Genome-wide identification of asparagine synthetase genes (CsASNs) and their potential roles in nitrogen remobilization from mature leaves to new shoots of tea plants (Camellia sinensis L.)

Asparagine synthetase (AS) is a key enzyme in plant nitrogen (N) metabolism, regulating N assimilation and remobilization. Its product, asparagine, acts as a major N transport and storage compound. Although AS genes (ASNs) have been studied in many plants, their roles in tea plants (Camellia sinensis L.) remains unclear. In this study, the CsASNs family was confirmed to comprise three members, namely CsASN1, CsASN2, and CsASN3. The CsASNs had distinct untranslated regions (UTRs) but conserved ASN domains, encoding evolutionarily conserved proteins that were cytoplasm-localized. Expression profiling showed that CsASN1 was an inducible gene, while the expressions of CsASN2 and CsASN3 were constitutive. Tissue specificity of CsASNs in tea cultivars with high and low N remobilization efficiency (NRE) suggested that the tissue specificity of CsASNs was distinct not only between the two cultivars LJ43 and ZC108, but also among tissues within the same tea cultivar. Furthermore, the 15N-labeling experiment confirmed that the enhanced N remobilization in mature leaves was closely correlated with the changes of CsASN1 expression under dark treatment, suggesting CsASN1 may play an important positive role in the regulation of N remobilization from mature leaves to new shoots (NSs) under dark treatment. This study provided some insights into the mechanism underlying dark-induced remobilization of storage N from mature leaves to NSs in tea plants, and help to promote the sustainable development of tea production.