In etiolated seedlings, light perceived by phytochrome promotes the expression of light-harvesting chlorophyll a/b protein of photosystem II (Lhcb) genes. However, excess of photosynthetically active radiation can reduce Lhcb expression. Here, we investigate the convergence and divergence of phytochrome, high-light stress and abscisic acid (ABA) signaling, which could connect these processes. Etiolated Arabidopsis thaliana seedlings bearing an Lhcb promoter fused to a reporter were exposed to continuous far-red light to activate phytochrome and not photosynthesis, and treated with ABA. We identified a cis-acting region of the promoter required for down-regulation by ABA. This region contains a CCAC sequence recently found to be necessary for ABI4-binding to an Lhcb promoter. However, we did not find a G-box-binding core motif often associated with the ABI4-binding site in genes promoted by light and repressed by ABI4. Mutations involving this motif also impaired the responses to reduced water potential, the response to high photosynthetic light and the response to methyl viologen but not the response to low temperature or to Norflurazon. We propose a model based on current and previous findings, in which hydrogen peroxide produced in the chloroplasts under high light conditions interacts with the ABA signaling network to regulate Lhcb expression. Since the mutation that affects high-light and methyl viologen responses does not affect phytochrome-mediated responses, the regulation by retrograde and phytochrome signaling can finally be separated at the target promoter level.

In plants, light signals caused by the presence of neighbors accelerate stem growth and flowering and induce a more erect position of the leaves, a developmental strategy known as shade-avoidance syndrome. In addition, mutations in the photoreceptors that mediate shade-avoidance responses enhance disease susceptibility in Arabidopsis thaliana. Here, we describe the Arabidopsis constitutive shade-avoidance1 (csa1) mutant, which shows a shade-avoidance phenotype in the absence of shade and enhanced growth of a bacterial pathogen. The csa1 mutant has a T-DNA inserted within the second exon of a Toll/Interleukin1 receptor–nucleotide binding site–leucine-rich repeat (TIR-NBS-LRR) gene, which leads to the production of a truncated mRNA. Arabidopsis plants transformed with the truncated TIR-NBS-LRR gene recapitulate the mutant phenotype, indicating that csa1 is a dominant-negative mutation that interferes with phytochrome signaling. TIR-NBS-LRR proteins have been implicated in defense responses in plants. RPS4, the closest homolog of CSA1, confers resistance to Pseudomonas syringae and complements the csa1 mutant phenotype, indicating that responses to pathogens and neighbors share core-signaling components in Arabidopsis. In Drosophila melanogaster and Caenorhabditis elegans, TIR domain proteins are implicated in both development and immunity. Thus, the dual role of the TIR domain is conserved across kingdoms.

SummaryPhytochrome A (phyA) and phytochrome B (phyB) share the control of many processes but little is known about mutual signaling regulation. Here, we report on the interactions between phyA and phyB in the control of the activity of an Lhcb1*2 gene fused to a reporter, hypocotyl growth and cotyledon unfolding in etiolated Arabidopsis thaliana. The very-low¯uence responses (VLFR) induced by pulsed far-red light and the high-irradiance responses (HIR) observed under continuous far-red light were absent in the phyA and phyA phyB mutants, normal in the phyB mutant, and reduced in the fhy1 mutant that is defective in phyA signaling. VLFR were also impaired in Columbia compared to Landsberg erecta. The low-¯uence responses (LFR) induced by red-light pulses and reversed by subsequent far-red light pulses were small in the wild type, absent in phyB and phyA phyB mutants but strong in the phyA and fhy1 mutants. This indicates a negative effect of phyA and FHY1 on phyB-mediated responses. However, a pre-treatment with continuous far-red light enhanced the LFR induced by a subsequent red-light pulse. This enhancement was absent in phyA, phyB, or phyA phyB and partial in fhy1. The levels of phyB were not affected by the phyA or fhy1 mutations or by far-red light pre-treatments. We conclude that phyA acting in the VLFR mode (i.e. under light pulses) is antagonistic to phyB signaling whereas phyA acting in the HIR mode (i.e. under continuous far-red light) operates synergistically with phyB signaling, and that both types of interaction require FHY1.

Incubation of GDP-['4C]mannose with liver microsomes and magnesium ions led to the formation of dolichol monophosphate mannose and of other acidlabile compounds that contain oligosaccharides. Formation of these compounds was enhanced by addition of an acceptor lipid in the same fractions of DEAE-cellulose chromatography where bound dolichol is found. Alkaline treatment of the oligosaccharides, obtained by acid methanolysis, followed by paper electrophoresis, gave rise to the formation of two positively charged substances believed to be formed by deacetylation of hexosamine residues. Incubation of the oligosaccharide-containing compounds with liver microsomes and manganese ions resulted in a transfer to endogenous protein. The role of dolichol derivatives in glycoprotein synthesis is discussed.Incubation of liver microsomes with UDP-Glc and dolichol-P leads to the formation of dolichol-P-Glc and a substance believed to be dolichol-PP-oligosaccharide (1, 2). This substance, which has been referred to as glucosylated endogenous acceptor, can also be obtained by direct transfer of glucose from dolichol-P-Glc and acts as donor in oligosaccharide transfer to protein (3).The structure of the oligosaccharide portion of glucosylated endogenous acceptor is not known, but studies with the substance labeled in the glucose moiety have given some information. Measurements of molecular weight by ion exclusion gave values of 3500, and the rate of migration on paper chromatography corresponds to that of a maltooligosaccharide of about 17 units (4). The oligosaccharide, therefore, has around 20 monosaccharide units.When the oligosaccharide prepared by acid methanolysis was subjected to electrophoresis, it behaved as an uncharged substance and on treatment with alkali, it gave rise to two positively charged substances (5). These became neutral after treatment with acetic anhydride under conditions that led to N-acetylation. From these facts it was tentatively concluded that the oligosaccharide of glucosylated endogenous acceptor contains two hexosamine residues.In experiments where UDP-['4C]GlcNAc was used as donor and dolichol-P as acceptor, it was found that substances that appear to be dolichol-PP-GlcNAc (6) and dolichol-PP-N,N'-diacetylchitobiose are formed (7). The transfer of both the sugar residue and phosphate from UDP-GlcNAc to a lipid had been described by Molnar et al. (8).These findings can be related to the fact that two N-acetylglucosamine residues are present in the inner portion of the oligosaccharide of many glycoproteins (9). In these, an N,N'-diacetylchitobiose residue is linked to the amide N of asparagine and chains of mannose residues are linked to it.In experiments with GDP-Man as donor, lipid-bound mannose has been detected (10-12). Its formation was enhanced by addition of dolichol-P (6, 13), and the structure of the product has been unambiguously proved to be dolichol-PMIan by Hemming's group (14).In previous experiments it was found that, besides dolichol-P-Man, other substances are formed which,...