Sugar content is a key determinant of fruit quality, and sugars also act as signalling molecules that regulate ripening processes, including anthocyanin accumulation. However, the molecular mechanisms underlying sugar accumulation and sugar signal-mediated ripening remain incompletely understood. In this study, we identify FvMAPK6 as an important phosphorylation hub that coordinates both sugar and anthocyanin accumulation in strawberry fruit. FvMAPK6 forms a phosphorylation cascade with FvMAPKK4, which directly phosphorylates the transcription factors FvMYB44.1 and FvMYB44.2. This phosphorylation reduces the stability and transcriptional activity of these proteins, attenuates their repression of downstream target genes such as FvCHI, FvSPS3 and FvSWEET1, thereby coordinating anthocyanin and sugar accumulation. Furthermore, FvMAPK6 increases the protein abundance of the hexose transporter FvSWEET1 in strawberry fruits and alters its transport activity through phosphorylation. We demonstrate that sucrose treatment activates FvMAPK6, reinforcing its regulation of FvMYB44s and FvSWEET1 and thus amplifying sugar and anthocyanin accumulation. These findings establish FvMAPK6 as a key regulator that integrates both sugar accumulation and signalling at both transcriptional and post-transcriptional levels. Although FvMAPK6 promotes sugar accumulation, it significantly reduces fruit yield and vegetative growth. To overcome this limitation, we screen for downstream targets of FvMAPK6 and identify FvSPS3 as a promising breeding target: modulating FvSPS3 improves fruit quality without compromising vegetative growth or yield. Collectively, our findings reveal novel regulatory pathways modulating sugar accumulation and signalling in strawberry while providing a valuable molecular target for the simultaneous improvement of fruit quality and agricultural productivity.
Feng et al. (Fri,) studied this question.