Biomedical subjects
Publications and source records attributed to J Chory.
Endogenous brassinosteroids in the shoots of Arabidopsis thaliana were investigated. Castasterone, 6-deoxocastasterone, typhasterol and 6-deoxotyphasterol were identified by GC-MS. The co-occurrence of 6-deoxo-brassinosteroids and 6-oxo-brassinosteroids suggests that there are both early and late C6-oxidation pathways of brassinosteroids in A. thaliana.
Plant responses to red and far-red light are mediated by a family of photoreceptors called phytochromes. Arabidopsis thaliana seedlings lacking one of the phytochromes, phyB, have elongated hypocotyls and other tissues, suggesting that they may have an alteration in hormone physiology. We have studied the possibility that phyB mutations affect seedling gibberellin (GA) perception and metabolism by testing the responsiveness of wild-type and phyB seedlings to exogenous GAs. The phyB mutant elongates more than the wild type in response to the same exogenous concentrations of GA3 or GA4, showing that the mutation causes an increase in responsiveness to GAs. Among GAs that we were able to detect, we found no significant difference in endogenous levels between wild-type and phyB mutant seedlings. However, GA4 levels were below our limit of detectability, and the concentration of that active GA could have varied between wild-type and phyB mutant seedlings. These results suggest that, although GAs are required for hypocotyl cell elongation, phyB does not act primarily by changing total seedling GA levels but rather by decreasing seedling responsiveness to GAs.
Chloroplast development requires the coordinated expression of nuclear and chloroplastic genes. A hypothesized signal from the chloroplast couples the transcription of certain nuclear genes encoding photosynthetic proteins with chloroplast function. We have previously described an Arabidopsis thaliana mutant, gun1, which has a defect in the signal transduction pathway coupling such nuclear and plastidic gene expression. Here we show that gun1 seedlings are also defective in establishing photoautotrophic growth. gun1 seedlings develop normally in the dark, but, based on morphological criteria and the kinetics of chlorophyll accumulation, photosynthetic mRNA accumulation, and the differentiation of etioplasts to chloroplasts, are retarded in their ability to de-etiolate. Therefore, we propose that the GUN1 gene plays an important role in the transition from heterotrophic to photoautotrophic growth, suggesting an important physiological role for the plastid-nucleus signaling pathway during chloroplast biogenesis.
All metazoan cells are able to make decisions about cell division or cellular differentiation based, in part, on environmental cues. Accordingly, cells express receptor systems that allow them to detect the presence of hormones, growth factors and other signals that manipulate the regulatory processes of the cell. In plants, an unusual signal-light-is required for the induction and regulation of many developmental processes. Past physiological and molecular studies have revealed the variety and complexity of plant responses to light but until recently very little was known about the mechanisms of those responses. Two major breakthroughs have allowed the identification of some photoreceptor signalling intermediates: the identification of photoreceptor and signal transduction mutants in Arabidopsis, and the development of single-cell microinjection assays in which outcomes of photoreceptor signalling can be visualized. Here, we review recent genetic advances which support the notion that light responses are not simply endpoints of linear signal transduction pathways, but are the result of the integration of a variety of input signals through a complex network of interacting signalling components.
Transgenic Arabidopsis plants expressing a luciferase gene fused to a circadian-regulated promoter exhibited robust rhythms in bioluminescence. The cyclic luminescence has a 24.7-hour period in white light but 30- to 36-hour periods under constant darkness. Either red or blue light shortened the period of the wild type to 25 hours. A phytochrome-deficient mutation lengthened the period in continuous red light but had little effect in continuous blue light, whereas seedlings carrying mutations that activate light-dependent pathways in darkness maintained shorter periods in constant darkness. These results suggest that both phytochrome- and blue light-responsive photoreceptor pathways control the period of the circadian clock.
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Signalling intermediates in plant phototransduction pathways have been identified. These new results suggest how plants maintain a hight photosyntheic efficiency despite changes in their light environment.
The mechanisms by which plants integrate light signals to modify endogenous developmental programs are largely unknown. One candidate for a signal transduction component that may integrate light with developmental pathways is the Arabidopsis DET1 gene product. Here we report the positional cloning of the DET1 locus and show that DET1 is a unique nuclear-localized protein. An analysis of a number of det1 mutants indicates that mutants with partial DET1 activity develop as light-grown plants in the dark. det1 null mutants share this phenotype, but also display severe defects in temporal and spatial regulation of gene expression. These results suggest that DET1 acts in the nucleus to control the cell type-specific expression of light-regulated promoters.
Light regulates the development of Arabidopsis seedlings in a variety of ways, including inhibition of hypocotyl growth and promotion of leaf development, chloroplast differentiation, and light-responsive gene expression. Mutations that uncouple most or all of these responses from light control have been described, for example, det1, det2, and cop1. To identify regulatory components that define downstream branches in the light-regulated signal transduction pathway, mutants specifically affected in only one light-regulated response were isolated. A screen was designed to isolate mutants that overexpressed the CAB (photosystem II type I chlorophyll a/b-binding proteins) genes in the dark, by use of transgenic line containing a T-DNA construct with two CAB3 promoter-reporter fusions. Eight mutants that showed aberrant expression of both CAB3 promoters were isolated and were designated doc mutants (for dark overepression of CAB). All of the mutants have normal etiolated morphology in the dark. Genetic and phenotypic analyses indicate that most of the mutations are recessive and define at least three loci (doc1, doc2, doc3). Unlike det1 and det2 mutants, which affect the expression of CAB and RBCS (the small subunit of RuBP carboxylase) to approximately the same extent, all three doc mutations are much more specific in derepressing the expression of CAB. The phenotypes of doc mutants suggest that morphological changes can be genetically separated from changes in CAB gene expression. Moreover, the regulation of CAB gene expression can be separated further from the regulation of RBCS gene expression. Epistasis studies suggest that DOC1 and DET3 act downstream from DET1 on two separate branches in the phototransduction pathway. In contrast, DOC2 appears to act on a distinct pathway from DET1. Mutations in doc1, doc2, or doc3 also impair plant growth under short-day conditions.
Arabidopsis mutants with decreased responses to light and mutants showing light responses in the dark have both been characterized. Some of the former mutants lack specific photoreceptors, such as the red/far-red light receptor phytochrome A, phytochrome B, or a putative blue light receptor, HY4. These have allowed the assessment of physiological functions of these photoreceptors. The mutants with light responses in the dark include some, such as det1 and cop1, that appear to identify light signal transduction components, and others, such as fus6, that may be less directly related to normal control of light responses. Double mutant studies suggest how the different gene products might interact.
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Chloroplast development requires coordinate nuclear and chloroplast gene expression. A putative signal from the chloroplast couples the transcription of certain nuclear genes encoding photosynthesis-related proteins with chloroplast function. We have identified at least three Arabidopsis nuclear genes (GUN1, GUN2, and GUN3) necessary for coupling the expression of some nuclear genes to the functional state of the chloroplast. Homozygous recessive gun mutations allow nuclear gene expression in the absence of chloroplast development and furthermore may interfere with the switch from dark-grown to light-grown development. Other reports suggest this intracellular cross-talk also involves mitochondrial interactions. The GUN genes thus define steps in one specific branch of a complex interorganellar regulatory network.
Plant growth and development are governed by complex interactions between environmental signals and endogenous developmental programs. Mutations that disrupt light signal perception or transduction have provided an important resource to begin dissection of the pathways that control light-regulated development in young seedlings.
The past year has seen significant advances in the biochemical, genetic and molecular dissection of the light signal transduction and developmental pathways that lead to photoregulated gene expression in higher plants. A major part of recent research has focused on the assignment of biological functions to the various photoreceptors, the genetic dissection of the photoreceptor action pathways, and the identification of the cis-acting sequences and trans-acting factors that regulate the downstream light-regulated genes.
Phytochromes are a family of plant photoreceptors that mediate physiological and developmental responses to changes in red and far-red light conditions. In Arabidopsis, there are genes for at least five phytochrome proteins. These photoreceptors control such responses as germination, stem elongation, flowering, gene expression, and chloroplast and leaf development. However, it is not known which red light responses are controlled by which phytochrome species, or whether the different phytochromes have overlapping functions. We report here that previously described hy3 mutants have mutations in the gene coding for phytochrome B (PhyB). These are the first mutations shown to lie in a plant photoreceptor gene. A number of tissues are abnormally elongated in the hy3(phyB) mutants, including hypocotyls, stems, petioles, and root hairs. In addition, the mutants flower earlier than the wild type, and they accumulate less chlorophyll. PhyB thus controls Arabidopsis development at numerous stages and in multiple tissues.
When grown in the absence of light, the det1 mutants of Arabidopsis thaliana (L.) Heynh. develop characteristics of light-grown plants as determined by morphological, cellular, and molecular criteria. Further, in light-grown plants, mutations in the DET1 gene affect cell-type-specific expression of light-regulated genes and the chloroplast developmental program. Here we show that the addition of exogenously added cytokinins (either 2-isopentenyl adenine, kinetin, or benzyladenine) to the growth medium of dark-germinated wild-type seedlings results in seedlings that resemble det1 mutants, instead of having the normal etiolated morphology. Like det1 mutants, these dark-grown seedlings now contain chloroplasts and have high levels of expression of genes that are normally 'light'-regulated. These results suggest an important role for cytokinins during greening of Arabidopsis, and may implicate abnormal cytokinin levels or an increased sensitivity to cytokinins as explanations for some of the observed phenotypes of det1 mutants.
Light affects both the development and the metabolism of plants. In addition to the role of light in providing energy for photosynthesis, light signals cause profound changes in the morphology of the developing young seedling, including cotyledon expansion, leaf development, inhibition of stem growth, and production of chlorophyll in the photosynthetically competent chloroplast. The light-dependent development of plants (photomorphogenesis) is a complex process resulting from the combined action of several photoreceptors. This review summarizes what is known of the red- and blue-light photoreceptors that regulate dicotyledonous seedling development and the complexity of the downstream responses. Special emphasis is placed on the recent progress made toward genetic and biochemical dissection of the signal transduction pathways.