Conditional Synergism between Cryptochrome 1 and Phytochrome B Is Shown by the Analysis of phyA, phyB, andhy4 Simple, Double, and Triple Mutants in Arabidopsis

276

213

Photoperiodism is a day-length-dependent seasonal change of physiological or developmental activities that is widely found in plants and animals. Photoperiodic flowering in plants is regulated by photosensory receptors including the red͞far-red light-receptor phytochromes and the blue͞UV-A light-receptor cryptochromes. However, the molecular mechanisms underlying the specific roles of individual photoreceptors have remained poorly understood. Here, we report a study of the day-length-dependent response of cryptochrome 2 (cry2) and phytochrome A (phyA) and their role as day-length sensors in Arabidopsis. The protein abundance of cry2 and phyA showed a diurnal rhythm in plants grown in short-day but not in plants grown in long-day. The short-day-specific diurnal rhythm of cry2 is determined primarily by blue light-dependent cry2 turnover. Consistent with a proposition that cry2 and phyA are the major day-length sensors in Arabidopsis, we show that phyA mediates far-red light promotion of flowering with modes of action similar to that of cry2. Based on these results and a finding that the photoperiodic responsiveness of plants depends on light quality, a model is proposed to explain how individual phytochromes and cryptochromes work together to confer photoperiodic responsiveness in Arabidopsis.P hotoperiodic flowering in plants was the first photoperiodism phenomenon documented (1). The flowering of longday (LD) or short-day (SD) plants occurs or is accelerated in the LD or SD condition, respectively. Arabidopsis is a facultative LD plant for which flowering-time regulation has been extensively studied (2-5). Although the detailed mechanism underlying photoperiodism is not well understood, extensive plant physiological studies support a hypothesis referred to as the external coincidence model (6-8). According to this hypothesis, the light signal must interact at the appropriate time of the day (or ''coincide'') with the photoperiodic response rhythm (PRR) of a cellular activity to confer photoperiodic responsiveness. It has been found that mRNA expression of flowering-time genes in Arabidopsis, including CO, GI, and FT, exhibited circadian rhythms, which have different phase shapes in plants grown in LD compared with plants grown in SD (9-12). Therefore, the day-length-dependent circadian expression of one or more flowering-time genes may represent the PRR.Arabidopsis relies on at least nine photosensory receptors, including five phytochromes (phyA-phyE), two cryptochromes (cry1 and cry2), and two phototropins (phot1 and phot2), to regulate most of its light responses (13-16). Among these photoreceptors, phytochromes and cryptochromes are known to regulate flowering time (5). It has also been found that phyA and cry2 protein abundance is regulated by light (17, 18) and that cry2 expression changes in response to photoperiod (19). These studies indicate that cry2 and phyA may act as major day-length sensors. Indeed, it has been found that the coincidence of light perception by cry2 and phyA with the peak circadian ...

Section: Figmentioning

confidence: 99%

Regulation of photoperiodic flowering byArabidopsisphotoreceptors

Mockler

Yang

et al. 2003

276

213

“…Based on classic photobiological experiments, Hans Mohr (9) had proposed that the sole action of blue light perceived by specific photoreceptors (now identified as cryptochromes) was to amplify the response to Pfr (i.e., cryptochrome action requires Pfr). Under suboptimal light input conditions, cryptochrome action requires phyB activity (10), and under those conditions, phyB Pfr has recently been shown to act downstream of cryptochrome as predicted by Mohr's model (11). Under prolonged exposures to blue light, cryptochromes operate independently of phyA and phyB (12), but they could depend on other members of the phytochrome family (8,12).…”

mentioning

confidence: 99%

Arabidopsis thaliana life without phytochromes

Strasser

Sánchez‐Lamas

Yanovsky

et al. 2010

172

150

Plants use light as a source of energy for photosynthesis and as a source of environmental information perceived by photoreceptors. Testing whether plants can complete their cycle if light provides energy but no information about the environment requires a plant devoid of phytochromes because all photosynthetically active wavelengths activate phytochromes. Producing such a quintuple mutant of Arabidopsis thaliana has been challenging, but we were able to obtain it in the flowering locus T ( ft ) mutant background. The quintuple phytochrome mutant does not germinate in the FT background, but it germinates to some extent in the ft background. If germination problems are bypassed by the addition of gibberellins, the seedlings of the quintuple phytochrome mutant exposed to red light produce chlorophyll, indicating that phytochromes are not the sole red-light photoreceptors, but they become developmentally arrested shortly after the cotyledon stage. Blue light bypasses this blockage, rejecting the long-standing idea that the blue-light receptors cryptochromes cannot operate without phytochromes. After growth under white light, returning the quintuple phytochrome mutant to red light resulted in rapid senescence of already expanded leaves and severely impaired expansion of new leaves. We conclude that Arabidopsis development is stalled at several points in the presence of light suitable for photosynthesis but providing no photomorphogenic signal.

“…All affect gross morphological changes in the seedling as it makes the transition from heterotrophic growth in the dark to photoautotrophic growth in the light (1). Genetic analyses demonstrate a complex web of interactions between these photoreceptor signaling pathways (2)(3)(4)(5).…”

mentioning

confidence: 99%

BAS1 : A gene regulating brassinosteroid levels and light responsiveness in Arabidopsis

Neff

Nguyen

Malancharuvil

et al. 1999

354

312

The Arabidopsis bas1-D mutation suppresses the long hypocotyl phenotype caused by mutations in the photoreceptor phytochrome B (phyB). The adult phenotype of bas1-D phyB-4 double mutants mimics that of brassinosteroid biosynthetic and response mutants. bas1-D phyB-4 has reduced levels of brassinosteroids and accumulates 26-hydroxybrassinolide in feeding experiments. The basis for the mutant phenotype is the enhanced expression of a cytochrome P450 (CYP72B1). bas1-D suppresses a phyB-null allele, but not a phyA-null mutation, and partially suppresses a cryptochrome-null mutation. Seedlings with reduced BAS1 expression are hyperresponsive to brassinosteroids in a light-dependent manner and display reduced sensitivity to light under a variety of conditions. Thus, BAS1 represents one of the control points between multiple photoreceptor systems and brassinosteroid signal transduction.