Figure 3.

Auxin interactions with other hormones during shoot development. (A) Auxin and cytokinin (CK) oppose one another in the regulation of shoot branching. Strigolactone (SL) acts as a second messenger for auxin signaling: auxin from the shoot promotes SL production in the root and shoot, which then acts to inhibit branching. (B) Auxin acts to promote differentiation in the shoot apical meristem while CK promotes stem cell division. (C) During branching, shoot-derived auxin travels down the main stem where it promotes SL biosynthesis. This SL then travels up the shoot and inhibits CK production and polar auxin transport (PAT) in the shoot, thereby blocking branching. Auxin regulates local metabolism of CK, reducing CK levels, while CK promotes auxin efflux from the developing bud. CK signaling in the bud promotes bud release and branch outgrowth. CK and SL also oppose one another in regulating the expression of BRC1, a negative regulator of bud outgrowth. Red denotes auxin and its actions; blue denotes CK and its actions; magenta denotes SL and its actions. Arrowheads represent positive regulation; blunt arrows represent negative regulation; in panels B and C, dashed lines represent indirect regulation.