Concluding Remarks

As we have described in this chapter, there is a growing list of genes that have been shown to influence organ size with the potential for regulation. These factors can affect growth by proliferation and/or expansion, and both positive and negative regulators have been identified. Also, initial insights have been gained into how developmental processes (patterning, cell and organ identity) and evolutionary changes alter growth patterns to make organs larger or smaller. However, one of the most crucial questions for the control of organ size is yet to gain a satisfactory answer: what is it that growing organs are measuring? As discussed above, plants with altered organ sizes mostly show changes in the timing of growth. However, given a constant rate of growth, time and size or mass of the organ are strictly coupled. Thus, experimental manipulation of the rate of growth would be needed to distinguish between whether organs are measuring time or size/mass to decide for or against further growth. Understanding this and additional fundamental growth-related questions in plants can be expected to ultimately lead to novel tools for rationally manipulating the sizes of economically relevant plant organs.

Acknowledgements We thank members of the Lenhard lab for critical reading and helpful comments on the manuscript. Work in the authors' lab is supported by the Deutsche Forschungsgemeinschaft and by a BBSRC David Phillips Fellowship.

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