Acts as a direct positive regulator of leaf size but not leaf shape. Expressed in seedlings, roots, stems, leaves, flowers and siliques. The protein kinase domain is predicted to be catalytically inactive. Plants show a reduction in leaf size, but no apparent defect in pollen development or fertility. Cannot functionally replace STRUBBELIG. Over-expression of SRF4 led to an increased leaf surface and to male-sterility in both cv. Landsberg and cv. Columbia. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.