maize and barley; the oilseed crops canola, Indian mustard and safflower; pasture species like perennial ryegrass, tall fescue and white clover; sugarcane and horticultural crops including banana, papaya, pineapple and grapevines.

Image credit: ©Pencho Tihov/Dollar Photo Club