What drives fruit growth?

Cell division, endoreduplication (an increase in nuclear DNA content without cell division) and cell expansion are important processes for growth. It is debatable whether organ growth is driven by all three cellular processes. Alternatively, all could be part of a dominant extracellular growth regulatory mechanism. Cell level processes have been studied extensively and a positive correlation between cell number and fruit size is commonly reported, although few positive correlations between cell size or ploidy level and fruit size have been found. Here, we discuss cell-level growth dynamics in fruits and ask what drives fruit growth and during which development stages. We argue that (1) the widely accepted positive correlation between cell number and fruit size does not imply a causal relationship; (2) fruit growth is regulated by both cell autonomous and noncell autonomous mechanisms as well as a global coordinator, the target of rapamycin; and (3) increases in fruit size follow the neocellular theory of growth.

Fruit illumination stimulates cell division but has no detectable effect on fruit size in tomato (Solanum lycopersicum).

Light affects plant growth through assimilate availability and signals regulating development. The effects of light on growth of tomato fruit were studied using cuvettes with light-emitting diodes providing white, red or blue light to individual tomato trusses for different periods during daytime. Hypotheses tested were as follows: (1) light-grown fruits have stronger assimilate sinks than dark-grown fruits, and (2) responses depend on light treatment provided, and fruit development stage. Seven light treatments [dark, 12-h white, 24-h white, 24-h red and 24-h blue light, dark in the first 24 days after anthesis (DAA) followed by 24-h white light until breaker stage, and its reverse] were applied. Observations were made between anthesis and breaker stage at fruit, cell and gene levels. Fruit size and carbohydrate content did not respond to light treatments while cell division was strongly stimulated at the expense of cell expansion by light. The effects of light on cell number and volume were independent of the combination of light color and intensity. Increased cell division and decreased cell volume when fruits were grown in the presence of light were not clearly corroborated by the expression pattern of promoters and inhibitors of cell division and expansion analyzed in this study, implying a strong effect of posttranscriptional regulation. Results suggest the existence of a complex homeostatic regulatory system for fruit growth in which reduced cell division is compensated by enhanced cell expansion.

A multilevel analysis of fruit growth of two tomato cultivars in response to fruit temperature.

Fruit phenotype is a resultant of inherent genetic potential in interaction with impact of environment experienced during crop and fruit growth. The aim of this study was to analyze the genetic and physiological basis for the difference in fruit size between a small ('Brioso') and intermediate ('Cappricia') sized tomato cultivar exposed to different fruit temperatures. It was hypothesized that fruit heating enhances expression of cell cycle and expansion genes, rates of carbon import, cell division and expansion, and shortens growth duration, whereas increase in cell number intensifies competition for assimilates among cells. Unlike previous studies in which whole-plant and fruit responses cannot be separated, we investigated the temperature response by varying fruit temperature using climate-controlled cuvettes, while keeping plant temperature the same. Fruit phenotype was assessed at different levels of aggregation (whole fruit, cell and gene) between anthesis and breaker stage. We showed that: (1) final fruit fresh weight was larger in 'Cappricia' owing to more and larger pericarp cells, (2) heated fruits were smaller because their mesocarp cells were smaller than those of control fruits and (3) no significant differences in pericarp carbohydrate concentration were detected between heated and control fruits nor between cultivars at breaker stage. At the gene level, expression of cell division promoters (CDKB2, CycA1 and E2Fe-like) was higher while that of the inhibitory fw2.2 was lower in 'Cappricia'. Fruit heating increased expression of fw2.2 and three cell division promoters (CDKB1, CDKB2 and CycA1). Expression of cell expansion genes did not corroborate cell size observations.