Understanding plant responses to environmental change is crucial for assessing the vulnerability of forest ecosystems to climate change. Tropical species may be particularly vulnerable due to their potentially narrower thermal tolerance ranges, meaning that even small environmental changes can significantly affect fitness and lead to substantial population declines. Despite the need for investigations at the genetic level, few studies have used molecular approaches to investigate the effects of environmental change on tropical plants. Furthermore, comparisons between tropical and temperate species to assess the vulnerability of tropical species remain limited. In this study, we used a molecular phenology approach to test whether tropical trees show significant physiological responses to even small environmental changes. We conducted a field transcriptome study using leaf tissues of Rubroshorea leprosula, collected biweekly in Malaysia from June 2017 to June 2018, to generate molecular phenology data. We then analysed the relationship between molecular phenology and environmental factors, including solar radiation, rainfall and minimum temperature. Additionally, we performed gene ontology (GO) enrichment analysis to identify biological functions that were significantly enriched for specific gene expression profiles. To compare molecular phenology between tropical and temperate species, we used field transcriptome data from two Fagaceae species (Quercus glauca and Lithocarpus edulis) collected monthly in Fukuoka from October 2019 to April 2021. We extracted one-to-one orthologous genes to compare expression profiles and functions across tropical and temperate trees.
Gene expression analysis of R. leprosula revealed four clusters and six functional groups. The cluster with the highest gene expression levels was associated with significantly lower minimum temperatures than the other clusters. However, no significant differences in solar radiation or precipitation were observed between the clusters. Focusing on genes associated with low temperature response, GO analysis showed that gene expression associated with photosynthetic function decreased and gene expression associated with metabolic function increased at low temperature. Clustering analysis of gene expression from the three species revealed five clusters and showed that the pronounced low temperature response　(approximately 21°C) of R. leprosula was similar to the winter temperature (approximately 4.7℃) response observed in the two temperate Fagaceae species. Furthermore, the normal gene expression profile in R. leprosula was similar in spring and autumn in the temperate zone. These interspecific comparisons indicate that tropical trees are acutely sensitive to small decreases in temperature, highlighting their vulnerability to environmental change. Based on these results, this presentation will discuss the seasonal responses of tropical trees.