Proteasomal degradation of MaMYB60 mediated by the E3 ligase MaBAH1 causes high temperature-induced repression of chlorophyll catabolism and green ripening in banana

MaHsf24, a novel negative modulator, regulates cold tolerance in banana fruits by repressing the expression of HSPs and antioxidant enzyme genes

Transcriptional regulation mechanisms underlying chilling injury (CI) development have been widely investigated in model plants and cold-sensitive fruits, such as banana (Musa acuminata). However, unlike the well-known NAC and WRKY transcription factors (TFs), the function and deciphering mechanism of heat shock factors (HSFs) involving in cold response are still fragmented. Here, we showed that hot water treatment (HWT) alleviated CI in harvested banana fruits accomplishing with reduced reactive oxygen species (ROS) accumulation and increased antioxidant enzyme activities. A cold-inducible but HWT-inhibited HSF, MaHsf24, was identified. Using DNA affinity purification sequencing (DAP-seq) combined with RNA-seq analyses, we found three heat shock protein (HSP) genes (MaHSP23.6, MaHSP70-1.1 and MaHSP70-1.2) and three antioxidant enzyme genes (MaAPX1, MaMDAR4 and MaGSTZ1) were the potential targets of MaHsf24. Subsequent electrophoretic mobility shift assay (EMSA), chromatin immunoprecipitation coupled with quantitative PCR (ChIP-qPCR) and dual-luciferase reporter (DLR) analyses demonstrated that MaHsf24 repressed the transcription of these six targets via directly binding to their promoters. Moreover, stably overexpressing MaHsf24 in tomatoes increased cold sensitivity by suppressing the expressions of HSPs and antioxidant enzyme genes, while HWT could recover cold tolerance, maintaining higher levels of HSPs and antioxidant enzyme genes, and activities of antioxidant enzymes. In contrast, transiently silencing MaHsf24 by virus-induced gene silencing (VIGS) in banana peels conferred cold resistance with the upregulation of MaHSPs and antioxidant enzyme genes. Collectively, our findings support the negative role of MaHsf24 in cold tolerance, and unravel a novel regulatory network controlling bananas CI occurrence, concerning MaHsf24-exerted inhibition of MaHSPs and antioxidant enzyme genes.

KIN10-mediated HB16 protein phosphorylation and self-association improve cassava disease resistance by transcriptional activation of lignin biosynthesis genes

Cassava bacterial blight significantly affects cassava yield worldwide, while major cassava cultivars are susceptible to this disease. Therefore, it is crucial to identify cassava disease resistance gene networks and defence molecules for the genetic improvement of cassava cultivars. In this study, we found that MeHB16 transcription factor as a differentially expressed gene in cassava cultivars with contrasting disease resistance, positively modulated disease resistance by modulating defence molecule lignin accumulation. Further investigation showed that MeHB16 physically interacted with itself via the leucine-Zippe domain (L-Zip), which was necessary for the transcriptional activation of downstream lignin biosynthesis genes. In addition, protein kinase MeKIN10 directly interacted with MeHB16 to promote its phosphorylation at Ser6, which in turn enhanced MeHB16 self-association and downstream lignin biosynthesis. In summary, this study revealed the molecular network of MeKIN10-mediated MeHB16 protein phosphorylation improved cassava bacterial blight resistance by fine-tuning lignin biosynthesis and provides candidate genes and the defence molecule for improving cassava disease resistance.

E3 ubiquitin ligase MaRZF1 modulates high temperature-induced green ripening of banana by degrading MaSGR1

High temperatures (>24°C) prevent the development of a yellow peel on bananas called green ripening, owing to the inhibition of chlorophyll degradation. This phenomenon greatly reduces the marketability of banana fruit, but the mechanisms underlining high temperature-repressed chlorophyll catabolism need to be elucidated. Herein, we found that the protein accumulation of chlorophyll catabolic enzyme MaSGR1 (STAY-GREEN 1) was reduced when bananas ripened at high temperature. Transiently expressing MaSGR1 in banana peel showed its positive involvement in promoting chlorophyll degradation under high temperature, thereby weakening green ripening phenotype. Using yeast two-hybrid screening, we identified a RING-type E3 ubiquitin ligase, MaRZF1 (RING Zinc Finger 1), as a putative MaSGR1-interacting protein. MaRZF1 interacts with and targets MaSGR1 for ubiquitination and degradation via the proteasome pathway. Moreover, upregulating MaRZF1 inhibited chlorophyll degradation, and attenuated MaSGR1-promoted chlorophyll degradation in bananas during green ripening, indicating that MaRZF1 negatively regulates chlorophyll catabolism via the degradation of MaSGR1. Taken together, MaRZF1 and MaSGR1 form a regulatory module to mediate chlorophyll degradation associated with high temperature-induced green ripening in bananas. Therefore, our findings expand the understanding of posttranslational regulatory mechanisms of temperature stress-caused fruit quality deterioration.

The transcription factor PbbHLH164 is destabilized by PbRAD23C/D.1 and mediates ethylene biosynthesis during pear fruit ripening

The phytohormone ethylene plays an important role in climacteric fruit ripening. However, the knowledge on molecular regulation of ethylene biosynthesis remains limited in pear fruit. Herein, a new basic helix-loop-helix transcription factor, PbbHLH164, was identified based on the transcriptome analysis of different developing and ripening fruits of two pear cultivars 'Sucui No. 1' and 'Cuiguan'. PbbHLH164 was more highly expressed in ripening fruit than in developing fruit and positively correlated with ethylene production in both cultivars. PbbHLH164 could directly bind to the promoter of 1-aminocyclopropane-1-carboxylate synthase, PbACS1b, to enhance the expression, leading to the increase of ethylene production and the acceleration of fruit ripening. Interestingly, PbbHLH164 physically interacted with an ubiquitin-like/ubiquitin-associated protein PbRAD23C/D.1, and the interaction of PbbHLH164 with PbRAD23C/D.1 attenuated the function of PbbHLH164 in enhancing the activity of the PbACS1b promoter. Notably, PbRAD23C/D.1 was involved in the degradation of PbbHLH164, and this degradation was inhibited by an ubiquitin proteasome inhibitor MG132. Different from PbbHLH164, PbRAD23C/D.1 was more highly expressed in developing fruit than in ripening fruit of both cultivars. These results suggest that the increase of ethylene production during pear fruit ripening results from the up-regulated expression of PbbHLH164 and the down-regulated expression of PbRAD23C/D.1. This information provided new insights into the molecular regulation of ethylene biosynthesis during fruit ripening.

AcHZP45 is a repressor of chlorophyll biosynthesis and activator of chlorophyll degradation in kiwifruit

The degradation of chlorophyll during fruit development is essential to reveal a more 'ripe' color that signals readiness to wild dispersers of seeds and the human consumer. Here, comparative biochemical analysis of developing fruit of Actinidia deliciosa cv. Xuxiang ('XX', green-fleshed) and Actinidia chinensis cv. Jinshi No.1 ('JS', yellow-fleshed) indicated that variation in chlorophyll content is the major contributor to differences in flesh color. Four differentially expressed candidate genes were identified: the down-regulated genes AcCRD1 and AcPOR1 involved in chlorophyll biosynthesis, and the up-regulated genes AcSGR1 and AcSGR2 driving chlorophyll degradation. Prochlorophyllide and chlorophyllide, the metabolites produced by AcCRD1 and AcPOR1, progressively reduced in 'JS', but not in 'XX', indicating that chlorophyll biosynthesis was less active in yellow-fleshed fruit. AcSGR1 and AcSGR2 were verified to be involved in chlorophyll degradation, using both transient expression in tobacco and stable overexpression in kiwifruit. Furthermore, a homeobox-leucine zipper (HD-Zip II), AcHZP45, showed significantly increased expression during 'JS' fruit ripening, which led to both repressed expression of AcCRD1 and AcPOR1 and activated expression of AcSGR1 and AcSGR2. Collectively, the present study indicated that different dynamics of chlorophyll biosynthesis and degradation coordinate the changes in chlorophyll content in kiwifruit flesh, which are orchestrated by the key transcription factor AcHZP45.

Two adjacent NAC transcription factors regulate fruit maturity date and flavor in peach

Although maturity date (MD) is an essential factor affecting fresh fruit marketing and has a pleiotropic effect on fruit taste qualities, the underlying mechanisms remain largely unclear. In this study, we functionally characterized two adjacent NAM-ATAF1/2-CUC2 (NAC) transcription factors (TFs), PpNAC1 and PpNAC5, both of which were associated with fruit MD in peach. PpNAC1 and PpNAC5 were found capable of activating transcription of genes associated with cell elongation, cell wall degradation and ethylene biosynthesis, suggesting their regulatory roles in fruit enlargement and ripening. Furthermore, PpNAC1 and PpNAC5 had pleiotropic effects on fruit taste due to their ability to activate transcription of genes for sugar accumulation and organic acid degradation. Interestingly, both PpNAC1 and PpNAC5 orthologues were found in fruit-producing angiosperms and adjacently arranged in all 91 tested dicots but absent in fruitless gymnosperms, suggesting their important roles in fruit development. Our results provide insight into the regulatory roles of NAC TFs in MD and fruit taste.

MaNAC19-MaXB3 regulatory module mediates sucrose synthesis in banana fruit during ripening

Sucrose, a predominant sweetener in banana (Musa acuminata) fruit, determines sweetness and consumer preferences. Although sucrose phosphate synthase (SPS) is known to catalyze starch conversion into sucrose in banana fruit during the ripening process, the SPS regulatory mechanism during ripening still demands investigation. Hence, this study discovered that the MaSPS1 expression was promoted during ethylene-mediated ripening in banana fruit. MaNAC19, recognized as the MaSPS1 putative binding protein using yeast one-hybrid screening, directly binds to the MaSPS1 promoter, thereby transcriptionally activating its expression, which was verified by transient overexpression experiments, where the sucrose synthesis was accelerated through MaNAC19-induced transcription of MaSPS1. Interestingly, MaXB3, an ethylene-inhibited E3 ligase, was found to ubiquitinate MaNAC19, making it prone to proteasomal degradation, inhibiting transactivation of MaNAC19 to MaSPS1, thereby attenuating MaNAC19-promoted sucrose accumulation. This study's findings collectively illustrated the mechanistic basis of a MaXB3-MaNAC19-MaSPS1 regulatory module controlling sucrose synthesis during banana fruit ripening. These outcomes have broadened our understanding of the regulation mechanisms that contributed to sucrose metabolism occurring in transcriptional and post-transcriptional stages, which might help develop molecular approaches for controlling ripening and improving fruit quality.

Natural variation in STAYGREEN contributes to low-temperature tolerance in cucumber

Low-temperature (LT) stress threatens cucumber production globally; however, the molecular mechanisms underlying LT tolerance in cucumber remain largely unknown. Here, using a genome-wide association study (GWAS), we found a naturally occurring single nucleotide polymorphism (SNP) in the STAYGREEN (CsSGR) coding region at the gLTT5.1 locus associated with LT tolerance. Knockout mutants of CsSGR generated by clustered regularly interspaced palindromic repeats (CRISPR)/CRISPR-associated nuclease 9 exhibit enhanced LT tolerance, in particularly, increased chlorophyll (Chl) content and reduced reactive oxygen species (ROS) accumulation in response to LT. Moreover, the C-repeat Binding Factor 1 (CsCBF1) transcription factor can directly activate the expression of CsSGR. We demonstrate that the LT-sensitive haplotype CsSGRHapA , but not the LT-tolerant haplotype CsSGRHapG could interact with NON-YELLOW COLORING 1 (CsNYC1) to mediate Chl degradation. Geographic distribution of the CsSGR haplotypes indicated that the CsSGRHapG was selected in cucumber accessions from high latitudes, potentially contributing to LT tolerance during cucumber cold-adaptation in these regions. CsSGR mutants also showed enhanced tolerance to salinity, water deficit, and Pseudoperonospora cubensis, thus CsSGR is an elite target gene for breeding cucumber varieties with broad-spectrum stress tolerance. Collectively, our findings provide new insights into LT tolerance and will ultimately facilitate cucumber molecular breeding.

MaMADS1-MaNAC083 transcriptional regulatory cascade regulates ethylene biosynthesis during banana fruit ripening

The hormone ethylene is crucial in the regulation of ripening in climacteric fruit, such as bananas. The transcriptional regulation of ethylene biosynthesis throughout banana fruit ripening has received much study, but the cascaded transcriptional machinery of upstream transcriptional regulators implicated in the ethylene biosynthesis pathway is still poorly understood. Here we report that ethylene biosynthesis genes, including MaACS1, MaACO1, MaACO4, MaACO5, and MaACO8, were upregulated in ripening bananas. NAC (NAM, ATAF, CUC) transcription factor, MaNAC083, a ripening and ethylene-inhibited gene, was discovered as a potential binding protein to the MaACS1 promoter by yeast one-hybrid screening. Further in vitro and in vivo experiments indicated that MaNAC083 bound directly to promoters of the five ethylene biosynthesis genes, thereby transcriptionally repressing their expression, which was further verified by transient overexpression experiments, where ethylene production was inhibited through MaNAC083-modulated transcriptional repression of ethylene biosynthesis genes in banana fruits. Strikingly, MaMADS1, a ripening-induced MADS (MCM1, AGAMOUS, DEFICIENS, SRF4) transcription factor, was found to directly repress the expression of MaNAC083, inhibiting trans-repression of MaNAC083 to ethylene biosynthesis genes, thereby attenuating MaNAC083-repressed ethylene production in bananas. These findings collectively illustrated the mechanistic basis of a MaMADS1-MaNAC083-MaACS1/MaACOs regulatory cascade controlling ethylene biosynthesis during banana fruit ripening. These findings increase our knowledge of the transcriptional regulatory mechanisms of ethylene biosynthesis at the transcriptional level and are expected to help develop molecular approaches to control ripening and improve fruit storability.

Low temperature-induced regulatory network rewiring via WRKY regulators during banana peel browning

Banana (Musa spp.) fruits, as typical tropical fruits, are cold sensitive, and lower temperatures can disrupt cellular compartmentalization and lead to severe browning. How tropical fruits respond to low temperature compared to the cold response mechanisms of model plants remains unknown. Here, we systematically characterized the changes in chromatin accessibility, histone modifications, distal cis-regulatory elements, transcription factor binding, and gene expression levels in banana peels in response to low temperature. Dynamic patterns of cold-induced transcripts were generally accompanied by concordant chromatin accessibility and histone modification changes. These upregulated genes were enriched for WRKY binding sites in their promoters and/or active enhancers. Compared to banana peel at room temperature, large amounts of banana WRKYs were specifically induced by cold and mediated enhancer-promoter interactions regulating critical browning pathways, including phospholipid degradation, oxidation, and cold tolerance. This hypothesis was supported by DNA affinity purification sequencing, luciferase reporter assays, and transient expression assay. Together, our findings highlight widespread transcriptional reprogramming via WRKYs during banana peel browning at low temperature and provide an extensive resource for studying gene regulation in tropical plants in response to cold stress, as well as potential targets for improving cold tolerance and shelf life of tropical fruits.

Banana MKK1 modulates fruit ripening via the MKK1-MPK6-3/11-4-bZIP21 module

The mitogen-activated protein kinase (MAPK) cascade consisting of MKKK, MKK, and MPK plays an indispensable role in various plant physiological processes. Previously, we showed that phosphorylation of MabZIP21 by MaMPK6-3 is involved in banana fruit ripening, but the regulatory mechanism by which MKK controls banana fruit ripening remains unclear. Here, ripening-induced MaMKK1 from banana fruit is characterized, and transiently overexpressing and silencing of MaMKK1 in banana fruit accelerates and inhibits fruit ripening, respectively, possibly by influencing phosphorylation and activity of MPK. MaMKK1 interacts with and phosphorylates MaMPK6-3 and MaMPK11-4 mainly at the pTEpY residues, resulting in MPK activation. MaMPK11-4 phosphorylates MabZIP21 to elevate its transcriptional activation ability. Transgenic tomato fruit expressing MabZIP21 ripen quickly with a concomitant increase in MabZIP21 phosphorylation. Additionally, MabZIP21 activates MaMPK11-4 and MaMKK1 transcription to form a regulatory feedback loop. Collectively, here we report a regulatory pathway of the MaMPK6-3/11-4-MabZIP21 module in controlling banana fruit ripening.