An Overview of PRR- and NLR-Mediated Immunities: Conserved Signaling Components across the Plant Kingdom That Communicate Both Pathways

Abstract

Cell-surface-localized pattern recognition receptors (PRRs) and intracellular nucleotide-binding domain and leucine-rich repeat receptors (NLRs) are plant immune proteins that trigger an orchestrated downstream signaling in response to molecules of microbial origin or host plant origin. Historically, PRRs have been associated with pattern-triggered immunity (PTI), whereas NLRs have been involved with effector-triggered immunity (ETI). However, recent studies reveal that such binary distinction is far from being applicable to the real world. Although the perception of plant pathogens and the final mounting response are achieved by different means, central hubs involved in signaling are shared between PTI and ETI, blurring the zig-zag model of plant immunity. In this review, we not only summarize our current understanding of PRR- and NLR-mediated immunities in plants, but also highlight those signaling components that are evolutionarily conserved across the plant kingdom. Altogether, we attempt to offer an overview of how plants mediate and integrate the induction of the defense responses that comprise PTI and ETI, emphasizing the need for more evolutionary molecular plant–microbe interactions (EvoMPMI) studies that will pave the way to a better understanding of the emergence of the core molecular machinery involved in the so-called evolutionary arms race between plants and microbes.

Tuning the Wavelength: Manipulation of Light Signaling to Control Plant Defense

“…Plants perceive cell damage (damage-associated molecular patterns, DAMPs) and detect exposed and highly conserved molecules from different classes of microbe/pathogen-associated molecular patterns (MAMPs/PAMPs). These damage and non-self molecules, as well as some apoplastic effectors, are recognized by plasma-membrane-localized pattern-recognition receptors (PRRs), inducing a heightened state of immunity known as PAMP-triggered immunity (PTI) which is well reviewed [ 8 , 10 , 11 , 12 ]. Pests and microbes that have coevolved with their hosts are able to secrete effectors into host tissues rapidly enough to suppress PTI and allow infection.…”

“…However, pathogen effectors are also subject to detection by the host via membrane PRRs and intracellular receptors belonging to the nucleotide-binding site leucine-rich repeat (NBS-LRR) family, known as NLRs or R (resistance) proteins. An effector can be either recognized directly, through protein interactions, or indirectly, by exploitative activities on particular guarded host targets in a process known as effector-triggered immunity (ETI), which is also very well reviewed [ 8 , 12 , 13 ].…”

“…Briefly, both PTI and ETI involve similar signaling molecules that transmit cell-to-cell communications, such as reactive oxygen species (ROS) accumulation, Ca2+ influx, and nitric oxide (NO) production [ 12 , 14 ]. The induced signaling cascades, during both PTI and ETI, are regulated by a range of protein oxidases, kinases, phosphatases, and ubiquitin ligases to name but a few.…”

“…Both ETI and some PTI responses evoke a localized form of programmed cell death aimed to confine biotrophic pathogens that require living tissue at the site of infection, with ETI being considered to proceed with more speed and intensity. PTI has recently been shown to be involved in potentiating ETI responses, which can, in turn, reinforce PTI [ 8 , 12 , 14 , 20 ].…”

Molecular mechanisms of Phytophthora sojae avirulence effectors escaping host recognition

“…The type of recognition is known as effector-triggered immunity (ETI), a part of the plant immune system ( Jones and Dangl, 2006 ). ETI frequently undergo hypersensitive response (HR) to protect plant healthy from pathogen invasion, forming a “barrier wall” to block pathogen nutrient intake, resulting in rapid local programmed cell death (PCD; Ramírez-Zavaleta et al, 2022 ). However, pathogens, on the other hand, have developed strategies to suppress or evade ETI ( Wang et al, 2011 ).…”