Plant pathogens have been a serious challenge to crop production worldwide. External use of chemicals to fight crop diseases is now considered to be detrimental to human health and the environment. Encouragingly, genetic engineering now provides a more sustainable and desirable alternative to boost plant disease resistance.
Do plants have their own immune system?
One of the ways plant immune system acts is by the production of a small phenolic compound salicylic acid (SA). Plant’s pattern-triggered immunity (PTI) is based on the recognition of “microbe-associated molecular patterns (MAMPs) which stimulates SA production.
SA-activated immunity has been extensively studied in the model plant Arabidopsis. Here, a protein called “non-expressor of pathogenesis-related genes 1 (NPR1) serves as a master regulator of such immune response.
Normally, NPR1 protein molecules reside in the cell cytoplasm as oligomers. In response to SA-induced redox changes the protein is liberated from its quiescent oligomeric state. Subsequently, the monomers translocate into the cell nucleus.
In the nucleus, NPR1 serves as a coactivator in complex with a transcription factor, TGA. The NPR1-TGA complex is responsible for the expression of the pathogenesis-related 1 (PR1) gene. As a favorable consequence of PR1 gene expression, the plant is able to counteract the pathogenic challenge.
Boosting plant disease resistance
Most often, plant’s own immune system is not sufficiently robust to counter pathogenic onslaught. So, it is imperative to empower the plants with enhanced and durable resistance against a broad range of pathogens. Expressing antimicrobial peptides (AMPs) in transgenic plants has been found to be an effective approach to boost plant disease resistance.
AMPs belong to the innate immune system of a multitude of animals and plants. Most of them act by lysing the membrane of invading microorganisms. Two of the well-know AMPs are cecropin and melittin. Cecropin A was originally isolated from the hemolymph of the giant silk moth, Hyalophora cecropia. Melittin was isolated from bee venom.
When expressed in plants, any of these AMPs increases host resistance against bacterial infection. Interestingly, a cecropin A-melittin hybrid peptide (CEMA) turned out to be the most effective.
Nevertheless, uncontrolled expression of an AMP may have an adverse effect on the growth and yield of the crop. If the gene is regulated by SA-NPR1-TGA complex, the AMP is expressed only when the plant is invaded by pathogens.
How are genes transferred to plant cells?
The most widely used strategy for gene transfer to plant cells relies on the soil phytopathogen, Agrobacterium tumefaciens. The bacterium infects wound sites in a variety of plants and causes tumorigenic transformation known as crown gall disease.
Plant tumorigenesis is caused by only virulent Agrobacterium strains. Such strains harbor large tumor-inducing plasmids (Ti-plasmids). A short segment of the Ti-plasmid, called T DNA (transferred DNA) carries oncogenes required for tumorigenesis. The T-DNA is transferred from the infecting bacterium to the host plant cell. Here, it integrates at an apparently random site of the host genome.
Notably, genes that the T-DNA carries are non-essential for the transfer mechanism and can, therefore, be replaced with foreign DNA. Transfer-related genes are located in a separate segment of the Ti-plasmid called the vir (virulence) region.
It is evident that the Ti-plasmid is a natural vector for plant genetic engineering. However, to regenerate plants efficiently, its T-DNA needs to be “disarmed” by deleting all the oncogenes. Such a disarmed T-DNA, carrying the CEMA gene expression construct, can remain stably integrated in the plant genome and express the hybrid AMP. As reported, up to four copies of the transgene are integrated at different sites of the host genome. Evidently, a high level of CEMA production in the transgenic plant cells can be expected.
Can plant disease resistance be further boosted?
Additional resistance against even a broader range of pathogens can be conferred on plants by the inclusion of a jasmonic acid (JA)-responsive regulatory element in the transgene construct. JA is another hormone synthesized in pathogen-invaded plants. Enabled by JA-signaling, the concerned responsive element in the plant genome recruits a transcription factor MYC2. This leads to the expression of another set of plant defense genes.
As expected, a composite of SA- and JA-responsive elements is able to recruit both TGA and MYC. The use of such a composite to drive the expression of a hybrid AMP can further boost plant disease resistance.
Commercialization prospects?
Enthusiasm of scientists and protagonists notwithstanding, commercialization of transgenic crops has been perceptibly slow. Controversies over genetically modified (GM) food, plant or animal, persist and, certainly, are not going to die down. The arguments, in favor or against, only reverberate the entire history of technological developments which came about weighing intended benefits against possible hazards. The momentum towards GM food is most unlikely to stop – this is a reality. What are needed alongside the forward movement are unwavering vigilance and judicious regulations.