Organic acids are present in all plants, supporting numerous and varied facets of
cellular metabolism. The type of organic acid found, and the levels to which they
accumulate are extremely variable between species, developmental stages and tissue
types. Acidity plays important roles in the organoleptic properties of plant tissues,
where examples of both enhanced and reduced palatability can be ascribed to the presence
of specific organic acids. In fruits, sourness is generally attributed to proton release
from acids such as citric, malic, oxalic, quinic, succinic and tartaric, while the
anion forms each contribute a distinct taste. Acidity imposes a strong influence on
crop quality, and is an important factor in deciding the harvest date, particularly
for fruits where acidity is important for further processing, as in wine grapes. In
the grape, as for many other fruits, malate is one of the most prevalent acids, and
is an important participant in numerous cellular functions. The accumulation of malate
is thought to be due in large part to de novo synthesis in fruits such as the grape,
through metabolism of assimilates translocated from leaf tissues, as well as photosynthetic
activity within the fruit itself. During ripening, the processes through which malate
is catabolised are of interest for advancing metabolic understanding, as well as for
potential crop enhancement through agricultural or molecular practices. A body of
literature describes research that has begun to unravel the regulatory mechanisms
of enzymes involved in malate metabolism during fruit development, through exploration
of protein and gene transcript levels. Datasets derived from a series of recent microarray
experiments comparing transcript levels at several stages of grape berry development
have been revisited, and are presented here with a focus on transcripts associated
with malate metabolism. Developmental transcript patterns for enzymes potentially
involved in grape malate metabolism have shown that some flux may occur through pathways
that are less commonly regarded in ripening fruit, such as aerobic ethanol production.
The data also suggest pyruvate as an important intermediate during malate catabolism
in fruit. This review will combine an analysis of microarray data with information
available on protein and enzyme activity patterns in grapes and other fruits, to explore
pathways through which malate is conditionally metabolised, and how these may be controlled
in response to developmental and climatic changes. Currently, an insufficient understanding
of the complex pathways through which malate is degraded, and how these are regulated,
prevents targeted genetic manipulation aimed at modifying fruit malate metabolism
in response to environmental conditions.