Biotechnology provides indispensable tools for plant protection. Plant cell cultures, in vitro selection for resistance, embryo rescue, protoplast fusion, double haploids and other techniques have advanced resistance breeding long before genetic engineering became feasible in crop plants. Although these techniques are still being used, plant biotechnology is often understood as a synonym for the use of genetically engineered (GM) crops, which will be the focus of this presentation. Many strategies for genetic engineering of crops for resistance against pathogens exist but only few were developed to maturity. Production of enzymes digesting fungal cell walls in GM crops was historically the first strategy, inspired by natural defence of plants. Antimicrobial peptides followed with a ramification to peptides from sources that might frighten consumers (scorpions) and to non-enzymatic effects of enzymes (lysozyme). Fusion proteins consisting of antimicrobial peptides and pathogen-specific antibodies attracted considerable attention, promising to prevent Fusarium head blight in wheat.
Enzymatic detoxification of fungal virulence factors was coined in the 1980th in Japan, targeting fusaric acid. Although fusaric acid acts as a virulence factor in many diseases, in planta detoxification as a resistance strategy has yet to be demonstrated. Detoxification was however exploited successfully against pathogens producing oxalic acid and turned out to be one of the most efficient GM-based resistances available against fungi, entering patent portfolios of major breeding companies. The detoxification of mycotoxin deoxynivalenol was shown to protect grain crops from Fusarium infection. First fungal genes were used; the industrial development of GM wheat resistant to Fusarium failed due to a wrong choice of the gene. More recently genes of plant origin have been employed for the detoxification of deoxynivalenol.
Apart from rendering crops resistant to fungal infection, detoxification of fungal metabolites may improve food safety status of crops when the targeted compounds are mycotoxins. A detoxification strategy aiming at the reduction of exposure to mycotoxin fumonisin was developed by a major seed company to maturity but the GM varieties have not been commercialized, apparently because of concerns about the effect of negatively loaded keywords "GM" and "mycotoxin" on public perception. A surprising effect of a strategy targeting maize pests was discovered in Iowa and confirmed in many countries since: GM maize producing Bt protein, protecting the plant from pests, benefit from significantly reduced content of fumonisins and other mycotoxins.
The most recent strategy of genetic engineering of crops for resistance is transkingdom gene silencing by RNA interference. The efficiency is surprisingly high but there are concerns about the stability of the strategy because fungal mutants impaired in RNA interference are supposed to break the resistance fast. Finally, CRISPR/CAS equipped plant biotechnology with the capability to edit plant genomes in situ in a precise way, offering new options for the enhancement of plant native defence especially against biotrophic pathogens. While in Europe irrational fears and anti-GM lobbies essentially prevented applications of GM in plant production, plant biotechnology will continue advancing plant production in North America, Africa and on the Asian continent.
Take Away Notes:
• Plant biotechnology is more than genetically modified crops.
• Which strategies are available to make crop plants resistant to fungal pathogens?
• Apart from improving resistance to fungal diseases, can plant biotechnology improved food safety?
• Why are food products made from GM maize varieties resistant to pests (so called Bt-maize) healthier than products from non-GM maize?
• How can RNA interference help creating disease-resistant crops?