Creating A New Plant Immune System: Pikobody

Plants lack an adaptive immune system and therefore rely on innate immune receptors to detect invading pathogens. Recombination of the immune system is largely limited to the modification of natural components to produce new biologically active substances. Although these approaches have achieved promising results, these modifications rely on substances isolated from pathogens, making it difficult to be broad-spectrum. In addition, pathogenic bacteria evolve very rapidly, making it difficult to develop resistance. Therefore, there is a great need for an adaptive immune system in which bioengineering can be performed on demand to generate genotypes against pathogenic bacteria.

A class of immune proteins that are very suitable as templates for transformation in plants is the intracellular nucleotide-binding, leucine-rich repeat immune receptors (NLRs) protein with unconventional domains. These domains usually affect the recognition of pathogenic bacteria through direct or indirect means, thereby affecting the immune response. These NLR proteins with unconventional domains usually need to combine with conventional NLR proteins to exert immune activity.

Recently, Sophien Kamoun's research group at the University of East Anglia published an article entitled "NLR immune receptor–nanobody fusions confer plant disease resistance" in Science. Inspired by the animal's humoral immune system, which can produce a variety of antibodies against exposed pathogens, the authors focused on the smallest antigen-binding fragment of a single-domain heavy-chain antibody. Such fragments normally fold normally within cells and have many useful properties for biotechnological applications.

To test their conjecture, the authors constructed orthogonal Pik-1 sensors in which the integrated heavy metal-associated (HMA) domain was swapped with green fluorescent protein (GFP)- or mCherry-binding nanobodies. In the absence of ligand, mutations in the Pik-1 HMA domain often lead to autoimmune activities, like immune signaling in response to effector recognition, which is dependent on the presence of Pik-2. Tobacco experiments confirmed that the Pikm-1 nanobody fusion is functional and can be endowed with new natural activities. The authors coined the term Pikobody for the combination of Pikm-2 and Pikm-1 nanobody fusions. After modifying its core part, Pikobody can be inactivated. Pikm-2 is the core of Pikobody's function.

To confirm that Pikobody can produce immune responses against pathogenic bacteria, the authors constructed Pikobody-enhanced and inactivated materials, respectively. Through a series of experiments, it was confirmed that Pikobody does have disease-resistant activity. And the accumulation of Pikobody may enhance the recognition and response of plants to pathogenic bacteria without autoimmunity. Finally, the authors constructed Pikobody overexpression material, which has specific resistance to Potato virus X (PVX) similar to that of the R gene.

In summary, based on the NLR disease resistance system, the author transferred the animal immune system mechanism to plants and created a new plant immune system, Pikobody. Although this immune system still has some limitations, such as the need for pathogenic bacterial proteins to be delivered into the plant to be recognized by the system, the system can generate specific resistance against any pathogen or pest and has very wide applications and research significance.

Reference:

Kourelis, J.; et al. NLR immune receptor–nanobody fusions confer plant disease resistance. Sicence 379(6635), 934-939 (2023).