Tomato high pigment (hp) mutants are characterized by their exaggerated photoresponsiveness. Light-grown hp mutants display elevated levels of anthocyanins, are shorter and darker than wild-type plants, and have dark green immature fruits due to the overproduction of chlorophyll pigments. It has been proposed that HP genes encode negative regulators of phytochrome signal transduction. We have cloned the HP-2 gene and found that it encodes the tomato homolog of the nuclear protein DEETIOLATED1 (DET1) from Arabidopsis. Mutations in DET1 are known to result in con-stitutive deetiolation in darkness. In contrast to det1 mutants, tomato hp-2 mutants do not display any visible phenotypes in the dark but only very weak phenotypes, such as partial chloroplast development. Furthermore, whereas det1 mutations are epistatic to mutations in phytochrome genes, analysis of similar double mutants in tomato showed that manifestation of the phenotype of the hp-2 mutant is strictly dependent upon the presence of active phytochrome. Because only one DET1 gene is likely to be present in each of the two species, our data suggest that the phytochrome sig-naling pathways in which the corresponding proteins function are regulated differently in Arabidopsis and tomato.

Tomato high pigment ( hp ) mutants are characterized by their exaggerated photoresponsiveness. Light-grown hp mutants display elevated levels of anthocyanins, are shorter and darker than wild-type plants, and have dark green immature fruits due to the overproduction of chlorophyll pigments. It has been proposed that HP genes encode negative regulators of phytochrome signal transduction. We have cloned the HP-2 gene and found that it encodes the tomato homolog of the nuclear protein DEETIOLATED1 (DET1) from Arabidopsis. Mutations in DET1 are known to result in constitutive deetiolation in darkness. In contrast to det1 mutants, tomato hp-2 mutants do not display any visible phenotypes in the dark but only very weak phenotypes, such as partial chloroplast development. Furthermore, whereas det1 mutations are epistatic to mutations in phytochrome genes, analysis of similar double mutants in tomato showed that manifestation of the phenotype of the hp-2 mutant is strictly dependent upon the presence of active phytochrome. Because only one DET1 gene is likely to be present in each of the two species, our data suggest that the phytochrome signaling pathways in which the corresponding proteins function are regulated differently in Arabidopsis and tomato. INTRODUCTIONLight is a critical environmental signal controlling many aspects of plant development. For example, dark-grown plants display a typical etiolated morphology with elongated hypocotyls, closed apical hooks, and unexpanded cotyledons, whereas plants grown in the light have short hypocotyls, opened apical hooks, and expanded photosynthetically active cotyledons. Light is perceived by a series of photoreceptors that can detect light within a wide spectral range. The phytochromes are the best characterized of these photoreceptors and are able to intercept light primarily within the red and far-red regions of the spectrum (Furuya and Schäfer, 1996). They exist as multigene families, for example, PHYA to PHYE in Arabidopsis, and each phytochrome is likely to have a specific photoperceptory function during plant development (Quail et al., 1995). In addition, plants contain blue/UV-A-absorbing cryptochromes and UV-Babsorbing photoreceptors.Several models for light signal transduction in plants have been proposed. One has been deduced largely by microinjection experiments with tomato and involves G proteins, calcium, and cGMP (Bowler et al., 1994b;Mustilli and Bowler, 1997). Others are based on the genetic analysis of Arabidopsis mutants, such as deetiolated ( det ) and constitutively photomorphogenic ( cop ), which display characteristics of light-grown plants when grown in complete darkness, for example, reduced hypocotyl length, cotyledon opening and expansion, chloroplast development, and expression of light-induced genes (Chory et al., 1989(Chory et al., , 1996Deng et al., 1991;Chamovitz and Deng, 1996).Although several COP and DET genes have been identified, it is not clear how the activities of their gene products are regulated by phytochrome or by the calcium-an...

SummaryThe tomato HIGH PIGMENT-2 gene encodes an orthologue of the Arabidopsis nuclear protein DE-ETIOLATED 1 (DET1). From genetic analyses it has been proposed that DET1 is a negative regulator of light signal transduction, and recent results indicate that it may control light-regulated gene expression at the level of chromatin remodelling. To gain further understanding about the function of DET1 during plant development, we generated a range of overexpression constructs and introduced them into tomato. Unexpectedly, we only observed phenotypes characteristic of DET1 inactivation, i.e. hyper-responsiveness to light. Molecular analysis indicated in all cases that these phenotypes were a result of suppression of endogenous DET1 expression, due to post-transcriptional gene silencing. DET1 silencing was often lethal when it occurred at relatively early stages of plant development, whereas light hyper-responsive phenotypes were obtained when silencing occurred later on. The appearance of phenotypes correlated with the generation of siRNAs but not DNA hypermethylation, and was most efficient when using constructs with mutations in the DET1 coding sequence or with constructs containing only the 3¢-terminal portion of the gene. These results indicate an important function for DET1 throughout plant development and demonstrate that silencing of DET1 in fruits results in increased carotenoids, which may have biotechnological potential.

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Plants have developed flexible mechanisms to respond appropriately to environmental signals. These stimuli are transduced by largely unknown signalling pathways that are likely to be modulated by endogenous developmental signals to produce an integrated response that coordinately regulates gene expression. Light is a critical environmental signal that controls many aspects of plant development via a series of photoreceptors that are able to respond to different light wavelengths. Light is also the principal energy source for photosynthesis. The photosynthetic products are carbohydrates which are translocated in the form of sucrose from the photosynthetic (source) to non‐photosynthetic (sink) organs. Consequently, the control of photoregulated genes must integrate developmental inputs with signals derived from the photoreceptors, from the photosynthetic apparatus and from metabolites such as sucrose.