TCP8 transcription factor negatively regulates Arabidopsis seed germination and antagonizes the function of TCP14

TEOSINTE BRANCHED1/CYCLOIDEA/PROLIFERATING CELL FACTORS (TCPs) are plant-specific transcription factors that regulate key biological processes. To date, only TCP14 and TCP15, two members of the TCP Class I subfamily, have been characterized as positive regulators of seed germination. In this study, we determined that the Class-I TCP8 negatively regulates seed germination in Arabidopsis. Seeds of the tcp8.1 mutant exhibited weaker primary dormancy together with altered expression of genes related to ABA and GA. Transcriptomic analyses of seeds imbibed for 24 h identified 270 differentially expressed genes associated with seed dormancy and germination in tcp8.1 seeds compared with the wild type, with genes preferentially expressed in the endosperm being notably over-represented. The germination phenotypes of seeds of the tcp8.1 and tcp14.4 mutants were abolished in the tcp8.1 tcp14.4 double-mutant, indicating that TCP14 and TCP8 have antagonistic functions. As previously reported for TCP14, TCP8 interacted with and was phosphorylated by the MAP KINASE MPK8. Knocking out MPK8 in the tcp8.1, tcp14.4, and tcp8.1 tcp14.4 backgrounds did not modify the phenotypes of these mutants, suggesting that MPK8 functions upstream of TCP8 and TCP14. Overall, our results identify a new regulator of germination capacity, and shed light on opposite functions for TCP8 and TCP14 in this process that might be coordinated by MPK8.