Higher Level Control of Organ Size and Plant Wide Integration

Tomato fruit size also serves to exemplify the control of plant organ growth by integrative mechanisms that operate on the level of a part of or the entire plant. It has long been known that pruning to reduce the total number of fruits borne by a plant causes an increase in the size of the remaining fruits, indicating that individual organs compete with each other for some limiting factor(s), such as possibly photoassimilates and nutrients. Recent work has shown that a reduction in fruit load leads to altered expression of key cell cycle regulators and of fw2.2, suggesting that resource availability may have a rather direct influence on growth and proliferation (Baldet et al. 2006). How this is sensed by individual cells and translated into the appropriate growth response is discussed by Doerner and Tsukaya in the present volume.

Changing not the number of developing sink organs but their sink strength can also influence their final size: apoplastic expression of a yeast invertase, which cleaves sucrose into glucose and fructose and is thought to influence sink strength, was found to increase tuber size in potato tubers (Sonnewald et al. 1997). However, the effect was compartment-specific, and targeting in-vertase to the cytosol actually had the opposite effect.

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