Volatile profiles of members of the USDA Geneva Malus Core Collection: utility in evaluation of a hypothesized biosynthetic pathway for esters derived from 2-methylbutanoate and 2-methylbutan-1-ol

Transcriptome and Metabolomics Integrated Analysis Reveals MdMYB94 Associated with Esters Biosynthesis in Apple (Malus × domestica)

Volatile esters are major aromas contributing to the organoleptic quality of apple fruit. However, the molecular mechanisms underlying the regulation of volatile ester biosynthesis in apple remain elusive. This study investigated the volatile profiles and transcriptomes of 'Qinguan' (QG) apple fruit during development and/or postharvest storage. Although the constitution of volatiles varied widely between the peel and flesh, the volatile profiles of the peel and flesh of ripening QG fruit were dominated by volatile esters. WGCNA results suggested that 19 genes belonging to ester biosynthesis pathways and 11 hub transcription factor genes potentially participated in the biosynthesis and regulation of esters. To figure out key regulators of ester biosynthesis, correlation network analysis, dual-luciferase assays, and yeast one-hybrid assay were conducted and suggested that MdMYB94 trans-activated the MdAAT2 promoter and participated in the regulation of ester biosynthesis. This study provides a framework for understanding ester biosynthesis and regulation in apple.

Amino acids metabolism as a source for aroma volatiles biosynthesis

Aroma volatiles are essential for plant ecological fitness and reproduction. Plants produce and use volatiles to attract pollinators and seed dispersers, repel herbivores and recruit their natural enemies, and communicate with other plants. Amino acids and their biosynthetic intermediates play key roles as precursors for the biosynthesis of plant volatiles. Different plants utilize different strategies and biosynthetic pathways to meet their specific biological needs. This review focuses on the different biosynthetic pathways that plants utilize to form amino acid-derived aroma volatiles, emphasizing their common and unique aspects and stressing the importance of the limiting enzymes residing in the primary-specialized metabolism interface. We also briefly review how biotechnology has used this interface and point to promising future directions for improving the quality of agricultural produce and the production of key volatiles.

Functional Characteristics of Aldehyde Dehydrogenase and Its Involvement in Aromatic Volatile Biosynthesis in Postharvest Banana Ripening

Butanol vapor feeding to ripe banana pulp slices produced abundant butyl butanoate, indicating that a portion of butanol molecules was converted to butanoate/butanoyl-CoA via butanal, and further biosynthesized to ester. A similar phenomenon was observed when feeding propanol and pentanol, but was less pronounced when feeding hexanol, 2-methylpropanol and 3-methylbutanol. Enzymes which catalyze the cascade reactions, such as alcohol dehydrogenase (ADH), acetyl-CoA synthetase, and alcohol acetyl transferase, have been well documented. Aldehyde dehydrogenase (ALDH), which is presumed to play a key role in the pathway to convert aldehydes to carboxylic acids, has not been reported yet. The conversion is an oxygen-independent metabolic pathway and is enzyme-catalyzed with nicotinamide adenine dinucleotide (NAD⁺) as the cofactor. Crude ALDH was extracted from ripe banana pulps, and the interference from ADH was removed by two procedures: (1) washing off elutable proteins which contain 95% of ADH, but only about 40% of ALDH activity, with the remaining ALDH extracted from the pellet residues at the crude ALDH extraction stage; (2) adding an ADH inhibitor in the reaction mixture. The optimum pH of the ALDH was 8.8, and optimum phosphate buffer concentration was higher than 100 mM. High affinity of the enzyme was a straight chain of lower aldehydes except ethanal, while poor affinity was branched chain aldehydes.

Citramalate synthase yields a biosynthetic pathway for isoleucine and straight- and branched-chain ester formation in ripening apple fruit

A plant pathway that initiates with the formation of citramalate from pyruvate and acetyl-CoA by citramalate synthase (CMS) is shown to contribute to the synthesis of α-ketoacids and important odor-active esters in apple (Malus × domestica) fruit. Microarray screening led to the discovery of a gene with high amino acid similarity to 2-isopropylmalate synthase (IPMS). However, functional analysis of recombinant protein revealed its substrate preference differed substantially from IPMS and was more typical of CMS. MdCMS also lacked the regulatory region present in MdIPMS and was not sensitive to feedback inhibition. 13C-acetate feeding of apple tissue labeled citramalate and α-ketoacids in a manner consistent with the presence of the citramalate pathway, labeling both straight- and branched-chain esters. Analysis of genomic DNA (gDNA) revealed the presence of two nearly identical alleles in "Jonagold" fruit (MdCMS_1 and MdCMS_2), differing by two nonsynonymous single-nucleotide polymorphisms (SNPs). The mature proteins differed only at amino acid 387, possessing either glutamine387 (MdCMS_1) or glutamate387 (MdCMS_2). Glutamate387 was associated with near complete loss of activity. MdCMS expression was fruit-specific, increasing severalfold during ripening. The translated protein product was detected in ripe fruit. Transient expression of MdCMS_1 in Nicotiana benthamiana induced the accumulation of high levels of citramalate, whereas MdCMS_2 did not. Domesticated apple lines with MdCMS isozymes containing only glutamate387 produced a very low proportion of 2-methylbutanol- and 2-methylbutanoate (2MB) and 1-propanol and propanoate (PROP) esters. The citramalate pathway, previously only described in microorganisms, is shown to function in ripening apple and contribute to isoleucine and 2MB and PROP ester biosynthesis without feedback regulation.