BTB-BACK-TAZ domain protein MdBT2-mediated MdMYB73 ubiquitination negatively regulates malate accumulation and vacuolar acidification in apple

Abstract

As an important primary metabolite, malate plays a key role in regulating osmotic pressure, pH homeostasis, stress tolerance, and fruit quality of apple. The R2R3-MYB transcription factor (TF) MdMYB73 was identified as a protein that plays a critical role in determining malate accumulation and vacuolar acidification by directly regulating the transcription of aluminum-activated malate transporter 9 (MdALMT9), vacuolar ATPase subunit A (MdVHA-A), and vacuolar pyrophosphatase 1 (MdVHP1) in apple. In addition, the bHLH TF MdCIbHLH1 interacts with MdMYB73 and enhances the transcriptional activity of MdMYB73. Our previous studies demonstrated that the BTB-BACK-TAZ domain protein MdBT2 can degrade MdCIbHLH1 to influence malate accumulation and vacuolar acidification. However, the potential upstream regulators of MdMYB73 are currently unknown. In this study, we found that MdBT2 directly interacts with and degrades MdMYB73 through the ubiquitin/26S proteasome pathway to regulate malate accumulation and vacuolar acidification. A series of functional assays with apple calli and fruit showed that MdBT2 controls malate accumulation and vacuolar acidification in an MdMYB73-dependent manner. Overall, our findings shed light on the mechanism by which the BTB-BACK-TAZ domain protein MdBT2 regulates malate accumulation and vacuolar acidification by targeting MdMYB73 and MdCIbHLH1 for ubiquitination in apple. This information may help guide traditional breeding programs and fruit tree molecular breeding, and lead to improvements in fruit quality and stress tolerance.

Classifying Cider Apple Germplasm Using Genetic Markers for Fruit Acidity

“…However, the dominance of the Ma1 allele is incomplete, which was indicated by the wide range of TA values for the heterozygous Mama allele, suggesting that both additive and dominant effects of the Ma1 allele exist (Verma et al, 2019;Xu et al, 2012). Recently, it was found that in response to excess nitrate accumulation, the MdBT2 protein modulated and downregulated the expression of MdCIbHLH1 and MdMYB73, which regulate malate-related genes, thus reducing acidity in apples (Zhang et al, 2020).…”

Nitrate‐inducible MdBT2 acts as a restriction factor to limit apple necrotic mosaic virus genome replication in Malus domestica

“…BT2 is also a nitrate‐responsive protein, and the expression of the BT2‐encoding gene is induced by nitrate in both Arabidopsis (Mandadi et al, 2009) and apple (Figure a). Recent findings suggest that MdBT2 regulates the accumulation of anthocyanin and malate in apple by interacting with and degrading MdMYB1 (Wang et al, 2018), MdCIbHLH1 (Zhang et al, 2020), and MdMYB73 (Zhang et al, 2020) in response to nitrate. Here, we found that nitrate treatment promoted the degradation of viral protein 1a in vitro through the proteasome pathway (Figures 2 and ).…”

“…It responds to circadian rhythms, light, stresses, and nutrients, suppresses sugar signalling, modulates plant hormone responses by suppressing abscisic acid (ABA) signalling while enhancing auxin signalling, and regulates telomerase activity by acting downstream of TAC1 (TELOMERASE ACTIVATOR1) (Kunz et al, 2015; Mandadi et al, 2009; Misra et al, 2018; Ren et al, 2007). MdBT2, a homolog of AtBT2, has been shown to function as a signal hub to regulate anthocyanin biosynthesis, leaf senescence, iron homeostasis, and malate accumulation in response to multiple hormonal and environmental signals (An, Zhang, et al, 2019; An, Wang, et al, 2020; Zhao et al, 2016; Zhang, Gu, Cheng, et al, 2020; Zhang, Gu, Wang, et al, 2020). For example, MdBT2 interacts with and promotes the ubiquitination and degradation of MdMYB1 and MdCIbHLH1 (cold‐induced basic helix‐loop‐helix1) to inhibit the accumulation of anthocyanin (Wang et al, 2018) and malate (Zhang, Gu, Cheng, et al, 2020; Zhang, Gu, Wang, et al, 2020), respectively, in response to nitrate.…”

“…MdBT2, a homolog of AtBT2, has been shown to function as a signal hub to regulate anthocyanin biosynthesis, leaf senescence, iron homeostasis, and malate accumulation in response to multiple hormonal and environmental signals (An, Zhang, et al, 2019; An, Wang, et al, 2020; Zhao et al, 2016; Zhang, Gu, Cheng, et al, 2020; Zhang, Gu, Wang, et al, 2020). For example, MdBT2 interacts with and promotes the ubiquitination and degradation of MdMYB1 and MdCIbHLH1 (cold‐induced basic helix‐loop‐helix1) to inhibit the accumulation of anthocyanin (Wang et al, 2018) and malate (Zhang, Gu, Cheng, et al, 2020; Zhang, Gu, Wang, et al, 2020), respectively, in response to nitrate. In addition, MdBT2 functions to delay leaf senescence by interacting with and promoting the ubiquitination and degradation of MdbHLH93 (basic helix‐loop‐helix93) and MdMYC2 (myelocytomatosis protein2) in apple (An, Zhang, et al, 2019; An et al, 2021).…”

Multiple-model GWAS identifies optimal allelic combinations of quantitative trait loci for malic acid in tomato

“…The MA content in fruit is determined by its synthesis anddegradation and by malate-related transporters, which are controlled by a complex molecular regulation system [ 9–12 ]. In addition to the synthesis and degradation pathways of MA representedin glycolysis and the tricarboxylic acid cycle, malate transportersplay essential roles in the final MA concentration [ 7 , 9 , 13 , 14 ]. The transcription factor MMYB1 directly regulates the transcription ofmalate transporters MdVHA-B s, MdVHA-E2 , and MdtDT12 to control MA content in apple [ 14 ].…”