Contribution of ornithine aminotransferase in proline metabolism response to high temperature in Gracilariopsis lemaneiformis (Rhodophyta).

The seaweed Gracilariopsis lemaneiformis is used for agar and abalone feed, but high summer temperature in the southern oceans of China limits the cultivation industry. Therefore, enhancing the heat tolerance of G. lemaneiformis is crucial for prolonging its growth period along with increasing production and economic performance. Proline, a key osmotic regulator, plays an essential role in stress responses, including drought, heat, cold, and high salinity. Here, we investigated the dynamic changes in proline accumulation and its metabolic enzymes response to high-temperature stress in G. lemaneiformis. Results showed that elevated temperatures significantly increased proline accumulation, stimulated the activities of its synthetases pyrroline-5-carboxylate synthetase (P5CS) and ornithine aminotransferase (OAT), while concurrently suppressed the activity of its degrading enzyme proline dehydrogenase (ProDH). Pearson correlation analysis displayed a stronger positive association between proline level and OAT activity than with P5CS or pyrroline-5-carboxylate reductase (P5CR) activity, as well as a negative correlation with ProDH activity. Furthermore, exogenous 24-epibrassinolide, trehalose, and ornithine increased OAT activity or its gene transcription, while salicylic acid induced P5CS activity and P5CR2 transcriptional levels, which all resulted in the enhanced proline accumulation. Finally, bioinformatics analysis revealed that GlOAT contains a predicted PLP-binding domain, exhibits a high likelihood of mitochondrial localization, and shows a high degree of phylogenetic conservation. In summary, this study demonstrated that OAT is the critical enzyme involved in proline synthesis under high-temperature stress, which might provide a foundation for future transgenic breeding to improve high-temperature tolerance in G. lemaneiformis.