Monochromatic green light programs the chicken embryo intestinal microbiota and mucosal barrier via the melatonin-Nrf2-nitrate-tryptophan pathway.

Newly hatched chicks are susceptible to opportunistic pathogens because of their immature intestinal barrier and developing microbiota. This study investigated whether continuous in ovo monochromatic light exposure was associated with intestinal microbial and mucosal phenotypes at hatch. Compared with the other light treatments, green-light exposure was associated with increased pineal Arylalkylamine N-acetyltransferase (AANAT) expression, elevated plasma melatonin, enhanced phospho- Nuclear Factor Erythroid 2-Related Factor 2 (p-Nrf2)-related antioxidant signaling, reduced Inducible Nitric Oxide Synthase (iNOS) expression, and lower intestinal nitrate concentrations. These changes were accompanied by limited respiratory substrate availability for facultative anaerobes, promoting the competitive enrichment of obligate probiotics (Clostridium), increased tryptophan-related metabolites, enhanced Aryl hydrocarbon receptor (AhR)/Interleukin (IL)-22-related markers, reduced LPS/Toll Like Receptor 4 (TLR4)/Myeloid Differentiation Primary Response Gene 88 (MyD88)/nuclear factor kappa-B (NF-κB)-associated inflammatory signaling, and improved intestinal barrier-associated phenotypes. In vitro experiments further showed that indole-3-acetic acid (IAA) attenuated LPS-induced epithelial injury through AhR-related signaling. Together, these findings support a working model in which continuous in ovo green-light exposure may promote early intestinal mucosal maturation through coordinated host redox, microbial, and metabolic responses. However, functional perturbation and longitudinal studies are required to establish the causality and post-hatch persistence of these associations.