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Linoleic acid (LA; C18:2) and α-linolenic acid (ALA; C18:3) are two essential unsaturated fatty acids that play indispensable roles in maintaining membrane integrity in cold stress, and ω-3 fatty acid desaturases (FADs) are responsible for the transformation of LA into ALA. However, how this process is regulated at transcriptional and posttranscriptional levels remains largely unknown. In this study, an MYB transcription factor, MaMYB4, of a banana fruit was identified and found to target several ω-3 MaFADs , including MaFAD3-1 , MaFAD3-3 , MaFAD3-4 and MaFAD3-7 , and repress their transcription. Intriguingly, the acetylation levels of histones H3 and H4 in the promoters of ω-3 MaFADs were elevated in response to cold stress, which was correlated with the enhancement in the transcription levels of ω-3 MaFADs and the ratio of ALA/LA. Moreover, a histone deacetylase MaHDA2 physically interacted with MaMYB4, thereby leading to the enhanced MaMYB4-mediated transcriptional repression of ω-3 MaFADs. Collectively, these data demonstrate that MaMYB4 might recruit MaHDA2 to repress the transcription of ω-3 MaFADs by affecting their acetylation levels, thus modulating fatty acid biosynthesis. Our findings provided new molecular insights into the regulatory mechanisms of fatty acid biosynthesis in cold stress in fruits. [ABSTRACT FROM AUTHOR]

Bananas are good sources of nutrients, magnesium and vitamins, and they are also rich in many antioxidant compounds, such as flavonoids. Flavonoids are secondary metabolites with antioxidant properties that play an effective role in detoxification of reactive oxygen species (ROS). Stresses, such as low temperature stress, lead to excessive accumulation of ROS and stimulate the increase of activity of enzymes and genes related to flavonoid biosynthesis and thereby the flavonoid contents in a variety of plants. However, the mechanism underlying the transcriptional regulation of flavonoids production in banana fruit under chilling injury (CI) remains largely unknown. In this study, we found that both the contents of phloretin, naringenin chalcone and dihydrokaempferol (a member of the flavanonols) and the expression levels of chalcone and flavanonol biosynthesis genes MaCHSs and MaF3Hb significantly increased in banana peels during CI. It is vital that these genes expression levels and flavonoid contants contain remarkabe positive correlations. Additionally, a key myeloblastosis (MYB) transcriptional inhibitor MaMYB4, a homologue of AtMYB4 involved in flavonoids biosynthesis, was negatively correlated to the accumulated phloretin, naringenin chalcone, and dihydrokaempferol. MaMYB4 could bind to AC-rich elements in the promoters of MaCHSs and MaF3Hb to repress their transcriptional activities. However, the expression level of MaMYB4 is down-regulated in banana peels during CI. As a result, MaCHSs and MaF3Hb were released and activated, leading to the production of naringenin chalcone and dihydrokaempferol in banana peels. Taken together, our findings unveil an interesting transcriptional module in relation to flavonoid accumulation in peels of banana under chilling stress. • Flavonoids accumulated in peels of banana fruits. • Phloretin, naringenin chalcone and dihydrokaempferol increased in banana peels under CI. • Expression level of MaCHSs was correlated to the contents of phloretin, naringenin chalcone and dihydrokaempferol. • MaMYB4 binds to the promoters of MaCHSs and MaF3Hb to take action. [ABSTRACT FROM AUTHOR]

Starch is the major constituent of green banana fruit and is a resource for banana quality as it is degraded to soluble sugars during postharvest ripening. However, the molecular basis of starch degradation in different banana cultivars during fruit ripening remains largely unclear. In this study, the starch degradation of four banana cultivars (Baxi, Dajiao, Fengjiao, Gongjiao) during ripening was investigated. The contents of total starch, amylase, amylopectin, the amylose/amylopectin ratio, and the crystallinity were all decreased in all the four banana cultivars tested. In addition, the morphological structures of starch granules changed as ripening progressed due to starch degradation. The activity and gene expression of starch hydrolytic enzymes were increased. Importantly, a transcriptional regulator MabZIP21 was identified, which functions as an activator of starch degradation by upregulating the transcription of MaBAM4 , MaBAM7 , and MaAMY3. In addition, transient overexpression of MabZIP21 in banana or overexpression of MabZIP21 in tomato increased starch corruption by elevating the accumulation of starch degrading genes. Through this study, we expect to show that distinct patterns of starch degradation exist in different banana cultivars, and that MabZIP21 mediates starch breakdown by transcriptional activation of MaBAM4 , MaBAM7 , and MaAMY3. ● Starch breaks down during the ripening of banana fruit. ● Starch crystallinity and starch granule size decreased during ripening. ● Amylase and isoamylase activities and transcripts increased in ripening bananas. ● MabZIP21 trans-activates the transcription of MaBAM4 , MaBAM7 and MaAMY3. [ABSTRACT FROM AUTHOR]

• High temperature elevates carotenoid accumulation of banana. • High temperature induces expression of MaEIL9 and carotenogenic genes. • MaEIL9 targets promoters of carotenogenic genes and MaSPL16. • Overexpression of MaEIL9 in tomato enhances carotenoid production. Banana is a good source of carotenoids, which are bioactive metabolites with health beneficial properties for human. However, the molecular mechanism of carotenoid accumulation in banana fruit is largely unclear. In this study, we found that high temperature elevated carotenoid production in banana pulp, which is presumably due to upregulation of a subset of carotenogenic genes as well as a carotenoid biosynthesis regulator MaSPL16. Moreover, an ethylene signaling component MaEIL9 was identified, whose transcript and protein contents were also induced by high temperature. In addition, MaEIL9 positively regulates transcription of MaDXR1 , MaPDS1 , MaZDS1 and MaSPL16 through directly targeting their promoters. Overexpression of MaEIL9 in tomato fruit substantially increased the expression of carotenoid formation genes and elevated carotenoid content. Importantly, transiently silencing MaEIL9 in banana fruit weakened carotenoid production caused by high temperature. Taken together, these results indicate that high temperature induces carotenoid production in banana fruit, at least in part, through MaEIL9-mediated activation of MaDXR1 , MaPDS1 , MaZDS1 and MaSPL16 expression. [ABSTRACT FROM AUTHOR]

Banana is an important source of carotenoids, and the fruit accumulate carotenoids during the ripening process. However, the regulatory mechanisms underlying carotenoid production in banana fruit have not been well studied. Here, we report that MaERF124 is a negative transcriptional regulator of carotenogenesis genes in banana. The reduced accumulation of MaERF124 was accompanied with the increased contents of carotenoids during the ripening of the four banana varieties, suggesting a negative role of MaERF124 in carotenoid production. Further assays showed that MaERF124 acts as a transcriptional repressor and directly targets the promoters of carotenoid biosynthetic genes including MaDXR , MaGGPS , MaCRTISO and MaLCYB1.2. Moreover, ectopic expression of MaERF124 in tomato fruit repressed the transcript levels of carotenoid formation genes to inhibit carotenoid production. Taken together, these findings reveal that MaERF124 negatively modulates carotenoid formation via transcriptional repression of key genes involved in carotenoid synthesis, which provides a regulatory machinery for improving the contents of carotenoids in banana fruit. • Reduced expression of MaERF124 is inversely associated with carotenoid accumulation. • MaERF124 acts as a transcriptional repressor. • MaERF124 suppresses the transcription of carotenogenesis genes. • Overexpression of MaERF124 in tomato inhibits carotenoid production. [ABSTRACT FROM AUTHOR]

Ethylene is essential for the ripening of climacteric fruit such as banana, but the transcriptional modulation of ethylene formation in ripening banana is not well studied. In this work, two WRKY transcription factors MaWRKY49 and MaWRKY111 were identified from banana fruit. MaWRKY49 and MaWRKY111 were both nucleus-localized transcriptional activators, the transcription of which continued to increase following banana fruit ripening. MaWRKY49 and MaWRKY111 not only stimulated the transcription of themselves to constitute the feedback regulatory loop, but also promoted the transcription of ethylene biosynthesis genes MaACS1 and MaACO1 by directly targeting the W-box elements. Moreover, MaWRKY49 and MaWRKY111 interacted with each other to form the heterodimer, contributing to further enhancement of their target gene transcription. Importantly, both MaWRKY49 and MaWRKY111 interacted with another transcriptional activator MabZIP21, which synergistically strengthened the transcription of MaACS1 and MaACO1. Collectively, our findings suggest that MaWRKY49 and MaWRKY111 cooperate with MabZIP21 to positively regulate the ripening of banana by promoting transcription of ethylene biosynthesis genes MaACS1 and MaACO1. • MaWRKY49/111 transcactivate the transcription of MaACS1 , MaACO1 and themselves. • MaWRKY49 physically interacts with MaWRKY111. • MaWRKY49/111 interact with another transcriptional regulator MabZIP21. • Interaction between MaWRKY49/111 and MabZIP21 enhances target gene transcription. [ABSTRACT FROM AUTHOR]

• Ethylene alleviates CI of banana. • Ethylene inhibits MabHLH060/183 expression. • MabHLH060/183 target the promoters of PA formation genes. Chilling injury (CI) of banana represents a serious problem constraining postharvest preservation in the industry. Application of ethylene was found to efficiently attenuate CI of banana, however, the mechanism of which remains to be investigated. Here, we found that application of ethrel could ameliorate the CI of banana, as evidenced by lower CI index, electrolytic leakage and malondialdehyde content. Meanwhile, expression of phosphatidic acid (PA) formation genes MaPLDδ1 , MaPLDδ5 and MaDGK2 , as well as two transcriptional regulators MabHLH060 and MabHLH183 was significantly increased with the progression of CI, but decreased under ethrel treatment. MabHLH060 and MabHLH183 were shown to locate in nucleus and have transactivation ability. Importantly, these two transcriptional regulators MabHLH060/183 recognized the E-box elements in the promoters of MaPLDδ1 , MaPLDδ5 and MaDGK2 , and promoted their transcription. Together, these findings suggest that ethylene-alleviated CI of banana may involve the MabHLH060/183 module in modulating the expression of PA formation genes. [ABSTRACT FROM AUTHOR]