Distinct Plastids Trigger Local Signaling for Systemic Stress Response in Plants

Plastids comprise a complex set of organelles in plants that can undergo distinctive patterns of differentiation and redifferentiation during their lifespan. Plastids localized to the epidermis and vascular parenchyma are distinctive in size, structural features and functions. These plastids are termed ‘sensory’ plastids, and data show their proteome to be distinct from chloroplast, with specialized stress-associated features. The distinctive sensory plastid proteome derives from spatio-temporal regulation of nuclear genes encoding plastid-targeted proteins. Perturbation caused by depletion of the sensory plastid-specific protein MSH1conditioned local, programmed changes in gene networks controlling chromatin, stressrelated phytohormone and circadian clock behavior, and producing a global, systemic stress response in the plant. A similar systemic stress response resulted from over-expression of a second sensory plastid protein, PPD3. We posit that the sensory plastid participates in sensing of plant environmental stress, integrating this sensory function with epigenetic and gene expression circuitry to condition heritable stress memory.