Final organ size is regulated by coordinated cell proliferation and cell expansion, which control cell number and cell size, respectively. Little is known about how organ size is determined in plants, but now, on , Ran Xu and Yunhai Li report that MEDIATOR COMPLEX SUBUNIT 25 (MED25) regulates organ size in Arabidopsis thaliana. The researchers discover this new role for MED25 – a gene that controls shade avoidance and stress responses in Arabidopsis and is involved in transcriptional regulation – through a genetic screen for mutations that enhance the floral size of the da1-1 mutant; DA1 is a negative regulator of seed and organ size that restricts cell proliferation. Loss-of-function mutants in MED25 form large organs, they report, whereas plants overexpressing MED25 have small organs. These alterations in organ size are caused by changes in both cell number and cell size. Thus, the researchers suggest, MED25 acts within the transcriptional machinery to regulate plant organ size by restricting both cell proliferation and cell expansion.
