Fig. 4. Partition of hormone response within callus. (A) The pARR5::GFP reporter (green) was active in callus forming shoot meristems (arrowhead), but downregulated in primordia [incipient primordia (I)₁ and primordia (P)₁]. Chlorophyll autofluorescence is in red. (B) Regenerating meristems (arrowheads) marked by pPIN1::PIN1-GFP (green) expression, emerged from callus with low pDR5rev::3XVENUS-N7 (red) expression, while pDR5rev::3XVENUS-N7 signal was observed in peripheral callus and within initiating primordia flanking meristems. (C) pPIN1::PIN1-GFP reporter expression (green) within the shoot progenitors. (D,E) Strong pDR5rev::3XVENUS-N7 expression (blue) was later detected during the initiation (D) and outgrowth of organ primordia (P₁-P_3; E). Chlorophyll autofluorescence is in red. (F) pDR5::3XVENUS-N7 signal (red) was not observed 1 minute after application of auxin paste (arrows) to callus initiating shoot meristems (arrowhead), marked by pPIN1::PIN1-GFP (green). (G) Four hours after application of auxin paste (arrows) pDR5::3XVENUS- N7 signal (red) was observed throughout the callus in contrast to the meristem itself (arrowhead), in which pDR5::3XVENUS-N7 signal was only in developing primordia. Scale bars: 50 µm (A,C-E); 100 µm (B,F,G).