Fluorescence and photochemical properties of phytochromes in wild-type wheat and a transgenic line overexpressing an oat phytochrome A (PHYA) gene: functional implications

Etiolated seedlings of wild-type wheat and a transgenic line overexpressing an oat PHYA gene were investigated by the use of in situ low-temperature fluorescence spectroscopy. The red-absorbing phytochrome form, Pr, was characterized by (1) fluorescence emission spectrum; (2) total phytochrome content, and (3) by the extent of the Pr leads to lumi-R photoconversion at low temperature (gamma1), and of the Pr leads to Pfr photoconversion at ambient temperature (gamma2) as derived from emission data. All the characteristics were shown to be variable and to depend on (1) organ and tissue used; (2) seedling age; (3) transgenic wheat modification, and (4) continuous far-red irradiation of seedlings during their growth. These variations were interpreted in terms of the existence in wheat seedlings of the two phenomenological Pr types: (a), Pr' - major longer wavelength (687/673 nm, emission/absorption maxima) variable and light-labile with gamma1 approximately equal to 0.5; and (b), Pr" - minor, shorter wavelength (682/668 nm), relatively constant with its concentration not changing significantly with the increase of total phytochrome content in tissues and light-stable with gamma1 less than or equal to 0.05-0.1. Overexpression of oat phyA increases primarily the content of Pr' suggesting that it is comprised of phyA (phyA') whereas Pr" is believed to consist of the minor phyA fraction (phyA") and phyB. The transgenic wheat line has been demonstrated to have a modified phenotype--the appearance of the far-red high irradiance reaction (FR-HIR) (Shlumukov et al. Plant, Cell and Environment 24, 703-712). The increased content of phyA' in the transgenic line, whereas the total [phyA" + phyB] remains the same as in the wild type, indicates that the phyA' pool is primarily responsible for the observed modification of the phenotype and suggests that even in wild-type plants the phyA' component of the phyA pool may mediate the FR-HIR.