SQUAMOSA PROMOTER BINDING PROTEIN-LIKE 9 and 13 repress BLADE-ON-PETIOLE 1 and 2 directly to promote adult leaf morphology in Arabidopsis.

The juvenile-to-adult phase transition during vegetative development is a critical decision in a plant's life cycle. This transition is mediated by a decline in levels of miR156/157, and an increase in the activities of its direct targets, SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) proteins. In Arabidopsis, the juvenile-to-adult transition is characterized by an increase in the length to width ratio of the leaf blade (a change in the distal region of a leaf), but what mediates this change in lamina shape is not known. Here, we showed that ectopic expression of SPL9 and SPL13 produces enlarged and elongated leaves, resembling leaves from the blade-on-petiole1 (bop1) bop2 double mutant. The expression of BOP1/BOP2 are down regulated in successive leaves, correlating with the amount of miR156 and antagonistic to the expression of the SPL9 and SPL13 in leaves. SPL9 and SPL13 bind to the promoters of BOP1/BOP2 directly to repress their expression, resulting in delayed establishment of proliferative region in leaves that promotes more blade outgrowth (distal region of a leaf) and suppresses petiole development (proximal region of a leaf). Our results revealed a mechanism for leaf development along the proximal-distal axis, a heteroblastic character between juvenile leaves and adult leaves.