## 1 Introduction  
The highly dynamic epidemiological environment coupled with changing viral pathogens requires constant research on those emerging, their diagnosis, and prevention. The COVID-19 pandemic highlighted the demand for a rapid response from pharma companies to effectively produce large amounts of compounds to protect, treat, and diagnose new viral diseases. This global crisis has accelerated scientific advancements, facilitated cross-disciplinary cooperation, and enabled an agile advancement of technology [1,2]. Recombinant proteins for pharmaceutical and medical applications can be produced using a variety of expression systems, including bacteria, yeast, plants, insects, or animal cells [3,4]. These expressions, \"biofactories\", are instruments for a rapid response to pandemic threats and the production of vaccines and other biopharmaceuticals [5, 6, 7]. Several platforms for producing therapeutic and diagnostic proteins are based on plants and plant cells [8,9]. In recent years, plant-based expression systems have emerged as a promising alternative due to their unique advantages, such as cost-effectiveness, scalability, and the ability to produce large quantities of proteins safely and swiftly during viral outbreaks [10, 11, 12]. These plant \"biofactories\" are increasingly recognized as competitive platforms for producing valuable pharmaceuticals, critical for therapeutic and diagnostic applications targeting SARS-CoV-2 and other emerging viruses [13]. During the SARS-CoV-2 pandemic, the development of efficient and sensitive serological tests for monitoring the dynamics of the disease as well as the immune response after illness or vaccination was critical. Since the spike (S) and nucleoprotein (N) of SARS-CoV-2 are the most immunogenic proteins, serological testing usually identifies antibodies against these two proteins [14,15]. The S protein is composed of two subunits, S1 and S2, and is used by the virus to attach itself to the ACE-2 (angiotensin-converting enzyme 2) host cell receptor [16]. The S1 contains the surface-exposed receptor binding domain (RBD) that specifically recognizes the host cell receptor to gain entry into the cell and fuse the viral and lysosomal membrane [17]. The N protein is responsible for wrapping virus RNA and forming the ribonucleoprotein complex (RNP), and it has been highly expressed during infection, inducing early humoral and cellular response [18]. N is a 45.6 kDa phosphoprotein composed of two domains, the N-terminal domain (NTD) and the C-terminal domain (CTD), linked with intrinsically disordered amino acid stretches [19]. N protein is highly conserved, with 89.7% homology to the N of SARS-CoV-1 [20, 21, 22]. N protein is abundantly expressed during infection, inducing early humoral and cellular immune responses [23]. Antibodies against the N protein are intensely generated during infection, and even though they lack neutralizing activity, they may be used for serological diagnosis [24,25]. It has been proposed that antibodies targeting the S protein are more specific and an accurate indicator of protective immune response, while those against N are more sensitive, at the expense of specificity (the N protein of SARS-CoV-2 has high protein sequence similarity with the other _Coronaviridae_ nucleocapsid proteins [26]). As a fusion protein or alone, RBD was produced in plants with a maximal purifying yield of approximately 40 \(\upmu\)g per gram of fresh leaf tissue [27; 28; 29]. To increase the yields and stability of RBD, RBD was fused to the truncated capsid protein of the hepatitis E virus at the Tyr485 position [30]. The capsid protein of HEV can self-assemble into virus-like particles (VLPs) when expressed in _Nicotiana benthamiana_ plants and can be used as a carrier of immunogenic proteins [31; 32]. Further, we applied this strategy to target the fused HEV/RBD protein to the endoplasmic reticulum (ER), which can influence their expression efficiency, stability, and solubility [33].