**10. Transgenic plant expressing viral resistance**

Apart from their antifungal or antibacterial effects, PR proteins appear to be a promising candidate gene for producing virus-resistant transgenic crops based on *Pathogenesis-Related Proteins and Their Transgenic Expression for Developing Disease-Resistant… DOI: http://dx.doi.org/10.5772/intechopen.106774*

different studies of PR proteins, as given in **Table 2**. Antiviral activities of PR proteins such as defensins, thionins, peroxidase and lipid transfer proteins have been observed *in vitro* [115]. Antiviral activity has also been observed in ribosome-inactivating proteins (RIPs), which suppress translation by enzymatically damaging ribosomes [115]. Plant resistance to plant viruses was improved by a transformation study involving RIPs. In addition, CaPR10 from *Capsicum annuum* has been found to have increased ribonucleolytic activity against the Tobacco mosaic virus (TMV) RNA, allowing it to break viral RNAs [116].

### **11. Synergistic effect of transgenic PR proteins**

In transgenic plants, the synergistic action of two or more PR genes reduces susceptibility to various pathogens. Researchers have reported that *β-1,3-glucanases* and *chitinases* synergistically inhibited the growth of *Fusarium oxysporum* by using *in planta* transformation [135]. Transgenic potato plants co-expressing chitinase (*BjCHI1*) and β-1,3-glucanase (*HbGLU*) suppressed *Rhizoctonia solani* and showed healthier root growth [70]. In another study, transgenes carrying the chitinase gene (*chi11*) and the thaumatin-like protein gene (*tlp*) from rice were introduced by co-bombardment, and overexpression of these antifungal *chi* and *tlp* proteins provided resistance to fungal infections in barley [136]. Likewise, in transgenic carrots, the synergistic action of three different PR-protein genes such as chitinase, β-1,3 glucanase and peroxidase, conferred disease resistance to necrotrophic pathogens namely, *Botrytis cinerea* and *Sclerotinia sclerotiorum* [137]. Amian *et al* [138] reported the development of transgenic pea plants with stable integration of two genes *viz* β-1,3-glucanase (*Hordeum vulgare*) and chitinase gene (*Streptomyces olivaceoviridis*) via *Agrobacterium*-mediated gene transformation and hence produced suppression of fungal spore germination. Chhikara *et al* [139] used *Agrobacterium*-mediated transformation to co-express the barley antifungal genes chitinase and ribosomeinactivating protein in Indian mustard, protecting against Alternaria leaf spot disease. Furthermore, transgenic potato plants expressing *rip30* and *chiA* genes transformed by *A. tumefaciens* strain *GV3101* showed improved resistance to *Rhophitulus solani* [140]. In the case of Oriental melon (*Cucumis melo* Makuwa Group), the fusion of chitinase (*CHI*) and antifungal protein (*AFP*) genes confers enhanced protection against *Rhizoctonia solani* and *Fusarium oxysporum* [141]. Rice plants co-transformed with chitinase (*OsCHI11*) and oxalate oxidase (*OsOXO4*), which are defense-related genes, showed improved resistance to the pathogen that causes sheath blight [142]. Boccardo *et al* [143] suggested co-expression of PR proteins *AP24* and *β-1,3 glucanase* enhanced resistance against *Rhizoctonia solani* in greenhouse conditions and *Peronospora hyoscyami* f.sp. *tabacina* and *phytophthora nicotianae* pathogens in field conditions.

### **12. Challenges faced by transgenic expression with PR proteins**

Since the advent of plant genetic engineering, PR proteins have consistently been the top choice among scientists when creating transgenic plants to increase disease resistance against a variety of diseases. PR proteins expressed either singly or synergistically in transgenic plants can provide broader and more effective disease resistance against different pathogens as described above.

Aside from these successful outcomes, many studies have described the challenges of using PR proteins in transgenic technology. In contrast to the above findings, numerous studies have suggested that the transgenic expression of PR proteins did not lead to increased tolerance to pathogens. Szwacka *et al* [144] reported no relationship between transgenic protein expression level and increased tolerance against the pathogen. Transgenic cucumber plants with stably integrated thaumatin II cDNA under the control of the CAM35S promotor via *Agrobacterium* did not exhibit tolerance to *Pseudoperonospora cubensis*. Moravckova *et al* [145] co-introduced chitinase and glucanase into *Solanum tuberosum* to increase resistance to *R. solani* infection, but hyphal extension assay revealed that transformants did not affect *R. solani* growth in vitro.

Various transgenic plant modifications have been described, with varying degrees of protection against certain fungal and oomycete infections. However, the resulting resistance levels were frequently insufficient for breeding [146]. Furthermore, constitutive expression of PR proteins can lead to the spontaneous production of lesions that look like HR lesions in the absence of a pathogen), which can be an unfavorable outcome [147]. Disease resistance techniques must control specific diseases without affecting crop yield and quality.

Moreover, most researchers have used constitutive promoters to control the expression of PR genes in agricultural plants to enhance resistance, resulting in homology-dependent gene silencing. As a result, unregulated and untimely activation of PR genes or AMPs harms plant growth and development. Human allergenicity is one of the main issues hindering the success of transgenic technology with PR genes. According to the current classification, there are 19 different classes of PR-Proteins, and 8 of them have been confirmed to cause allergic reactions in humans by using *in-silco* approaches. These proteins have been known to trigger allergenic symptoms such as food allergens depending upon their mode of entry into the human body [148], dermatitis, airborne, asthma, airway allergy etc. and if all these allergens have been consumed in greater amount, the gastrointestinal symptoms are also triggered.

### **13. Conclusion**

The goal of this chapter was to review the role of PR-proteins in plant defense and how transgenic expression of PR-proteins in agricultural plants resulted in increased resistance to stresses. Biotic stress has become a significant concern in modern agriculture and many research institutions are actively researching to generate resistant cultivars using PR proteins. PR proteins have become a highlighted topic between scientists because of their effectiveness against biotic agents. Genetic engineering is considered the best way to develop transgenic resistant plants using PR proteins. To increase agronomic characteristics worldwide, new inventions or novel approaches in PR protein transgenic technology are necessary and will continue to improve plant health in the future. Another future concern is that the formation of virulent phytopathogen strains increases as the global climatic change rate increases. So, to cope with such significant obstacles, it is necessary to define and identify novel PR genes functionally. Advances in genomics, transcriptomics, phenomics, proteomics, metabolomics, and ionomics, will substantially aid our understanding of the complex network of PR genes and the interaction of PR proteins with other proteins from plants and pathogens. Therefore, PR proteins could be utilized to develop crop plants

*Pathogenesis-Related Proteins and Their Transgenic Expression for Developing Disease-Resistant… DOI: http://dx.doi.org/10.5772/intechopen.106774*

more resistant to various stresses. They could also be employed as candidate genes for genetically engineering crop multi-trait factors. Future research is needed to assess the PR transgenic plants' responses to various traits, including biotics, plant development and yield.
