Metabolomics to understand metabolic regulation underpinning fruit ripening, development, and quality.

Classically fruit ripening and development was studied using genetic approaches, with understanding of metabolic changes that occurred in concert largely focused on a handful of metabolites including sugars, organic acids, cell wall components, and phytohormones. The advent and widespread application of metabolomics has, however, led to far greater understanding of metabolic components that play a crucial role not only in this process but also in influencing the organoleptic and nutritive properties of the fruits. Here we review how the study of natural variation, mutants, transgenics, and gene-edited fruits has led to a considerable increase in our understanding of these aspects. We focus on fleshy fruits such as tomato but also review berries, receptacle fruits, and stone-bearing fruits. Finally, we offer a perspective as to how comparative analyses and machine learning will likely further improve our comprehension of the functional importance of various metabolites in the future.

Going broad and deep: sequencing-driven insights into plant physiology, evolution, and crop domestication.

Deep sequencing is a term that has become embedded in the plant genomic literature in recent years and with good reason. A torrent of (largely) high-quality genomic and transcriptomic data has been collected and most of this has been publicly released. Indeed, almost 1000 plant genomes have been reported (www.plabipd.de) and the 2000 Plant Transcriptomes Project has long been completed. The EarthBioGenome project will dwarf even these milestones. That said, massive progress in understanding plant physiology, evolution, and crop domestication has been made by sequencing broadly (across a species) as well as deeply (within a single individual). We will outline the current state of the art in genome and transcriptome sequencing before we briefly review the most visible of these broad approaches, namely genome-wide association and transcriptome-wide association studies, as well as the compilation of pangenomes. This will include both (i) the most commonly used methods reliant on single nucleotide polymorphisms and short InDels and (ii) more recent examples which consider structural variants. We will subsequently present case studies exemplifying how their application has brought insight into either plant physiology or evolution and crop domestication. Finally, we will provide conclusions and an outlook as to the perspective for the extension of such approaches to different species, tissues, and biological processes.

Strawberry fruit FanCXE1 carboxylesterase is involved in the catabolism of volatile esters during the ripening process.

Volatile compounds produced during ripening of strawberry are key determinants of fruit quality and consumer preference. Strawberry volatiles are largely esters which are synthesized by alcohol acyltransferases (AATs) and degraded by carboxylesterases (CXEs). Although CXE activity can have a marked influence on volatile contents in ripe strawberry fruits, CXE function and regulation in them are poorly known. Here, we report the biochemical and functional characterization of the fruit receptacle-specific and ripening-related carboxylesterase FanCXE1. The expression of the corresponding gene was found to be antagonistically regulated by auxins and abscisic acid, key hormones that regulate fruit growth and ripening in strawberry. In vitro, FanCXE1 was able to hydrolyze artificial ester substrates similar to those produced by ripe strawberry fruits. Transient suppression of the FanCXE1 gene by RNAi resulted in an increase of important volatile esters such as methyl hexanoate, methyl butanoate and ethyl hexanoate as well as a decrease of the alcohols hexenol and linanool. The results of this work enhance our understanding of the molecular basis for volatile syntheses and facilitate production of better flavored strawberry fruits by introduction of the relevant alleles into common cultivars.

Azacytidine arrests ripening in cultivated strawberry (Fragaria × ananassa) by repressing key genes and altering hormone contents.

BACKGROUND: Strawberry ripening involves a number of irreversible biochemical reactions that cause sensory changes through accumulation of sugars, acids and other compounds responsible for fruit color and flavor. The process, which is strongly dependent on methylation marks in other fruits such as tomatoes and oranges, is highly controlled and coordinated in strawberry. RESULTS: Repeated injections of the hypomethylating compound 5-azacytidine (AZA) into green and unripe Fragaria × ananassa receptacles fully arrested the ripening of the fruit. The process, however, was reversible since treated fruit parts reached full maturity within a few days after AZA treatment was stopped. Transcriptomic analyses showed that key genes responsible for the biosynthesis of anthocyanins, phenylpropanoids, and hormones such as abscisic acid (ABA) were affected by the AZA treatment. In fact, AZA downregulated genes associated with ABA biosynthetic genes but upregulated genes associated with its degradation. AZA treatment additionally downregulated a number of essential transcription factors associated with the regulation and control of ripening. Metabolic analyses revealed a marked imbalance in hormone levels, with treated parts accumulating auxins, gibberellins and ABA degradation products, as well as metabolites associated with unripe fruits. CONCLUSIONS: AZA completely halted strawberry ripening by altering the hormone balance, and the expression of genes involves in hormone biosynthesis and degradation processes. These results contradict those previously obtained in other climacteric and fleshly fruits, where AZA led to premature ripening. In any case, our results suggests that the strawberry ripening process is governed by methylation marks.

Comparative Molecular and Metabolic Profiling of Two Contrasting Wheat Cultivars under Drought Stress.

Drought is one of the most important threats to plants and agriculture; therefore, understanding of the mechanisms of drought tolerance is crucial for breeding of new tolerant varieties. Here, we assessed the effects of a long-term water deficit stress simulated on a precision phenotyping system on some morphological criteria and metabolite traits, as well as the expression of drought associated transcriptional factors of two contrasting drought-responsive African wheat cultivars, Condor and Wadielniel. The current study showed that under drought stress Wadielniel exhibits significant higher tillering and height compared to Condor. Further, we used gas chromatography and ultra-high performance liquid chromatography mass-spectrometry to identify compounds that change between the two cultivars upon drought. Partial least square discriminant analysis (PLS-DA) revealed that 50 metabolites with a possible role in drought stress regulation were significantly changed in both cultivars under water deficit stress. These metabolites included several amino acids, most notably proline, some organic acids, and lipid classes PC 36:3 and TAG 56:9, which were significantly altered under drought stress. Here, the results discussed in the context of understanding the mechanisms involved in the drought response of wheat cultivars, as the phenotype parameters, metabolite content and expression of drought associated transcriptional factors could also be used for potential crop improvement under drought stress.

The knowns and unknowns of intracellular partitioning of carbon and nitrogen, with focus on the organic acid-mediated interplay between mitochondrion and chloroplast.

The presence of specialized cellular compartments in higher plants express an extraordinary degree of intracellular organization, which provides efficient mechanisms to avoid misbalancing of the metabolism. This offers the flexibility by which plants can quickly acclimate to fluctuating environmental conditions. For that, a fine temporal and spatial regulation of metabolic pathways is required and involves several players e.g. organic acids. In this review we discuss different facets of the organic acid metabolism within plant cells with special focus to those related to the interactions between organic acids compartmentalization and the partitioning of carbon and nitrogen. The connections between organic acids and CO2 assimilation, tricarboxylic acid (TCA) cycle, amino acids metabolism, and redox status are highlighted. Moreover, the key enzymes and transporters as well as their function on the coordination of interorganellar metabolic exchanges are discussed.