Nitrate Modulates Fruit Lignification by Regulating CgLAC3 Expression in Pomelo

Lignification of juice sacs is a primary contributor to reductions in fruit quality, with impacts on taste and economic value of pomelo (Citrus grandis). To date, information on the regulation of fruit lignification remains fragmentary. In this study, we first analyzed the relationship between lignification and nutrient status of pomelo juice sacs, which revealed a significant positive correlation between nitrate (NO₃^-) concentration and lignin concentration, with over 60% of lignin accumulation explained by NO₃^- levels in three models of machine learning-based regression. Results from field trails in 11 pomelo orchards, as well as in pear fruits and soybean roots exposed to low or high NO₃^- supplies, further demonstrated that nitrate plays an important role in lignification. Transcriptomic analysis further showed that pomelo laccases (CgLACs) were more intensively up-regulated upon addition of NO₃^- than any of the genes encoding one of the other 12 enzymes involved in lignin biosynthesis. Among the nine identified CgLACs, CgLAC3 was the most significantly up-regulated CgLAC in high nitrate treated plants. Over-expressing CgLAC3 increased lignin concentrations in both pomelo albedo and soybean hairy roots. Taken together, we conclude that nitrate modulates fruit lignification in pomelo through regulation of CgLAC3 expression, which suggests that NO₃^--N fertilization may affect fruit lignification, and thereby can be managed to improve fruit quality.