The older the wiser, unless you are a banana: The NAP1-MADS1 network in the regulation of banana ripening

If you are a banana bread lover, having overripe bananas is hardly a problem. However, for manufacturers, overripe bananas pose significant challenges in terms of handling and storage. Characteristics such as peel lightening, fruit softening, and reduced resistance of the banana finger (the part we use to start peeling) can complicate post-harvest processing and transportation (Wang et al. 2018). Ethylene is well known to play a central role in the ripening of climacteric fruits like bananas, acting as a hormonal trigger that accelerates the ripening process (Barry and Giovannoni 2007). This hormone not only influences external changes, like peel color, but also affects the internal composition of the fruit, leading to altered texture, sugar content, and aroma. However, without fully understanding the underlying mechanisms, accurately manipulating or controlling this process remains a significant challenge for industries relying on the precise timing of ripening.

Recently, Hua Li and colleagues (Li et al. 2024) delved into the ripening process of banana fruits (Musa acuminata) to uncover the precise mechanisms that govern this tightly regulated process. Their phenotypic analysis revealed that after 5 days of ethylene exposure, significant changes in banana color and texture were observed, linked to increased activity of cell wall–related proteins. To identify the transcriptional changes driving this phenotype, the research team conducted RNA-sequencing analysis on 2 regions of the banana fruit, the central zone and the finger drop zone, before and after ripening. The analysis revealed transcriptional activation of ethylene biosynthesis genes, such as ACC synthases and ACC oxidases, along with cell wall–related genes, consistent with autocatalytic activity of ethylene and its impact on fruit softening. Additionally, transcription factors (TFs) from the NAC and MADS families were found to be highly upregulated after ripening, underscoring their crucial role in orchestrating the ripening process.

The authors went on to investigate changes in chromatin accessibility following ethylene treatment using DNase hypersensitive site sequencing, which shows highly differentially accessible regions that generally correspond to promoter regions. Through footprint analysis, which identifies potential TF binding sites, a significant enrichment for NAC and WRKY family binding sites was found after ripening, underscoring their pivotal role. Correlating these findings with transcriptomic data, 2 key players from these families were identified: MaNAP1 and MaWRKY49. To further validate the connection between ethylene signaling and these TFs, the direct binding of MaEIN3—a crucial ethylene signaling component in banana (Lü et al. 2018)—was examined using DNA Affinity Purification sequencing coupled with DNase hypersensitive site sequencing. As expected, MaEIN3 was shown to directly bind the promoter of MaNAP1, and, in addition, it was found to bind the promoter of the TF MaMADS1.

Transient overexpression of MaNAP1 and MaMADS1 confirmed their roles in banana ripening, as overexpression led to an early ripening phenotype compared with control samples, along with elevated expression of ripening markers. Through Chromatin Immunoprecipitation sequencing, the authors identified key binding sites for these TFs, revealing that MaNAP2, MaLOB41, MaWRKY49, and MaWRKY31 are primary targets of the MaNAP1-MaMADS1 regulatory module. To further validate these findings, DNA Affinity Purification sequencing analyses were conducted for all these TFs, suggesting roles in activating specific fruit ripening markers, ethylene biosynthesis, and a regulatory feedback loop. Additionally, histone epigenetic changes were examined to assess chromatin accessibility, and the results showed a correlation between the MaNAP1-MaMADS1 module and enhanced promoter accessibility for genes involved in the ripening process.

The comprehensive datasets generated through several next-generation sequencing approaches significantly enhances our understanding of the intricate regulation of banana ripening, ultimately reinforcing the experimentally validated model proposed by the authors (Fig.). The model is based on a robust empirical analysis and clarifies the roles of MaNAP1 and MaMADS1 while illustrating the key role of an EIN3-regulated hierarchical genetic network governing banana ripening. Furthermore, this work addresses critical gaps in our understanding of the complex and the finely tuned process of banana ripening. With the identification of these key players, this model opens new opportunities for developing innovative strategies to manipulate banana ripening during processing, allowing for easier handling without undesirable side effects.

Figure.

Proposed model of the EIN3-activated hierarchical transcriptional network regulated by the MaNAP1-MaMADS1 module during banana ripening. Image credit: Raul Sanchez-Muñoz.

Data availability

There are no new data associated with this article.

Dive Curated Terms

The following phenotypic, genotypic, and functional terms are of significance to the work described in this paper:

• ethylene CHEBI: CHEBI:29362