Combining growth-promoting genes leads to positive epistasis in Arabidopsis thaliana

Several genes have been described to positively influence final leaf size in Arabidopsis when mutated or overexpressed. However, the connections between these growth regulating genes are still poorly understood. Clearly such knowledge would significantly contribute to understand the biological processes driving leaf growth. In this study, we performed a combinatorial screen with thirteen transgenic Arabidopsis lines with an increased leaf size. Surprisingly, we found that from 61 analyzed combinations, 39% showed an additional increase in leaf size and most of these resulted from a positive epistasis on growth. Similar to what is found in other organisms in which such an epistasis assay was performed, only few genes were highly connected in synergistic combinations. We also observed a positive epistasis in the majority of the combinations with samba, BR11(OE) or SAUR19(OE), suggesting that these growth regulators are more prone to lead to synergistic effects in binary combinations. Furthermore, positive epistasis was not only found with combinations of genes with a similar mode of action, but also with genes which affect distinct processes, such as cell proliferation and cell expansion.