Phototropism

The cells on the plant that are farthest from the light contain a hormone called auxin that reacts when phototropism occurs.

[1] Most plant shoots exhibit positive phototropism, and rearrange their chloroplasts in the leaves to maximize photosynthetic energy and promote growth.

[2][3] Some vine shoot tips exhibit negative phototropism, which allows them to grow towards dark, solid objects and climb them.

[5] Auxins activate proton pumps, decreasing the pH in the cells on the dark side of the plant.

[6] Water then enters the cells along its osmotic gradient, leading to an increase in turgor pressure.

pin3 mutants had shorter hypocotyls and roots than the wild-type, and the same phenotype was seen in plants grown with auxin efflux inhibitors.

PIN3 is normally localized to the surface of hypocotyl and stem, but is also internalized in the presence of Brefeldin A (BFA), an exocytosis inhibitor.

Recent studies reveal that multiple AGC kinases, except for PHOT1 and PHOT2, are involved in plant phototropism.

Firstly, PINOID, exhibiting a light-inducible expression pattern, determines the subcellular relocation of PIN3 during phototropic responses via a direct phosphorylation.

In 2012, Sakai and Haga[12] outlined how different auxin concentrations could be arising on shaded and lighted side of the stem, giving birth to phototropic response.

Five models in respect to stem phototropism have been proposed, using Arabidopsis thaliana as the study plant.

And according to the study by Sakai and Haga, the observed asymmetric auxin distribution and subsequent phototropic response in hypocotyls seems most consistent with this fifth scenario.

[12] Phototropism in plants such as Arabidopsis thaliana is directed by blue light receptors called phototropins.

[citation needed] Cryptochromes are photoreceptors that absorb blue/ UV-A light, and they help control the circadian rhythm in plants and timing of flowering.