How Pathogenic Fungi Exploit Organic Acids to Promote Postharvest Fruit Deterioration

by Jorge Luis Alonso with ChatGPT Introduction Fungal infections are a major cause of postharvest fruit decay, with specific enzymes and compounds contributing to their virulence. Fungi can be divided into acidic and alkaline types. Acidic fungi, such as Penicillium and Botrytis species, secrete organic acids that enhance their infectivity by regulating the pH of the host environment. Organic acid secretion also activates virulence factors and increases the pathogenicity of these fungi. The relationship between acidification and pathogenicity is not fully understood, but research suggests that acidification depends on host carbon levels and gene expression. Knowledge of these mechanisms is critical to controlling postharvest diseases and minimizing fruit losses. This paper reviews postharvest acidic fungi and their organic acids and examines the role of organic acids in the pathogenicity and infection of plant tissues. Acidic fungi and the predominant organic?acids Fungal pathogens secrete pH-regulating compounds, such as organic acids or ammonia, to promote infection by lowering or raising the pH of the environment. As mentioned above, these fungi are classified as acidic or alkaline. Many postharvest fungi infect hosts by secreting organic acids such as oxalic, gluconic, and citric acids. Organic acid content is related to fungal pathogenicity; a decrease in organic acid accumulation leads to reduced virulence. Disabling genes involved in organic acid synthesis or encoding transcription factor genes reduces fungal pathogenicity. However, pathogenicity is not solely dependent on organic acid content. Biosynthesis of organic acids in acidic?fungi Organic acids play an important role in the virulence of acidophilic fungi by altering the apoplastic pH during pathogen attack. Oxalic, citric, and gluconic acids are essential for fungal pathogenicity because they activate other pathogenic factors such as polygalacturonases. Oxalic acid is required for pathogenic virulence, while gluconic acid triggers the biosynthesis and accumulation of patulin. Exogenous elicitors such as oxalic acid and ammonia highlight the importance of organic acids for pathogenic fungi. Artificial acidification of host tissues can enhance the virulence of certain pathogens. Regulation of organic acid synthesis or identification of elicitors for postharvest control of fruit acidity will be promising approaches to prevent postharvest fruit diseases. Regulation of organic acid production Pathogenic fungi use environmental pH signaling pathways to adjust the pH of their environment, thereby influencing their virulence and pathogenicity. The transcription factor PacC regulates organic acid production and accumulation in postharvest fungi, and its expression is affected by pH. Cyclic adenosine monophosphate and the mitogen-activated protein kinase pathway also regulate fungal appressorium formation and interact with environmental pH signaling. The production of organic acids by fungi is influenced by several factors, including the composition and conditions of the host medium. For example, maximum citric acid production by A. niger occurs at pH < 2, while gluconic acid is produced at pH 4.5?6.5 and oxalic acid above pH 5. Mechanisms of action of organic acids involved in fungal pathogenicity Oxalic acid Oxalic acid is a critical factor in the pathogenesis of S. sclerotiorum and B. cinerea. It promotes initial acid accumulation and decreases extracellular pH, chelates metal ions from plant tissues, alters redox reactions in plant tissues, induces reactive oxygen species (ROS) production, and inhibits defense responses. Oxalic acid can also stimulate early anion and K+ loss from plant cells, causing a programmed cell death (PCD) response, and inhibit plant cell autophagy, promoting successful infection by S. sclerotiorum. Gluconic acid Penicillium spp. and P. mangiferae secrete gluconic acid to enhance virulence, primarily by lowering host pH, which increases cell wall degrading enzyme activity and toxin accumulation. Citric acid Citric acid secreted by fungi during fruit infection enhances their virulence by (a) lowering the pH of host tissues, (b) creating an acidic environment for cell wall-degrading enzyme activity, (c) inhibiting H2O2 production in host tissues, and (d) weakening host cell wall functions by chelating Ca2+. Other organic acids Some fungi, such as F. oxysporum and P. digitatum, secrete organic acids such as fumaric acid, which can directly promote pathogenesis. However, not all secreted organic acids necessarily contribute to fungal virulence and pathogenicity. Conclusions Numerous studies have highlighted the increased pathogenicity of acidic fungi through the secretion of organic acids and the lowering of environmental pH. To further understand these complex acidification mechanisms, researchers should focus on several critical areas related to fungal adaptation, host interactions, and host resistance to invasion. First, scientists need to identify and study novel acidic fungal species, as only a few have been discovered among the numerous postharvest pathogens affecting fruits and vegetables. Expanding these databases can help improve understanding of the role of acidification in pathogenicity. Second, researchers should investigate the precise mechanisms behind organic acid secretion from fungal cells and explore the relationship between pH sensing and known pathogenic signaling modules. This will help elucidate the acidification process and its influence on fungal-host interactions. Third, comprehensive information on pathogenic mechanisms and regulatory networks of organic acid molecules during acidification must be demonstrated. This may provide valuable insights into the processes involved in fungal pathogenicity. Finally, researchers need to thoroughly investigate the transport and metabolic pathways of key organic acids during pathogenesis. This will contribute to a better understanding of the role of organic acids in the complex interactions between fungi and their hosts. By focusing on these areas, future research can shed light on the specific fungal species responsible for acidification, the types of organic acids secreted, and the pH signaling pathways. These findings will not only improve the understanding of pH change mechanisms on pathogenicity but also provide theoretical and technical guidance for the development of new disease control strategies. Ultimately, interfering with acidification processes may help reduce post-harvest losses and protect the global food supply. Source: Jiao, W., Liu, X., Li, Y., Li, B., Du, Y., Zhang, Z., Chen, Q., & Fu, M. (2022). Organic acid, a virulence factor for pathogenic fungi, causing postharvest decay in fruits. Molecular Plant Pathology, 23(2), 304?312. Author:?Jorge Luis?Alonso G. (with ChatGPT)?Maximizing Agribusiness Profits with Expert Postharvest Storage Strategies | Horticultural Writing Specialist.?This article was written exclusively for the business platform Postharvest. Image by?G?bor Adonyi?from?Pixabay?