Fruit respiration: putting alternative pathways into perspective

Over the past century, research has significantly advanced our understanding of fruit respiration, from (eco)physiological processes to molecular mechanisms. This review focuses on the functional relevance and regulatory roles of mitochondrial alternative respiratory pathways (ARPs) during fruit growth and ripening. We revisit classical distinctions between climacteric and nonclimacteric fruits, considering recent insights into the alternative oxidase, uncoupling proteins, and type II NAD(P)H dehydrogenases (NDIIs). These components are increasingly recognized as central to maintaining metabolic flexibility, energy balance, and redox homeostasis, supporting both primary and secondary metabolism. We highlight how CO₂ refixation and organic acid metabolism, often displaying C₄/CAM-like features, impose specific demands on mitochondrial electron transport, and how spatial heterogeneity in metabolism and O₂ availability across fruit tissues can shape respiratory activity. Interactions between fruit photosynthesis and respiration remain poorly understood, particularly under stress. The interplay between respiration, ethylene biosynthesis, and signaling is discussed, emphasizing feedback loops involving mitochondrial retrograde regulation and redox-sensitive control of ripening. Key knowledge gaps include in vivo flux analyses, tissue-resolved energy profiling, and functional characterization of underexplored ARP components. Finally, we outline postharvest and metabolic engineering strategies targeting ARPs as complementary to ethylene-centered approaches to improve fruit quality, stress resilience, and nutritional value.