Cross Kingdom Immunity: The Role of Immune Receptors and Downstream Signaling in Animal and Plant Cell Death

Abstract

Both plants and animals are endowed with sophisticated innate immune systems to combat microbial attack. In these multicellular eukaryotes, innate immunity implies the presence of cell surface receptors and intracellular receptors able to detect danger signal referred as damage-associated molecular patterns (DAMPs) and pathogen-associated molecular patterns (PAMPs). Membrane-associated pattern recognition receptors (PRRs), such as Toll-like receptors (TLRs), C-type lectin receptors (CLRs), receptor-like kinases (RLKs), and receptor-like proteins (RLPs) are employed by these organisms for sensing different invasion patterns before triggering antimicrobial defenses that can be associated with a form of regulated cell death. Intracellularly, animals nucleotide-binding and oligomerization domain (NOD)-like receptors or plants nucleotide-binding domain (NBD)-containing leucine rich repeats (NLRs) immune receptors likely detect effectors injected into the host cell by the pathogen to hijack the immune signaling cascade. Interestingly, during the co-evolution between the hosts and their invaders, key cross-kingdom cell death-signaling macromolecular NLR-complexes have been selected, such as the inflammasome in mammals and the recently discovered resistosome in plants. In both cases, a regulated cell death located at the site of infection constitutes a very effective mean for blocking the pathogen spread and protecting the whole organism from invasion. This review aims to describe the immune mechanisms in animals and plants, mainly focusing on cell death signaling pathways, in order to highlight recent advances that could be used on one side or the other to identify the missing signaling elements between the perception of the invasion pattern by immune receptors, the induction of defenses or the transmission of danger signals to other cells. Although knowledge of plant immunity is less advanced, these organisms have certain advantages allowing easier identification of signaling events, regulators and executors of cell death, which could then be exploited directly for crop protection purposes or by analogy for medical research.

Foliar application of liquiritin protects Chinese flowering cabbage against cucumber mosaic virus and increases health-promoting compounds

“…Both plants and animals are endowed with a conserved innate immune system able to neutralize pathogens and to contain the infection (Roudaire et al 2021). The essence of both plant and animal immunity is the recognition and protection against the foreign.…”

Can Plant Lectins Help to Elucidate Insect Lectin-Mediated Immune Response?

“…TLRs, the mammalian orthologs of the Drosophila immune receptor Toll, are important PRRs of mammalian cells that can be targeted by plant lectins. In vertebrates, 13 TRLs with different immune functions have been identified [131]. For example, cell-surface TLRs, including TLR1, 2, 4-6, and 10, recognize microbial PAMPs such as lipoproteins, lipids, and other microbial membrane fractions, while the endosomal TLRs, i.e., While the knowledge of the endogenous insect lectins is lagging, the knowledge of the molecular effects of plant lectins might be useful in elucidating the functions and modes of action of insect lectins in the immune system.…”

“…In vertebrates, 13 TRLs with different immune functions have been identified [131]. For example, cell-surface TLRs, including TLR1, 2, 4-6, and 10, recognize microbial PAMPs such as lipoproteins, lipids, and other microbial membrane fractions, while the endosomal TLRs, i.e., TLR3, 7-9, have been suggested to recognize the invading nucleotides from viruses and bacteria [131].…”

“…These TLRs are heterodimerized, which clearly improved host-cell sensitivity to PAMP stimuli such as LPS [132,135]. In addition, Con A induced humoral immunity through activation of the TLR-NF-κB pathway [131]. Some chimeric lectins have been shown to stimulate complex cellular responses.…”

“…First of all, in mammalian systems, plant lectins have been analyzed in more detail for their immunomodulatory effects, especially regarding receptors on the cell membrane and in the cytosol, in both the cellular and humoral immune responses [136,[141][142][143]. In addition, the signaling pathways behind these responses have been extensively studied in vertebrate models [131]. This knowledge can inspire the identification of orthologs in insects.…”