Phototropins (phot) are blue light receptors in plants which are involved in phototropism, stomatal opening, and chloroplast movements. Phototropin has two LOV domains (LOV1 and LOV2), and the LOV2 domain is responsible for activation of Ser/Thr kinase. There is an alpha-helix at the C-terminal side of the LOV2 domain, which is called the Jalpha helix. The functional importance of the Jalpha helix has been established for Arabidopsis phot1, where light-induced structural perturbation takes place in the Jalpha helix during the photocycle of LOV2 domains. However, the present FTIR study reports a different role of the Jalpha helix in light-induced signal transduction of LOV2 domains. Here we construct LOV2 domains with (LOV-Jalpha) and without (LOV-core) the Jalpha helix for Arabidopsis phot1 and phot2 and Adiantum neochrome 1 and compare their light-induced difference FTIR spectra. Light-induced protein structural changes differ significantly between LOV-Jalpha and LOV-core for Arabidopsis phot1 [Yamamoto, A., Iwata, T., Sato, Y., Matsuoka, D., Tokutomi, S., and Kandori, H. (2009) Biophys. J. 96, 2771-2778]. In contrast, the difference spectra are identical between LOV-Jalpha and LOV-core for Adiantum neochrome 1. In Arabidopsis phot2, the protein structural changes are intermediate between Arabidopsis phot1 and Adiantum neochrome 1. These results suggest that the conformational changes of the Jalpha helix and the interaction between the LOV-core and the Jalpha helix are different among phototropins. The role of the Jalpha helix for signal transduction in phototropins is discussed.