Overexpression of stress-related genes, BrERF4 and AtMYB44, in Arabidopsis thaliana alters cell expansion but not cell proliferation during leaf growth

We have previously shown that overexpression of BrERF4 (BrassicarapaETHYLENE-RESPONSIVEFACTOR4) increases salt and drought tolerance in Arabidopsis plants, and also retarded organ growth. In the present study, we investigated in detail the leaf growth retardation phenotype at the cellular level. Results showed that BrERF4-overexpressing Arabidopsis plants developed small leaves by reducing their cell size but not the cell number. Detailed kinematic analysis revealed that changes in cell size appeared from the very early stages of leaf development, directly affecting the size of leaf organs. RT-PCR analysis showed that expression of expansin genes was reduced in the overexpressors, whereas expression of the cell cycle gene, CYCB1;1, was not altered at all. In addition, overexpression of AtMYB44, another stress-related transcription factor gene, reduced leaf growth, which also resulted from reduction in cell size but not in cell number. These results suggest that overexpression of those transcription factors negatively affects cell expansion during leaf growth without altering cell number. We discuss about the advantages that the BrERF4- or AtMYB44-induced cell expansion retardation confers on plants under natural environmental adversity.