Cryptochrome 1 in the dark affects phytochrome A in etiolated Arabidopsis seedlings shifting the balance between its native types, phyA' and phyA″.

Plants during their evolution have acquired a highly efficient apparatus comprising several photoreceptor systems, the most effective in the transition from scoto- to photomorphogenesis being the blue-light-absorbing cryptochromes (crys) and the red - far-red-absorbing phytochromes (phys). Within the phytochrome family, the major ones are phyA and phyB, the former one comprising two photochemically and functionally distinct types - light-labile and soluble phyA' and relatively light-stable and amphiphilic phyA″. Cryptochromes are represented by cry1 and cry2. phyB and phyA were shown to interact with cry1, the substrate of their phosphorylation activity, modifying its photoregulatory responses. In this work, with the use of phytochrome fluorescence spectroscopy in vivo, we investigated the possible effects of phyA-cry1 interactions on the state of phyA and its native types, phyA' and phyA″ in etiolated Arabidopsis (Columbia) plants. In cry1 mutant (cry1) and cry1 overexpressor (CRY1OX), the spectroscopic properties and total content of phyA remained practically unchanged as compared to the wild type (WT), whereas the [phyA']/[phyA″] ratio was raised from approximately 0.8-1.0 in the WT and CRY1OX to 1.6 in cry1. This shift in the phyA pools' balance towards phyA' suggests that cry1 hampers the formation of phyA' from phyA″, possibly interfering with the phosphorylation of the latter. cry1 and phyA″ localized in the cytoplasm are likely to be responsible for the induction of the fast membrane depolarization effects (Folta and Spalding 2001), whereas the slow end-point photoregulation reactions may be connected with the nuclear fraction of cry1 interacting with phyA' and/or phyA″.