Hidden players in the regulation of secondary metabolism in tea plant: focus on non-coding RNAs

Non-coding RNAs (ncRNAs) are functional transcripts with minimal or no protein-coding capacity that comprise a large portion of the plant transcriptome. Among them, the microRNAs (miRNAs), linear long ncRNAs (lncRNAs), and circular long ncRNAs (circRNAs) have been widely proven to play essential regulatory roles in the biosynthesis of secondary metabolites (SMs) by modulating the expression of key synthesis-related genes in plants. Tea boasts numerous characteristic SMs, such as catechins, theanine, caffeine, volatile compounds, etc., which have distinguished health properties and largely determine the pleasant flavor quality. Thus, understanding how the tea plant produces these specialized metabolites is a hot topic. With the innovation and progress of biotechnologies in recent years, great progress has been made in the research on the regulation mechanism of SMs in tea plants at the DNA, mRNA, protein and metabolite levels. The release of the genome sequences of tea plants paves a path for precisely exploring ncRNAs and their functions in tea, and their great potential for the biosynthesis regulation of SMs gradually received attention. We herein summarize recent progress on miRNAs, lncRNAs, and circRNAs in tea plants and discuss their regulatory roles in the accumulation of SMs to enlighten the development of novel agronomic tools to enhance the quality of tea.