nPr4

NPR4, or Nonexpressor of PR Genes 4, is a plant protein in Arabidopsis thaliana and other angiosperms that functions in the salicylic acid (SA) signaling pathway governing plant innate immunity. It is one of two paralogs in Arabidopsis, along with NPR3, that regulate NPR1, a central transcription coactivator of defense genes.

NPR3 and NPR4 encode BTB/POZ-domain proteins that act as receptor/adaptor components for the CUL3-based E3 ubiquitin ligase complex. Binding to NPR1 targets NPR1 for ubiquitination and proteasomal degradation, a process modulated by SA. SA binding to NPR3 and NPR4 affects their interaction with NPR1 and the ligase, thereby tuning NPR1 stability in response to SA levels.

Consequences: NPR1 accumulation promotes expression of pathogenesis-related (PR) genes such as PR-1, leading to defense responses and systemic acquired resistance. NPR3 and NPR4 thus function as SA receptors that help convert the SA signal into NPR1 turnover, enabling dynamic control of immune responses.

Genomic context: NPR4 is widely conserved in flowering plants, with functional analyses mainly in Arabidopsis. Its activity is distinct from NPR3, with differential SA-binding affinities and degradation dynamics contributing to fine-tuning of NPR1 regulation.

Notes: As part of ongoing plant immunity research, NPR4 is studied for potential manipulation to enhance disease resistance in crops, while avoiding trade-offs with growth.