Comparative Study of Volatile Compounds and Expression of Related Genes in Fruit from Two Apple Cultivars during Different Developmental Stages

Molecular Regulatory Mechanisms Affecting Fruit Aroma

Aroma, an important quality characteristic of plant fruits, is produced by volatile organic compounds (VOCs), mainly terpenes, aldehydes, alcohols, esters, ketones, and other secondary metabolites, in plant cells. There are significant differences in the VOC profile of various fruits. The main pathways involved in the synthesis of VOCs are the terpenoid, phenylalanine, and fatty acid biosynthesis pathways, which involve several key enzyme-encoding genes, transcription factors (TFs), and epigenetic factors. This paper reviews the main synthetic pathways of the main volatile components in fruit, summarizes studies on the regulation of aroma formation by key genes and TFs, summarizes the factors affecting the fruit aroma formation, describes relevant studies on the improvement of fruit flavor quality, and finally proposes potential challenges and prospects for future research directions. This study provides a theoretical basis for the further precise control of fruit aroma quality and variety improvement.

Advancements in Reference Gene Selection for Fruit Trees: A Comprehensive Review

Real-time quantitative polymerase chain reaction (qRT-PCR) has been widely used in gene expression analyses due to its advantages of sensitivity, accuracy and high throughput. The stability of internal reference genes has progressively emerged as a major factor affecting the precision of qRT-PCR results. However, the stability of the expression of the reference genes needs to be determined further in different cells or organs, physiological and experimental conditions. Methods for evaluating these candidate internal reference genes have also evolved from simple single software evaluation to more reliable and accurate internal reference gene evaluation by combining different software tools in a comprehensive analysis. This study intends to provide a definitive reference for upcoming research that will be conducted on fruit trees. The primary focus of this review is to summarize the research progress in recent years regarding the selection and stability analysis of candidate reference genes for different fruit trees.

Advances in detecting fruit aroma compounds by combining chromatography and spectrometry

Fruit aroma is produced by volatile compounds, which can significantly enhance fruit flavor. These compounds are highly complex and have remarkable pharmacological effects. The synthesis, concentration, type, and quantity of fruit aroma substances are affected by various factors, both abiotic and biotic. To fully understand the aroma substances of various fruits and their influencing factors, detection technology can be used. Many methods exist for detecting aroma compounds, and approaches combining multiple instruments are widely used. This review describes and compares each detection technology and discusses the potential use of combined technologies to provide a comprehensive understanding of fruit aroma compounds and the factors influencing their synthesis. These results can inform the development and utilization of fruit aroma substances. © 2023 Society of Chemical Industry.

Physicochemical Attributes, Volatile Profile and Sensory Quality of Organic Crimson Crisp Apples during On-Tree Maturation

When new apple cultivars are planted, knowledge of their maturation and ripening behavior and, as a consequence, the best time for their harvest is of utmost importance for providing fruits of the highest quality to consumers. In this study, we followed the on-tree maturation and ripening behavior of organic Crimson Crisp apples over a period of almost 3 months. With the weekly analyses of basic fruit quality attributes (weight, fruit firmness, total soluble solids, titratable acids, starch degradation) in combination with the fruit volatilome (primary and secondary volatiles) and sensory analysis, we obtained a holistic picture of the maturation and ripening properties of this new variety. We could show that at the recommended harvest window, which is based on the degree of starch degradation alone, the fruit development is not finished. Synthesis of aroma volatiles—which is strongly related to the expression of pronounced fruity, apple-like flavor—requires two additional weeks of on-tree ripening. Results indicate an upregulation of 13-LOX at very early maturation stages, while upregulation of 9-LOX and enzyme systems involved in the β-oxidation pathway requires a prolonged on-tree maturation period. The results of this study demonstrate that the analysis of basic fruit quality attributes is not sufficient for understanding the properties of apples. However, we demonstrate that the analysis of the fruit volatilome is a valuable and necessary tool for optimizing the quality of new apple varieties.

Transcriptome and Metabolome Analyses Provide Insights Into the Composition and Biosynthesis of Grassy Aroma Volatiles in White-Fleshed Pitaya

Aroma is one of the major inherent quality characteristics in fruits. Understanding the composition of aroma volatiles and their biosynthesis mechanism is crucial to improving fruit quality. However, the biosynthesis mechanism of aroma volatiles has not been characterized yet in white-fleshed pitaya (Hylocereus undatus). This study was performed to investigate aroma volatiles and related gene expression patterns in the pulp of "mild grassy" and "strong grassy" aroma cultivars. Analysis of volatile composition and concentration showed that aldehydes, alcohols, esters, and alkenes were predominant in both cultivars. However, comparative analysis revealed a significant difference in the concentration of several metabolites, particularly hexanal and 1-hexanol. The results of the comparative transcriptome identified a large number of aroma-related differentially expressed genes. The majority of these genes were enriched in fatty acid and isoleucine degradation pathways. According to integrative analyses, changes in the expression of lipoxygenase pathway genes, specifically FAD, LOXs, HPLs, and ADHs, probably lead to the difference in strength of "grassy" aroma between both cultivars. The qRT-PCR of 18 aroma-related genes was performed to validate the transcriptome analysis. Our results identified key genes and pathways connected with the biosynthesis of aroma volatiles in white-fleshed pitaya. These results will be useful to dissect the genetic mechanism of fruit aroma in white-fleshed pitaya.