Biochemistry: Specialized Metabolites
Flavonoids, which represent a large group of phenolic plant specialized metabolites, have diverse functions in plants, including pigmentation, signaling, and protection against UV radiation and oxidative stress. Although anthocyanin biosynthesis is well characterized, several aspects remain unclear, including transport and storage. In Arabidopsis, the glutathione S-transferase TRANSPARENT TESTA 19 (TT19) is essential for anthocyanin accumulation, although the underlying mechanism is unknown and does not involve conjugation of glutathione to the pigments. To better understand the functions of TT19 on anthocyanin pathway regulation and product sequestration, we screened for genetic suppressors that allow anthocyanins to accumulate in a tt19 mutant. Here, we report that all six identified tt19 suppressors correspond to components of the RDR6-SGS3-DCL4 small RNA system. Moreover, our metabolic analyses show that the combination of tt19 and mutations in RDR6, SGS3, or DCL4 results in a three to five fold increase in carbon flux into flavonoid production compared to wild type. In seedlings, this carbon derives from sucrose through central metabolism, and from the down-regulation of other metabolic pathways, as evidenced from mRNAs, small RNAs, and metabolic profiling in the various single and double mutants. In addition, we show that the synergistic interaction between the impaired RDR6-SGS3-DCL4 system and flavonoid pathway dysfunction is not specific to the loss of TT19 as a second anthocyanin pathway mutant (tds4-4) shows a similar increased carbon flow effect when combined with an rdr6 mutation.Thus, our search for tt19 phenotypic suppressors resulted in the identification of a novel cross-talk mechanism between central metabolism and flavonoid biosynthesis that involves the small RNA system controlled by RDR6-SGS3-DCL4.
Funding for this project is provided in part by NSF MCB-1822343.