Patterns of Expression and Normalized Levels of the Five Arabidopsis Phytochromes

Abstract

Using monoclonal antibodies specific for each apoprotein and full-length purified apoprotein standards, the levels of the five Arabidopsis phytochromes and their patterns of expression in seedlings and mature plants and under different light conditions have been characterized. Phytochrome levels are normalized to the DNA content of the various tissue extracts to approximate normalization to the number of cells in the tissue. One phytochrome, phytochrome A, is highly light labile. The other four phytochromes are much more light stable, although among these, phytochromes B and C are reduced 4- to 5-fold in red- or white-light-grown seedlings compared with dark-grown seedlings. The total amount of extractable phytochrome is 23-fold lower in light-grown than dark-grown tissues, and the percent ratios of the five phytochromes, A:B:C:D:E, are measured as 85:10:2:1.5:1.5 in etiolated seedlings and 5:40:15:15:25 in seedlings grown in continuous white light. The four light-stable phytochromes are present at nearly unchanging levels throughout the course of development of mature rosette and reproductive-stage plants and are present in leaves, stems, roots, and flowers. Phytochrome protein expression patterns over the course of seed germination and under diurnal and circadian light cycles are also characterized. Little cycling in response to photoperiod is observed, and this very low amplitude cycling of some phytochrome proteins is out of phase with previously reported cycling ofPHY mRNA levels. These studies indicate that, with the exception of phytochrome A, the family of phytochrome photoreceptors in Arabidopsis constitutes a quite stable and very broadly distributed array of sensory molecules.

Diurnal Regulation of the Brassinosteroid-Biosynthetic CPD Gene in Arabidopsis

“…Although in wild-type background we could also observe a slight increase of LUC activity following blue light exposure, which was used within the photobiologically active fluence range (Devlin and Kay, 2000), this may be attributed to PHYA effect or/and the aforementioned nonspecific light-induced increase of LUC activity, rather than cryptochrome-mediated signals. In green seedlings PHYB is the most abundant phytochrome species, whereas PHYA is the least (Sharrock and Clack, 2002). Therefore, the dramatic decrease of light inducibility in plants deficient for both PHYA and PHYB seems to indicate a predominant role for PHYB in controlling CPD expression.…”

Evolutionary Studies Illuminate the Structural-Functional Model of Plant Phytochromes

“…Many of the determinants of functional specificity that distinguish phyA and phyB have been identified from the characterization of monogenic and multiple phytochrome mutants of Arabidopsis, such as those cited above (along with many others), from the dissection of signaling pathways (Castillon et al, 2007;Jiao et al, 2007;Bae and Choi, 2008), and from studies of expression patterns and protein levels (Goosey et al, 1997;Sharrock and Clack, 2002), dimerization (Sharrock and Clack, 2004;Clack et al, 2009), nuclear localization (Kircher et al, 2002), proteinchromophore interactions (Hanzawa et al, 2002), and phyA dark reversion rates (Eichenberg et al, 2000b). Despite these many advances, aspects of the structural-functional models for these photoreceptors remain elusive.…”

“…phyA is the predominant form in etiolated tissue and mediates germination, deetiolation, and daylength perception under continuous FR (Nagatani et al, 1993;Parks and Quail, 1993;Whitelam et al, 1993;van Tuinen et al, 1995;Shinomura et al, 2000;Takano et al, 2001), where it may also condition early neighbor detection . Due to rapid protein degradation and decreased transcription in the light (Sharrock and Clack, 2002), phyA has a reduced ability to antagonize shade avoidance responses under canopies once seedlings are established, although a persistent FR-HIR has been detected in woodland genotypes of Impatiens capensis (von Wettberg and Schmitt, 2005). In Arabidopsis, phyA may be the sole mediator of VLFR, detecting light conditions that the other phytochromes cannot distinguish from darkness, such as deep canopy shade, and transient exposures that result from tilling or disturbance (reviewed in Casal et al, 1997).…”

Lysine 206 inArabidopsisphytochrome A is the major site for ubiquitin-dependent protein degradation

“…The model plant, Arabidopsis thaliana, contains five genes (PHYA to PHYE) that encode phytochrome apoproteins (11,12). Phytochromes are classified as light labile phytochrome A (phyA) and light stable phytochromes B E (phyB-phyE) (13). Among the light stable phytochromes, phyB mediates the red high irradiance response (red HIR) and the red/far-red reversible low fluence response (red/far-red LFR) (14 16).…”