It's a long-term goal for researchers to precisely control genes to launch functional study and improving yield, quality and tolerance to stress in plants. The discovery of gene editing tools opens a new era for plant breeding.
With gene editing technology, plants are genetically modified for resistance to pathogen, insect and other organisms causing disease in plants. Compared with nuclear genome, chloroplast genome has advantages, including high-level expression of exogenous genes, muti-gene expression and avoiding damage from compound accumulation. Additionally, in most flowering plants, chloroplasts are not inherited from the male parent, which means transgenes can't be disseminated by pollen and will not bring potential genetic pollution.
Diseases caused by pathogen and insects will not only affect plant growth, but also reduce the yield and quality. It's been proved that chloroplast plays important role in plant immune system. By inserting exogenous genes into chloroplast genome resulted in their highest levels of expression, especially those gene related to tolerance and resistance.
Pathogen that causes diseases in plants includes fungi, oomycetes, bacteria, viruses, viroids, virus-like organisms, phytoplasmas, protozoa, nematodes and parasitic plants. By inserting genes involved in pathways of response to pathogen invasion into chloroplast of plant, tolerance to pathogen of plants can be improved.
Most phytopathogenic fungi belong to Ascomycota and Basidiomycota, which can be spread through water, air and soil. Biotrophic fungal pathogens colonize on plants tissue and obtain nutrients from plant, even kill the host plant. Thus, optimizing resistance to fungal pathogen in plants seems very important in plant breeding. Significant fungal plant pathogens and diseases it causes are listed as follows.
Ascomycetes
Basidiomycetes
Fungus-like Organisms
Such as oomycetes, including Phytophthora which causes potato late blight and sudden oak death, and Pythium cause root rot.
Bactria cause plant diseases by producing pathogenic factors like cell wall–degrading enzymes, toxins, effector proteins, phytohormones and exopolysaccharides.
Most plant viruses, viroids and virus-like organisms are asymptomatic or latent. Only some of them cause a loss of yield, such as bunchy top of banana and the upper leaves form a tight rosette.
Pests affect plant health by eating plant tissues, such as insects, mites, vertebrate or other pests. For example, sawfly and lepidopteran larvae eat leaves, and beetles larvae eat root.
Due to the rapid development of gene editing technology, chloroplasts of plants have been genetically modified with genes involved in pathogen responding. Here are pathogen tolerance related genes that have been studied in chloroplast genome.
| Genes | Insect or Pathogen Tolerance Traits |
|---|---|
| Bgl-1 | Resistance against whitefly and aphid |
| Pta | Broad-spectrum resistance against aphid, whitefly, Lepidopteran insects, bacterial and viral pathogens |
| PelB1, PelD2 | Resistance against Erwinia soft rot |
| RC1011, PG12 | Resistance to Erwinia soft rot and tobacco mosaic virus |
| cpo | Resistance to fungal pathogens in vitro |
| Bt cry2Aa2 operon | Resistance of cotton bollworm and beet armyworm |
| Bt cry9Aa2 | Resistance to Phthorimaea operculella |
| msi-99 | Resistance to fungal pathogens in planta |
| sporamin1, CeCPI2, and chitinase2 | Resistance against Spodoptera litura and Spodoptera exigua leaf spot, as well as soft rot diseases |
| MSI-99 | Resistance against rice blast fungus |
| cry1Ac | Resistance to larvae of Heliothis virescens, Helicoverpa zea, and Spodoptera exigua |
| cry1Ab | Resistance to caterpillar of Anticarsia gemmatalis |
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