Abstract
Plant nucleotide binding domain and leucine-rich repeat (NLR) receptors recognize pathogen effectors directly or indirectly and mediate innate immune responses. NLR-mediated immunity also has direct impacts on plant growth and development, as well as yield and survival. The levels of NLR proteins are therefore intricately controlled in plants to balance defense responses and other processes. In recent years, the ubiquitination-26S proteasome system and the HSP90 chaperones have emerged as having key functions in the regulation of NLR stability. The N-end rule pathway of protein degradation is also directly linked to NLR stability. Recent progress in the regulation of NLR stability and turnover is summarized here, focusing on the key components and pathways.
Highlights
Plants have evolved an innate immune system for pathogen recognition and subsequent defense activation[1]
We summarize the recent findings about the regulation of nucleotide binding domain and leucinerich repeat (NLR) stability and homeostasis in plants, focusing on the involvement of Ub-26S proteasome system, the N-end rule pathway and the Heat shock protein 90 (HSP90) chaperone machinery
It is believed that SRFR1 and SGT1 may work together to act as a scaffolding protein or chaperone in the SCFCPR1 complex to regulate the turnover of some NLRs[13] (Fig. 1(b))
Summary
Plants have evolved an innate immune system for pathogen recognition and subsequent defense activation[1]. The PRRs recognize pathogen-associated molecular patterns (PAMPs) and initiate PAMP-triggered immunity (PTI)[2], while the NLRs intercept, directly or indirectly, strain-specific pathogen effectors ( known as avirulent effectors) and mediate effector-triggered immunity (ETI)[3,4]. The N-end rule pathway engages the ubiquitin-proteasome system to target protein substrate for destruction by the 26S proteasome Protein substrates for this pathway contains an N-degron (as a degradation signal), which consists of a destabilizing Nt-residue, a downstream Lys as a ubiquitination site, and a structural conformation that exposes the N-terminus of the protein[25]. We summarize the recent findings about the regulation of NLR stability and homeostasis in plants, focusing on the involvement of Ub-26S proteasome system, the N-end rule pathway and the HSP90 chaperone machinery
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