The phytochrome response of the Lemna gibba NPR1 gene is mediated primarily through changes in abscisic acid levels

A small plant-specific protein family of ABI five binding proteins (AFPs) regulates stress response in germinating Arabidopsis seeds and seedlings

The transcription factor ABA-Insensitive5 (ABI5) is a key regulator of ABA signaling and stress response in Arabidopsis seeds and seedlings. Potential ABI5-interacting proteins were identified by a yeast two-hybrid screen; the most prevalent interactors were a family of four highly conserved plant-specific proteins with no domains of known function, but homology to a previously characterized ABI Five Binding Protein (AFP). This study compares expression and function of the family members. The AFPs are induced by ABA and/or dehydrating stresses in young seedlings, but the developmental timing of their induction differs. Mutations in AFP1 or AFP2 result in increased sensitivity to ABA and salt, whereas afp4 mutants are mildly ABA-resistant. AFP2, like AFP1, acts epistatically to ABI5. Reduced germination or seedling growth of the mutants under stress correlates with a higher level of ABI5 protein when compared to wild-type seedlings, but it is not clear whether this is a cause or effect of the reduced growth. Although both ABI5 and the AFPs are ABA-induced, the ABI5:AFP ratio increases at high ABA concentrations, maintaining growth inhibition under severe stress. An AFP2:GFP fusion, which complements the afp2 mutation, is nuclear-localized in seedlings exposed to stress, but becomes delocalized before being degraded following removal of stress. The AFPs may also interact to varying extents with many ABI5-related bZIP transcription factors. This study suggests that germination and seedling growth are regulated by antagonistic interactions among at least two functionally redundant families, the AFPs and the ABI5-related proteins, providing a mechanism to fine-tune seedling stress responses.

The early dark-response in Arabidopsis thaliana revealed by cDNA microarray analysis

Despite intense research on light responses in plants, the consequences of a simple shift from light to darkness remain poorly characterized. We have examined the transcriptome of Arabidopsis thaliana seedling leaves upon a shift from constant light to darkness for between 1 and 8 h, while excluding most effects associated with circadian oscillation. Expression clustering and gene ontology analyses identified about 790 responsive genes implicated in diverse cellular processes. Compared to the better-studied long-term dark adaptation response, the early response to darkness is partially overlapping yet clearly distinct, encompassing early transient, early sustained, and late response clusters. The repressor of photomorphogenesis, COP1 (constitutive photomorphogenic 1), is not a chief regulator of the early response to darkness, in contrast to its well-established role during long-term dark adaptation and etiolation. Only part of the early dark response can be understood as the opposite of the response following a dark-to-light transition and as a response to sugar deprivation. Bioinformatic comparisons with published microarray datasets further suggest that abscisic acid (ABA) signaling plays a prominent role in the early response to darkness, although this effect is not mediated by an increase in the ABA level. The potential basis for the co-regulation by darkness and ABA is discussed in light of sugar and redox signaling.

Clumping and dispersal of chloroplasts in succulent plants

Plants have evolved various photoprotective mechanisms to mitigate photodamage. Here we report the diurnal movement of chloroplasts in the leaves of succulent crassulacean acid metabolism (CAM) plants under combined light and water stress. In leaves of water-stressed plants, the chloroplasts became densely clumped in one or sometimes two areas in the cytoplasm under light and dispersed during darkness. The chloroplast clumping resulted in leaf optical changes, with a decrease in absorptance and an increase in transmittance. The plant stress hormone abscisic acid induced chloroplast clumping in the leaf cells under light. We suggest that the marked chloroplast movement in these CAM plants is a photoprotective strategy used by the plants subjected to severe water stress.

Knowing when to grow: signals regulating bud dormancy

Dormancy regulation in vegetative buds is a complex process necessary for plant survival, development and architecture. Our understanding of and ability to manipulate these processes are crucial for increasing the yield and availability of much of the world's food. In many cases, release of dormancy results in increased cell division and changes in developmental programs. Much can be learned about dormancy regulation by identifying interactions of signals in these crucial processes. Internal signals such as hormones and sugar, and external signals such as light act through specific, overlapping signal transduction pathways to regulate endo-, eco- and paradormancy. Epigenetic-like regulation of endodormancy suggests a possible role for chromatin remodeling similar to that known for the vernalization responses during flowering.

Phytochrome in cotyledons regulates the expression of genes in the hypocotyl through auxin-dependent and -independent pathways

To elucidate the mechanism of plant responses to shading, we identified three promoter/enhancer trap lines (M812, J53, J59) that exhibited reporter expression in the hypocotyl in response to the end-of-day far-red light treatment. Interestingly, we found auxin-responsive genes in the vicinities of the reporter insertion sites in M812 and J53. We examined the effects of auxin on the reporter expression in these lines together with a previously identified N35 line. The results indicated that the reporter expression was induced by exogenous auxin in N35 and J53. Furthermore, an auxin transport inhibitor inhibited the responses of these lines to the end-of-day far-red light treatment, suggesting the involvement of auxin in the responses of plants to shading. By contrast, neither auxin nor the transport inhibitor affected the response in M812 and J59. Interestingly, J59 responded to ABA. Hence, ABA might be involved in the response as well. Analysis of the photoreceptive sites for the responses revealed the cotyledons, not the hypocotyl, are the major photoreceptive sites both in the auxin-responsive and ABA-responsive lines. Hence, some signals appeared to be transmitted from the cotyledons to the hypocotyl.

Abiotic stress signal transduction in plants: Molecular and genetic perspectives

Low temperature, drought and salinity are major adverse environmental factors that limit plant productivity. Understanding the mechanisms by which plants perceive and transduce these stress signals to initiate adaptive responses is essential for engineering stress-tolerant crop plants. Molecular and biochemical studies suggest that abiotic stress signaling in plants involves receptor-coupled phosphorelay, phosphoinositol-induced Ca2+ changes, mitogen-activated protein kinase cascades and transcriptional activation of stress-responsive genes. In addition, protein posttranslational modifications and adapter or scaffold-mediated protein-protein interactions are also important in abiotic stress signal transduction. Most of these signaling modules, however, have not been genetically established to function in plant abiotic stress signal transduction. To overcome the scarcity of abiotic stress-specific phenotypes for conventional genetic screens, molecular genetic analysis using stress-responsive promoter-driven reporter is suggested as an alternative approach to genetically dissect abiotic stress signaling networks in plants.

SUGAR-INDUCED SIGNAL TRANSDUCTION IN PLANTS

Sugars have important signaling functions throughout all stages of the plant's life cycle. This review presents our current understanding of the different mechanisms of sugar sensing and sugar-induced signal transduction, including the experimental approaches used. In plants separate sensing systems are present for hexose and sucrose. Hexokinase-dependent and -independent hexose sensing systems can further be distinguished. There has been progress in understanding the signal transduction cascade by analyzing the function of the SNF1 kinase complex and the regulatory PRL1 protein. The role of sugar signaling in seed development and in seed germination is discussed, especially with respect to the various mechanisms by which sugar signaling controls gene expression. Finally, recent literature on interacting signal transduction cascades is discussed, with particular emphasis on the ethylene and ABA signal transduction pathways.

Formation and breakdown of ABA

The phytohormone, abscisic acid (ABA) is found in all photosynthetic organisms. The amount of ABA present is determined by the dynamic balance between biosynthesis and degradation: these two processes are influenced by development, environmental factors such as light and water stress, and other growth regulators. ABA is synthesized from a C40 carotenoid precursor and the first enzyme committed specifically to ABA synthesis is a plastid- localized 9-cis-epoxycarotenoid dioxygenase, which cleaves an epoxycarotenoid precursor to form xanthoxin. Subsequently, xanthoxin is converted to ABA by two cytosolic enzymes via abscisic aldehyde, but there appears to be at least one minor alternative pathway. The major catabolic route leads to 8'-hydroxy ABA and phaseic acid formation, catalyzed by the cytochrome P450 enzyme ABA 8'-hydroxylase. In addition, there are alternate catabolic pathways via conjugation, 4'-reduction and 7'-hydroxylation. As a consequence of recent developments, the mechanism by which the concentration of hormonally active ABA is controlled at the cellular, tissue and whole plant level can now be analyzed in detail.

Identification of a cis-regulatory element involved in phytochrome down-regulated expression of the pea small GTPase gene pra2

The pra2 gene encodes a pea (Pisum sativum) small GTPase belonging to the YPT/rab family, and its expression is down-regulated by light, mediated by phytochrome. We have isolated and characterized a genomic clone of this gene and constructed a fusion DNA of its 5'-upstream region in front of the gene for firefly luciferase. Using this construct in a transient assay, we determined a pra2 cis-regulatory region sufficient to direct the light down-regulation of the luciferase reporter gene. Both 5'- and internal deletion analyses revealed that the 93-bp sequence between -734 and -642 from the transcriptional start site was important for phytochrome down-regulation. Gain-of-function analysis showed that this 93-bp region could confer light down-regulation when fused to the cauliflower mosaic virus 35S promoter. Furthermore, linker-scanning analysis showed that a 12-bp sequence within the 93-bp region mediated phytochrome down-regulation. Gel-retardation analysis showed the presence of a nuclear factor that was specifically bound to the 12-bp sequence in vitro. These results indicate that this element is a cis-regulatory element involved in phytochrome down-regulated expression.