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PAG Plenary Points to Potential Strategy for Robust Pathogen Resistance in Crops

SAN DIEGO (GenomeWeb) – At a plenary presentation at the Plant and Animal Genomes conference here today, attendees heard about contributions that genetic and genomic research are making to establish more durable disease resistance in crop plants.

Plant pathologist Jan Leach, associate dean of research at Colorado State University's college of agriculture, discussed her team's search for genetic factors conferring long-lasting disease resistance in rice.

Such strategies are needed to protect crops from potential pests, since up to 40 percent of plant productivity in Africa and Asia is lost to pathogens, she noted. And troublingly, at least some single gene-based resistance strategies established in plants in the past have lost their effectiveness in as little as a few years due to pathogen adaptations or so-called gene breakdown.

As rice and other crop plants are faced with fungal, bacterial, or viral pathogens, Leach explained, the plants may martial two types of resistance.

Whereas single resistance gene, or R gene, based immunity tends to be relatively strong and largely specific to a given pathogen type, the pathogen-associated molecular pattern (PAMP) arm of the plant's defense response system appears to have broader but more subtle effects.

With that in mind, Leach and her team reasoned that it might be possible to achieve more durable, broad spectrum resistance in rice and other crops by exploring and, perhaps, tapping into plants' PAMP response pathways.

During her presentation, Leach offered an overview of several studies her team has done on that front, including studies of QTLs contributing to broad pathogen resistance in rice and other analyses of host-pathogen-environment interactions.

In a paper published in Plant Physiology in 2009, she and her colleagues described a chromosome 8 QTL related to broad-spectrum rice resistance that included a dozen so-called germin-like protein (OsGLP) coding genes contributing to resistance to the Magnaporthe oryzae fungus that causes rice blast disease.

The team's experiments suggest that when any of these OsGLP genes are silenced, the overall resistance declines in rice, supporting the notion that optimized broad spectrum resistance may augment plant responses to particular crop pests.

Even so, when the researchers looked at the genes that are up- and down-regulated during leaf blast and panicle blast in rice, they saw subtle differences in the sets of OsGLP genes that seem to respond to each of the two blast disease forms.

Likewise, Leach noted that some OsGLF alleles appear to be more effective than others at conferring resistance to particular pathogens, though field studies in rice plants grown over 11 years and 22 growing seasons suggest that optimized broad spectrum resistance genes can bolster panicle blast resistance in a durable manner.

Moreover, the team found clues that the same sorts of OsGLP-gene based pathogen resistance may exist in other important crop plants.

In the 2009 study, the group pointed out that "similarities of sequence, gene organization, and roles in disease resistance of GLP family members in rice and other cereals, including barley (Hordeum vulgare) and wheat (Triticum aestivum), suggest that resistance contributed by the [chromosome 8] OsGLP is a broad-spectrum, basal mechanisms conserved among the Gramineae [plant family]."

Leach also touched on results from more recent studies aimed at understanding and predicting the potential consequences of climate change on plant disease susceptibility and resistance.

Crop disease susceptibility is a concern as temperatures rise, she explained, since rice appears to have lost some of its resiliency against pathogens when exposed to higher-than-usual temperatures.

This may partly reflect a decline in broad spectrum pathogen resistance as the mercury climbs, Leach noted. And gene expression experiments suggest that at least some of the rice host genes that are dialed up by pathogen effectors show even greater expression jumps as the temperature ticks up.

Nevertheless, she said, at least one single gene resistance player seems to exhibit a blight response boost in the heat.

The team is interested in continuing to use genomic, transcriptomic, and other approaches to try to untangle relationships between host plant, pathogens, environmental factors, and other organisms that make up the plant's broader phytobiome community.