Climate Change and Sustainable Agriculture: Water, Soil, Carbon, Crops and Livelihoods in Transition

Plant innate immunity has for two decades been taught and modelled as a two-layer system in which surface-localised pattern-recognition receptors provide a first tier of defence and intracellular nucleotide-binding leucine-rich repeat receptors provide a second tier that can culminate in localised cell death. Evidence accumulated since 2021 has made strict separation of these systems difficult to sustain. Reciprocal experiments in Arabidopsis thaliana demonstrated that intracellular receptor signalling requires intact surface-receptor signalling for full expression and that surface-receptor responses recruit components previously assigned principally to intracellular immunity. Structural and biochemical work has since identified a shared calcium-signalling currency, overlapping RBOH-dependent oxidative machinery and lipase-like EDS1-family signalling nodes, while single-cell and spatial studies have exposed cellular heterogeneity that complicates bulk measurements. More recently, surface signalling has been shown to license intracellular receptor activation through alternative splicing in a solanaceous model. This critical narrative review evaluates that reorientation and asks how far it extends to crops and agronomic conditions. Literature was identified through structured searching and citation tracing, with bibliographic identities and digital object identifiers checked against authoritative records. The synthesis separates mechanistically well-supported convergence from stronger claims of universal reciprocal dependence. Confidence is highest for shared downstream signalling and reciprocal potentiation in Arabidopsis; it is lower for taxonomic generalisation because direct interdependence tests remain concentrated in a small number of model systems and receptor pairs. Crop evidence is expanding: rice now provides direct evidence for a functional TIR–EDS1–PAD4–ADR1 module and receptor-class stacking, wheat provides TaEDS1-dependent TIR-domain immunity, and solanaceous species reveal helper-receptor network constraints. These findings strengthen the network view while leaving its quantitative boundary conditions, field stability and fitness costs unresolved.