Primary metabolism as a major component of fruit growth and quality

One of the key goals of fruit biology is to understand the factors that influence fruit growth and quality, ultimately with the aim of manipulating them for improvement of fruit traits. Improvement is not a simple matter. Fruits are under attack from changing environmental factors as temperatures rise and pathogens spread. In a free access review in Annals of Botany, Bertrand Beauvoit and colleagues also note that changing consumer tastes also put distinct demands on breeders for improved crops based on factors like taste and durability.

They argue that biologists should examine metabolism, the chemical processes that run and build the cells in the fruit. Metabolism is a diverse process in flowering plants, but they argue that most of the differences are due to specialised metabolism and that primary metabolism is similar among plants. Writing in the article, they say: “[C]haracters such as sweetness, sourness, nitrogen content and the antioxidant properties of fruits, as well as fruit yield, are the result of a complex interplay between transport, compartmentalization and the transformation of metabolites.”

Metabolism diagram — Simplified representation of fruit primary metabolism. Major primary pathways and compounds involved in fruit growth and quality are represented. Sucrose and amino acids represent the major imported pools. Sucrose is first metabolized via sugar metabolism, which produces starch and precursors of cell wall components, and the major antioxidant ascorbate. A large fraction of sucrose and/or hexoses is stored in the vacuole. Hexose phosphates, which are intermediates of sucrose metabolism, are degraded via glycolysis to release energy and yield precursors of the TCA cycle and fatty acid metabolism. Alternatively, hexose phosphates are metabolized via the pentose phosphate pathway, which yields precursors of nucleotides and specialized metabolites. The TCA cycle coupled to respiration releases energy and provides precursors for amino acid synthesis. Imported amino acids, of which glutamine, glutamate, aspartate and asparagine are often the dominant forms, provide nitrogen and carbon skeletons for the synthesis of further amino acids. The major compounds that mainly influence fruit quality are in bold. Abbreviations: P, phosphate; UDP, uridine diphosphate; acetyl-CoA, acetyl-coenzyme A. Image Beauvoit *et al*. 2018. Click to enlarge.

Looking at the plants at a cellular level reverse the usual process of fruit improvement. People have looked at the phenotype, the physical expression of the genes, to see what the most desirable traits in a fruit and picked the best fruits for breeding. If you understand the interaction between genes, metabolism and product, then it’s possible to understand how the traits you are selecting for interact.

Beauvoit and colleagues also argue that this understanding could help improve modelling approaches to developing fruit. They conclude: “Whereas statistical approaches are now widely used in fruit research to model high-dimensional post-genomic data sets, mechanistic models of metabolism are just emerging. It is almost certain that once these mechanistic models have incorporated environmental factors, they will become extremely useful in advancing our knowledge of fruit metabolism. However, experimentation on fruit-producing crops is usually costly and time-consuming, especially when slow-growing fruits are studied. In consequence, anticipating as much as possible future needs in terms of modelling might prove very useful.”