Evolution of Plant Organ Size

Just as the factors that regulate organ identity ultimately modulate growth patterns, evolution has brought about an enormous range of different organ sizes and shapes. Over the last few years, we have begun to obtain first insights into how this has been achieved at the molecular level.

Studies of natural variation in organ size and shape and efforts at mapping quantitative trait loci (QTL) that govern this variation have identified numerous QTL for different aspects of leaf and floral organ dimensions (Jünger et al. 2000; Langlade et al. 2005; Perez-Perez et al. 2002). Surprisingly, comparative studies indicate that the QTL that affect leaves and those that influence floral organs are largely distinct, while the sizes and shapes of different floral organs (sepals and petals) are highly correlated and appear to be controlled by the same QTL (Frary et al. 2004; Jünger et al. 2005). Thus, evolution may have acted on distinct leaf and floral growth gene modules, or alternatively the presumed links between organ identity factors and growth control may have been targeted by evolutionary change.

The molecular basis for natural variation in organ size is probably best understood for tomato fruit size and the role of the fw2.2 gene mentioned above. Allelic variation at this locus has been reported to account for up to 30% of the differences in fruit weight between different tomato cultivars (Frary et al. 2000). The fw2.2 gene, which encodes a protein with structural similarity to the ras-oncoprotein, limits cell division in the pericarp of developing fruits. The large-fruit allele shows an early peak of fw2.2 expression in fruit development and produces a lower overall amount of mRNA, whereas expression from the small-fruit allele peaks later and leads to a higher total expression level (Cong et al. 2002; Liu et al. 2003). Thus, a heterochronic change in the promoter of an important growth regulator that also influences overall mRNA abundance seems to underlie the difference in fruit size be tween certain tomato varieties. It will be interesting to see how widespread such regulatory mutations are in the evolution of organ size differences, as opposed to structural changes in the encoded proteins.

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