Researchers from the Sainsbury Laboratory in Norwich present a preliminary proof-of-principle study, detailing how features of the animal adaptive immune system can be bioengineered to protect plants from pathogens. The hybrid fusion proteins, deemed Pikobodies, capitalise on the flexibility of the tailored antibody response, conferring a new-to-nature plastic immune response in plants.
Animal immune systems are bolstered by an adaptive immune response, through which antibodies are constantly synthesised against emerging antigens. In contrast, plant immune systems are fundamentally limited by the gene for gene mechanism through which virulence genes are silenced by innate R genes. Due to the rapid rate of evolution of pathogens, plants are handicapped in the genetic arms race against pathogenic invaders and lack the adaptability to keep pace with their assailants. Previous crop improvement studies often focus on increasing the R gene arsenal within crops, however Jiorgos Kourelis and his team approached the problem from an alternative angle.
It was previously established that nucleotide-binding leucine rich repeat immune receptors (NLRs) often work in pairs to recognise a feature of an invading pathogen and initiate PAMP triggered immunity. In rice, an NLR called Pik-1 binds Avr-pik proteins secreted by the blast fungus Magnaporthe oryzae and stimulates the oligomerisation of Pik-2. A key finding demonstrated that Pik-1 contains an unconventional heavy metal associated (HMA) integrated domain, which mediates pathogen detection. Thus, scientists were excited at the prospect of using Pik-1 as a scaffold for alternative integrated domains, tailoring the recognition system to any pathogen. By a stroke of ingenuity, researchers from The Sainsbury Laboratory engineered a novel Pikobody system by inserting a nanobody, a small antigen binding domain from an antibody, into the integrated domain of Pik-1.
Incredibly exciting in theory, the bioengineered tool needed to be tested for efficacy and safety. First, different fusion proteins, responsive to fluorescent proteins, were checked for autoimmune activity; those which did not induce an autoimmune response in the absence of the ligand were progressed. The Pikobodies were then expressed in the model plant Nicotiana benthamiana and treated with transgenic Potato Virus X, expressing either GFP or mCherry. Using fluorescence as a proxy for viral load, it was evident that the Pikobodies were successfully recognising the novel fluorescent protein on the virus and mounting an immune response. Additionally, as in natural effector- triggered immunity, hypersensitive immune responses were identified by patches of cell death on the leaves.
Whilst this finding requires substantiation in the field, the initial finding that Pikobodies can confer resistance to pathogens is monumental. The ability to adapt to the ever-changing threat posed by plant pathogens holds potential to diversify the recognition profile and increase the robustness of plant immune systems. Furthermore, it may act as an overlapping mechanism to reinforce the existing zig-zag pattern of plant immunity.
The impact of this innovation is immeasurable; once translated into agricultural systems, it could fortify plant defences against the growing threat of plant diseases. It is estimated that yield losses due to plant diseases amount to an annual cost of $220bn, and severely exacerbate global food insecurity. Due to climate change, the threat of plant disease is only predicted to grow as the distribution of diseases and their vectors expand. For example, Potato Late Blight, caused by a fungus which typically causes rot and lesions in potatoes and tomatoes, is adapting to warmer and wetter conditions and extending its range into Egypt. On top of this, the pseudo-adaptive immunity conferred by Pikobodies may counteract the extreme vulnerability of monocultural crops to devastating epidemics.
Therefore, this creative approach to agricultural bioengineering may reveal a novel toolkit for plant defence and have profound global impact for agricultural systems and food security. Follow up proof-of-application studies are ongoing, and the researchers remain optimistic that Pikobodies could challenge modern genetic applications and transform our view of plant immunity.