FvMYB79 Positively Regulates Strawberry Fruit Softening via Transcriptional Activation of FvPME38

Abstract

Strawberry is a soft fruit with short postharvest life, due to a rapid loss of firmness. Pectin methylesterase (PME)-mediated cell wall remodeling is important to determine fruit firmness and softening. Previously, we have verified the essential role of FvPME38 in regulation of PME-mediated strawberry fruit softening. However, the regulatory network involved in PME-mediated fruit softening is still largely unknown. Here, we identified an R2R3-type MYB transcription factor FvMYB79, which activates the expression level of FvPME38, thereby accelerating fruit softening. During fruit development, FvMYB79 co-expressed with FvPME38, and this co-expression pattern was opposite to the change of fruit firmness in the fruit of ‘Ruegen’ which significantly decreased during fruit developmental stages and suddenly became very low after the color turning stage. Via transient transformation, FvMYB79 could significantly increase the transcriptional level of FvPME38, leading to a decrease of firmness and acceleration of fruit ripening. In addition, silencing of FvMYB79 showed an insensitivity to ABA-induced fruit ripening, suggesting a possible involvement of FvMYB79 in the ABA-dependent fruit softening process. Our findings suggest FvMYB79 acts as a novel regulator during strawberry ripening via transcriptional activation of FvPME38, which provides a novel mechanism for improvement of strawberry fruit firmness.

Insights into transcription factors controlling strawberry fruit development and ripening

“…A similar positive regulatory feedback loop has also been reported for ABA and a MYB transcription factor, FvMYB79 , which is positively regulated by ABA. Thus, FvMYB79 silencing decreases the ABA content and delays fruit ripening, a phenotype that could not be reverted even after ABA treatment ( Cai et al., 2022 ). Another MYB transcription factor FaGAMYB has been shown to be an important regulator of strawberry fruit ripening, as it induces ABA biosynthesis by regulating NCED1 and NCED2 , as well as ABI5 and DREB1 , two TFs involved in ABA signaling.…”

“…Besides the knowledge of how all these different TFs regulate hormone biosynthesis and signaling ( Figure 1A ), several studies have also reported how some TFs are in turn regulated by different hormones ( Table 1 ) ( Encinas-Villarejo et al., 2009 ; Tisza et al., 2010 ; Daminato et al., 2013 ; Medina-Puche et al., 2015 ; Medina-Puche et al., 2016 ; Molina-Hidalgo et al., 2017 ; Carrasco-Orellana et al., 2018 ; Lu et al., 2018 ; Moyano et al., 2018 ; Medina-Puche et al., 2019 ; Xie et al., 2020 ; Zhang et al., 2020 ; Martín-Pizarro et al., 2021 ; Cai et al., 2022 ; Wang et al., 2022 ), highlighting the complex regulatory network between hormones and transcriptional regulators to control strawberry fruit development and ripening.…”

“…Besides MYB1 and MYB10 , other MYB TFs involved in the regulation of the phenylpropanoid pathway have been characterized or proposed as candidate regulators. Among them, FvMYB79 transient overexpression and silencing up- and downregulate the expression of MYB10 , CHS , CHI , DFR and UFGT ( Cai et al., 2022 ). Another MYB TF, FaMYB63 , which is related to the biosynthesis of phenylpropanoid-derived volatiles as we will discuss later, is also able to directly regulate MYB10 expression in dual-luciferase assays ( Wang et al., 2022 ).…”

“…Besides FaRIF, another NAC protein, FcNAC1 has been shown to directly induce PL expression, although its function in cell wall remodeling has not been studied yet ( Carrasco-Orellana et al., 2018 ). Another direct regulation has been described for FvMYB79 , which besides regulating the expression of phenylpropanoid-related genes, binds to the promoter of FaPME38 activating its expression ( Cai et al., 2022 ). Furthermore, the expression of PME , EXP , PL , PG and EGase genes was significantly downregulated in FvMYB79- RNAi fruits, and upregulated when this TF was overexpressed ( Cai et al., 2022 ).…”

“…Another direct regulation has been described for FvMYB79 , which besides regulating the expression of phenylpropanoid-related genes, binds to the promoter of FaPME38 activating its expression ( Cai et al., 2022 ). Furthermore, the expression of PME , EXP , PL , PG and EGase genes was significantly downregulated in FvMYB79- RNAi fruits, and upregulated when this TF was overexpressed ( Cai et al., 2022 ). Similar transient experiments allowed to identified cell wall-related genes differentially expressed in FvTCP9 silenced and overexpression lines, including β-Gal1/2/3 and EXP1/2/5 ( Xie et al., 2020 ).…”