Figure 6.

A model for the role of light on organ initiation. Light promotes cytokinin signaling in the central zone (step 1), which relieves CLV-mediated inhibition of meristem propagation (step 2), thereby supplying a source of cells for organogenesis. (Step 3) This cytokinin-dependent meristem growth promotes organ initiation in concert with the auxin signaling pathway. Light is also required for the establishment of the PIN1–auxin loop (red area), as PIN1 is internalized in the absence of light. (Step 4) Light may affect auxin signaling, transport, or biosynthesis either directly or indirectly via cytokinin/stem cell activation. (Step 5) Based on previous reports (Ori et al. 2000; Jasinski et al. 2005; Yanai et al. 2005), auxin is likely to antagonize cytokinin in developing primordia.