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Hierarchical coupling of phytochromes and cryptochromes reconciles stability and light modulation of Arabidopsis development

María Agustina Mazzella¹, Pablo D. Cerdán^2,, Roberto J. Staneloni² and Jorge J. Casal^1,*

¹ IFEVA, Facultad de Agronomía, Universidad de Buenos Aires, Avenida San Martín 4453, 1417-Buenos Aires, Argentina
² Instituto de Investigaciones Bioquímicas Fundación Campomar, Avenida Patricias Argentinas 435, 1405-Buenos Aires, Argentina
Present address: The Salk Institute for Biological Studies, La Jolla, California, 92037, USA

View larger version (14K): [in a new window]   Fig. 1. cry2 affects de-etiolation under high fluence rates of white light. Seedlings of the wild type and of the phyA, phyB, cry1 and cry2 single, double, triple and quadruple mutant combinations were grown in darkness (D) or under white light photoperiods (WL) for 7 days. No significant differences in morphology among genotypes were observed for dark controls (for this reason only the wild type is shown). The figure is based on the analysis of 10 replicate boxes (100 seedlings) per genotype. The only statistically significant effect of the cry2 mutation on hypocotyl growth was in the PHYA PHYB cry1 background. The insets show (from left to right) the cotyledons in the wild type, phyA phyB cry1 and phyA phyB cry1 cry2 seedlings.

View larger version (19K): [in a new window]   Fig. 2. cry1 and cry2 control GUS activity driven by a Lhcb1*2 gene promoter. One-day-old seedlings of the wild type and of the cry1, cry2, cry1 cry2, phyA phyB, phyA phyB cry1 and phyA phyB cry2 mutants, were exposed to continuous white light for 1, 2 or 3 days before harvest (A), or to orange light or white light for 3 days (B). GUS activity is expressed relative to the wild type, day 3 under white light. In dark controls GUS activities were below 0.01. Data are mean and s.e.m. of 6 (A) or 9 (B) replicates.

View larger version (16K): [in a new window]   Fig. 3. Time course of leaf production in the wild type, and the phyB and phyA phyB cry1 cry2 mutants. Data are means and s.e.m. of 6 plants.

View larger version (93K): [in a new window]   Fig. 4. Defective development of the quadruple phyA phyB cry1 cry2 mutant of Arabidopsis thaliana. (A) The wild type and the quadruple mutant 3 weeks after sowing. (B) The quadruple mutant 10 weeks after sowing. A 4-week-old wild type is displayed to illustrate the divergent morphology because 10 weeks after sowing the wild type had completed the cycle and fully senesced. (C) Defective development of early siliques (arrow) in the quadruple mutant. A normal wild type silique is shown for comparison.

View larger version (28K): [in a new window]   Fig. 5. Transition between vegetative and reproductive phases in the wild type and in the phyA, phyB, cry1 and cry2 single, double, triple and quadruple mutant combinations. (A) The total number of leaves (rosette plus stem) indicates the point of transition in a developmental scale. (B) The time to first visible flower bud provides an indication of the transition in a chronological scale. (C) The phyA mutation accelerates flowering in the cry2 background. Data are means and s.e.m. of 10-17 plants.

View larger version (22K): [in a new window]   Fig. 6. Response to long (inductive) photoperiods in wild-type seedlings and phyA, cry1 and cry2 single, double and triple mutant combinations. The seedlings were grown under short photoperiods for 21 days, transferred to long photoperiods for 5 days and returned to short days for the rest of the cycle. Control seedlings remained under short days. Data are means and s.e.m. of 7 plants.

View larger version (37K): [in a new window]   Fig. 7. Time between the first visible flower bud and anthesis in the wild type and the phyA, phyB, cry1 and cry2 single, double, triple and quadruple mutant combinations. Data are means and s.e.m. of 10-17 plants.

View larger version (14K): [in a new window]   Fig. 8 Proposed model of integration of the actions of phy and cry in the regulation of the transition from the vegetative to the reproductive phase in Arabidopsis thaliana. 1Reed et al., 1993, 2Devlin et al., 1999, 3Devlin et al., 1998, 4this work, 5Johnson et al., 1994, Blázquez M. and Weigel. D. (personal communication), 6Guo et al., 1998, 7Bagnall et al., 1996, 8based on the results of Bagnall et al., 1995.