Directional growth towards an asymmetrical
light source is known as phototropism. In
Arabidopsis seedlings, this is a blue light
response and is mediated by NPH1. NPH1 is
localized at the plasma membrane and
autophosphorylates in a blue-light-dependent
manner. This could mediate phospho-specific
interactions with downstream factors, such
as NPH2, NPH3 or NPH4. Alternatively,
NPH1 might phosphorylate NPH2, NPH3 or
NPH4. These two models are not mutually
exclusive and are hypothesized to initiate a
signaling cascade that ultimately results in
asymmetric growth.