Fig 7. A working model of how different light conditions and auxin regulate seedling growth through the functions of IAAs and PIFs.

The different responses of specific seedling tissues to changing light conditions and auxin reflect different patterns of IAA and PIF activity in these tissues. In young leaves, low light is perceived by photoreceptors and then triggers the accumulation of PIFs, leading to increased YUCCA expression and auxin abundance. The auxin in the leaves is transported to the hypocotyl, in which IAA3 is degraded through the TIR1-mediated proteasome pathway, releasing the inhibition of several transcription factors including PIFs, ARFs, and BZR1. The target genes of these transcription factors are activated to promote hypocotyl elongation. When the light conditions are sufficiently high, PIF accumulation in the leaves is reduced, leading to decreased YUCCA expression, and then reduced auxin levels in the leaves. Thus, the auxin in the hypocotyl which is transported from leaves is also reduced. Consequently, IAA3 represses PIFs, ARFs, and BZR1 in the hypocotyl. The blue arrows in the diagram represent auxin flow and the thickness of the arrows represent auxin level.