The phytochrome family of photoreceptors monitors the light environment and dictates patterns of gene expression that enable the plant to optimize growth and development in accordance with prevailing conditions. The enduring challenge is to define the biochemical mechanism of phytochrome action and to dissect the signaling circuitry by which the photoreceptor molecules relay sensory information to the genes they regulate. Evidence indicates that individual phytochromes have specialized photosensory functions. The amino-terminal domain of the molecule determines this photosensory specificity, whereas a short segment in the carboxyl-terminal domain is critical for signal transfer to downstream components. Heterotrimeric GTP-binding proteins, calcium-calmodulin, cyclic guanosine 5'-phosphate, and the COP-DET-FUS class of master regulators are implicated as signaling intermediates in phototransduction.

Emerging evidence suggests that individual members of the phytochrome family of photoreceptors may regulate discrete facets of plant photomorphogenesis. We report here the isolation of phytochrome A mutants of Arabidopsis using a novel screening strategy aimed at detecting seedlings with long hypocotyls in prolonged far-red light. Complementation analysis of 10 selected mutant lines showed that each represents an independent, recessive allele at a new locus, designated hy8. Immunoblot and spectrophotometric analyses of two of these lines, hy8-1 and hy8-2, showed that, whereas phytochromes B and C are expressed at wild-type levels, phytochrome A is undetectable, thus indicating that the long hypocotyl phenotype displayed by these mutants is caused by phytochrome A deficiency. A third allele, hy8-3, expresses wild-type levels of spectrally normal phytochrome A, suggesting a mutation that has resulted in loss of biological activity in an otherwise photochemically active photoreceptor molecule. Together with physiological experiments, these data provide direct evidence that endogenous phytochrome A is responsible for the "far-red high irradiance response" of etiolated seedlings, but does not play a major role in mediating responses to prolonged red or white light. Because the hy8 and the phytochrome B-deficient hy3 mutants exhibit reciprocal responsivity toward prolonged red and far-red light, respectively, the evidence indicates that phytochromes A and B have distinct photosensory roles in regulating seedling development.

Responses to the plant hormone ethylene are mediated by a family of five receptors in Arabidopsis that act in the absence of ethylene as negative regulators of response pathways. In this study, we examined the rapid kinetics of growth inhibition by ethylene and growth recovery after ethylene withdrawal in hypocotyls of etiolated seedlings of wild-type and ethylene receptor-deficient Arabidopsis lines. This analysis revealed that there are two phases to growth inhibition by ethylene in wild type: a rapid phase followed by a prolonged, slower phase. Full recovery of growth occurs approximately 90 min after ethylene removal. None of the receptor null mutations tested had a measurable effect on the two phases of growth inhibition. However, loss-of-function mutations in ETR1, ETR2, and EIN4 significantly prolonged the time for recovery of growth rate after ethylene was removed. Plants with an etr1-6;etr2-3;ein4-4 triple loss-of-function mutation took longer to recover than any of the single mutants, while the ers1;ers2 double mutant had no effect on recovery rate, suggesting that receiver domains play a role in recovery. Transformation of the ers1-2;etr1-7 double mutant with wild-type genomic ETR1 rescued the slow recovery phenotype, while a His kinase-inactivated ETR1 construct did not. To account for the rapid recovery from growth inhibition, a model in which clustered receptors act cooperatively is proposed.Ethylene regulates a number of developmental processes in higher plants, including growth in etiolated seedlings. Inhibition of growth in etiolated seedlings by ethylene is a convenient and useful bioassay that has been used to quantify the dose-response characteristics of ethylene (Chen and Bleecker, 1995) and, in mutant screens, to identify components in the ethylene signal transduction pathway (Bleecker et al., 1988; Guzman and Ecker, 1989). Mutational analysis of the ethylene signaling pathway has led to an increasingly refined model for signaling (Guo and Ecker, 2004). According to this model, responses to ethylene are mediated by a family of five receptors in Arabidopsis that are related to bacterial two-component receptors (Chang et al., 1993;Sakai et al., 1998). The ethylene receptors are thought to transduce signal via Ser/Thr kinase activity in CTR1 (Kieber et al., 1993;Huang et al., 2003). CTR1 may negatively regulate the ethylene response pathway by inhibiting activity of an Nramp-related protein, EIN2, which is required for responses to ethylene (Alonso et al., 1999). Ethylene binding to the receptors reduces the activity of the receptors, leading to reduced activity of CTR1 protein and an increase in activity of EIN2 protein along with subsequent signaling associated with it. At least some responses to ethylene, including the seedling growth response, are mediated by activation of a transcriptional cascade (Solano et al., 1998), suggesting that ethylene responses are mediated by differential gene activation and inactivation. In support of this, a number of genes have been shown to be ethylene res...

The hy1 and hy2 long hypocotyl mutants of Arabidopsis contain normal levels of immunochemically detectable phytochrome A, but the molecule is photochemically nonfunctional. We have investigated the biochemical basis for this lack of function. When the hy1 and hy2 mutants were grown in white light on a medium containing biliverdin IX[alpha], a direct precursor to phytochromobilin, the phytochrome chromophore, the seedlings developed with a morphological phenotype indistinguishable from the light-grown wild-type control. Restoration of a light-grown phenotype in the hy1 mutant was also accomplished by using phycocyanobilin, a tetrapyrrole analog of phytochromobilin. Spectrophotometric and immunochemical analyses of the rescued hy1 and hy2 mutants demonstrated that they possessed wild-type levels of photochemically functional phytochrome that displayed light-induced conformational changes in the holoprotein indistinguishable from the wild type. Moreover, phytochrome A levels declined in vivo in response to white light in rescued hy1 and hy2 seedlings, indicative of biliverdin-dependent formation of photochemically functional phytochrome A that was then subject to normal selective turnover in the far-red-light-absorbing form. Combined, these data suggest that the hy1 and hy2 mutants are inhibited in chromophore biosynthesis at steps prior to the formation of biliverdin IX[alpha], thus potentially causing a global functional deficiency in all members of the phytochrome photoreceptor family.