Banana is a fruit with high nutrient content and high economic importance. It is regarded as a main staple food in developing countries. As a typical climacteric fruit, banana will ripen and decay in one week after exogenous ethylene induction. The short shelf life of banana largely limits its storage, transportation and marketing, and causes great post‐harvest loss (Krishnakumar, T. and Thirupathi, V. 2014). The shelf life of banana is closely related to ethylene production, which is the first factor considered for developing postharvest preservation technology.

Banana (Musa acuminata, AAA group) is a representative climacteric fruit with essential nutrients and pleasant flavors. Control of its ripening determines both the fruit quality and the shelf life. NAC (NAM, ATAF, CUC2) proteins, as one of the largest superfamilies of transcription factors, play crucial roles in various functions, especially developmental processes. Thus, it is important to conduct a comprehensive identification and characterization of the NAC transcription factor family at the genomic level in M. acuminata. In this article, a total of 181 banana NAC genes were identified. Phylogenetic analysis indicated that NAC genes in M. acuminata, Arabidopsis, and rice were clustered into 18 groups (S1–S18), and MCScanX analysis disclosed that the evolution of MaNAC genes was promoted by segmental duplication events. Expression patterns of NAC genes during banana fruit ripening induced by ethylene were investigated using RNA-Seq data, and 10 MaNAC genes were identified as related to fruit ripening. A subcellular localization assay of selected MaNACs revealed that they were all localized to the nucleus. These results lay a good foundation for the investigation of NAC genes in banana toward the biological functions and evolution.

Fruit ripening is the last phase of fruit growth and development. The initiation and progression of fruit ripening are highly modulated by a plethora of key genes, such as transcription factor (TF) genes. The WRKY gene family is a large group of TFs that play important roles in various cellular processes; nevertheless, the role of WRKY TF on fruit ripening remains enigmatic. Here, we report that a banana WRKY TF, MaWRKY49 functions in ethylene-induced fruit ripening by modulating the expression of fruit softening-related genes. We found that the expression of MaWRKY49 is highly induced by ethephon and inhibited by 1-methylcyclopropene, which is synchronous with the ripening process. Moreover, based on transcriptome data on fruit ripening, two pectate lyase (PL) genes that are involved in fruit softening were determined, and their expression pattern is also consistent with the fruit ripening process. Yeast one-hybrid and dual-luciferase assay confirmed that MaWRKY49 activated the transcription of two PL genes. In addition, transient overexpression of MaWRKY49 in banana fruits can apparently accelerate fruit ripening processs. Taken together, our findings indicate that MaWRKY49 acts as a potential modulator of fruit ripening by direct regulation of PL expression. This work contributes to developing the technology for improving the shelf-life of banana fruit.

Background: Banana plant height is an important trait for horticultural practices and semi-dwarf cultivars show better resistance to damages by wind and rain. However, the molecular mechanisms controlling the pseudostem height remain poorly understood. Herein, we studied the molecular changes in the pseudostem of a semi-dwarf banana mutant Aifen No. 1 (Musa spp. Pisang Awak sub-group ABB) as compared to its wild-type dwarf cultivar using a combined transcriptome and metabolome approach. Results: A total of 127 differentially expressed genes and 48 differentially accumulated metabolites were detected between the mutant and its wild type. Metabolites belonging to amino acid and its derivatives, flavonoids, lignans, coumarins, organic acids, and phenolic acids were up-regulated in the mutant. The transcriptome analysis showed the differential regulation of genes related to the gibberellin pathway, auxin transport, cell elongation, and cell wall modification. Based on the regulation of gibberellin and associated pathway-related genes, we discussed the involvement of gibberellins in pseudostem elongation in the mutant banana. Genes and metabolites associated with cell wall were explored and their involvement in cell extension is discussed. Conclusions: The results suggest that gibberellins and associated pathways are possibly developing the observed semi-dwarf pseudostem phenotype together with cell elongation and cell wall modification. The findings increase the understanding of the mechanisms underlying banana stem height and provide new clues for further dissection of specific gene functions.

The CRISPR/Cas9-mediated genome editing system has been used extensively to engineer targeted mutations in a wide variety of species. Its application in banana, however, has been hindered because of the species’ triploid nature and low genome editing efficiency. This has delayed the development of a DNA-free genome editing approach. In this study, we reported that the endogenous U6 promoter and banana codon-optimized Cas9 apparently increased mutation frequency in banana, and we generated a method to validate the mutation efficiency of the CRISPR/Cas9-mediated genome editing system based on transient expression in protoplasts. The activity of the MaU6c promoter was approximately four times higher than that of the OsU6a promoter in banana protoplasts. The application of this promoter and banana codon-optimized Cas9 in CRISPR/Cas9 cassette resulted in a fourfold increase in mutation efficiency compared with the previous CRISPR/Cas9 cassette for banana. Our results indicated that the optimized CRISPR/Cas9 system was effective for mutating targeted genes in banana and thus will improve the applications for basic functional genomics. These findings are relevant to future germplasm improvement and provide a foundation for developing DNA-free genome editing technology in banana.