Do you have to put fruit in the fridge to keep it longer? Not for bananas! Because bananas turn brown in the fridge. Polyphenol oxidase (PPO) is widely considered as one of the important factors for the tissue browning of fruits and vegetables. However, the molecular mechanism of banana peel browning is still poorly understood.

In a new study published in New Phytologist, the research group led by Professor JIANG Yueming from South China Botanical Garden(SCBG) of Chinese Academy of Sciences (CAS), and the research group led by Professor XIA Rui from South China Agricultural University (SCAU), have jointly made progress in understanding the miRNA528-mediated regulation of ROS homeostasis in monocots. In this study, the researchers found that miR528 targets the PPO gene and plays a very important role in the cold stress response in banana. At low temperature, the decreased expression of miR528 led to a hundred-fold increase in PPO gene expression, which caused the increase of reactive oxygen species (ROS) level, and finally led to the appearance of banana peel browning. Now you know why bananas go brown in the fridge.

MicroRNA (miRNA) is a kind of small RNA that attracts the most attention in plant research. Its length is only 21-24 nt, but it is involved in many biological processes of plant growth and development and stress response. The researchers found that miR528 is present in almost all monocotyledons, including banana. But unlike known conserved miRNA families, miR528 has only one copy in most monocot species, with its mature sequence highly conserved, suggesting a unique evolutionary selection process.

Usually, the same miRNA regulates the expression of members of the same gene family. However, miR528 is quite unique in this respect. Through large-scale targeting prediction and data analysis, the researchers found that miR528 had evolved a distinct preference of target genes among different monocots, which seemed to be dispersed to a large number of genes (families), including AAO, LAC, CBP and PPO, etc. Interestingly, almost all of these miR528 targets are copper-containing proteins. Copper (Cu) is a necessary trace element in plants and it often exists in the active center of oxidoreductases. The contents and activities of these enzymes directly affect the ROS balance in plants, which is related to the normal development and stress response of plants.

MiR528 targets copper-containing protein coding genes in a broad spectrum, which makes it a hub regulator of ROS homeostasis in monocotyledons. On one hand, miR528 inhibits the increase of ROS by targeting PPO, AAO, AO, LAC, etc; on the other hand, it inhibits the decrease of ROS by targeting POD, SOD, etc, playing an important balancing role between the trade-off game of ROS production and elimination.

Figure. Model for the role of miR528 in cellular redox homeostasis. The “Yin-Yang” symbol represents the balance of ROS level, contributed by miR528 roles in both ROS generation and scavenging (Image by Prof. XIA Rui).

This study deepens our understanding of the miRNA regulation on plant ROS homeostasis, and provides a potential feasibility for improving fruit resistance through biotechnology. This is a collaborative research effort. ZHU Hong, Associate Professor of SCBG and CHEN Chengjie, Ph.D student of SCAU are co-first authors. Professor JIANG Yueming, Duan Xuewu and Professor XIA Rui are co-corresponding authors of this paper. For details, please refer to: https://doi.org/10.1111/nph.16130.