Functions of exogenous α-naphthaleneacetic acid and 6-benzylaminopurine in the macroalga Gracilariopsis lemaneiformis revealed by transcriptomic analysis

Gracilariopsis lemaneiformis is cultivated as feed for abalone and raw material for agar, and its economic benefits are strongly correlated with its yield. Plant growth regulators such as α-naphthaleneacetic acid (NAA) and 6-benzylaminopurine (6-BA), can effectively promote plant growth and biomass accumulation, however, their effects on macroalgae remain less thoroughly investigated. This study aimed to evaluate the roles of NAA and 6-BA on the G. lemaneiformis using physiological and transcriptomic analyses. Results showed that both NAA and 6-BA enhanced the relative growth rate, linear growth rate of main and lateral branches, as well as accumulation of phycobiliprotein, soluble polysaccharide and total lipid. Transcriptomic results corroborated the physiological performances. Both NAA and 6-BA upregulated carbohydrate metabolism, photosynthesis-related metabolism, amino acid biosynthesis and purine metabolism. In addition, NAA specifically increased the expression levels of the genes related to cell division process, including CDC20, CDC42 and expansin-like protein, while 6-BA upregulated the expression levels of ABC transporters, including ABCD3 and ABCG2. These findings will not only deepen our understanding of the NAA and 6-BA roles in promoting algal growth, but also be useful in cultivating the seedlings of G. lemaneiformis .