Arabidopsis Uracil DNA Glycosylase (UNG) Is Required for Base Excision Repair of Uracil and Increases Plant Sensitivity to 5-Fluorouracil

Uracil in DNA arises by misincorporation of dUMP during replication and by hydrolytic deamination of cytosine. This common lesion is actively removed through a base excision repair (BER) pathway initiated by a uracil DNA glycosylase (UDG) activity that excises the damage as a free base. UDGs are classified into different families differentially distributed across eubacteria, archaea, yeast, and animals, but remain to be unambiguously identified in plants. We report here the molecular characterization of AtUNG (Arabidopsis thaliana uracil DNA glycosylase), a plant member of the Family-1 of UDGs typified by Escherichia coli Ung. AtUNG exhibits the narrow substrate specificity and single-stranded DNA preference that are characteristic of Ung homologues. Cell extracts from atung^−/− mutants are devoid of UDG activity, and lack the capacity to initiate BER on uracil residues. AtUNG-deficient plants do not display any apparent phenotype, but show increased resistance to 5-fluorouracil (5-FU), a cytostatic drug that favors dUMP misincorporation into DNA. The resistance of atung^−/− mutants to 5-FU is accompanied by the accumulation of uracil residues in DNA. These results suggest that AtUNG excises uracil in vivo but generates toxic AP sites when processing abundant U:A pairs in dTTP-depleted cells. Altogether, our findings point to AtUNG as the major UDG activity in Arabidopsis.     

Binding of Cysteine Synthase to the STAS Domain of Sulfate Transporter and Its Regulatory Consequences

The sulfate ion (SO₄²⁻) is transported into plant root cells by SO₄²⁻ transporters and then mostly reduced to sulfide (S²⁻). The S²⁻ is then bonded to O-acetylserine through the activity of cysteine synthase (O-acetylserine (thiol)lyase or OASTL) to form cysteine, the first organic molecule of the SO₄²⁻ assimilation pathway. Here, we show that a root plasma membrane SO₄²⁻ transporter of Arabidopsis, SULTR1;2, physically interacts with OASTL. The interaction was initially demonstrated using a yeast two-hybrid system and corroborated by both in vivo and in vitro binding assays. The domain of SULTR1;2 shown to be important for association with OASTL is called the STAS domain. This domain is at the C terminus of the transporter and extends from the plasma membrane into the cytoplasm. The functional relevance of the OASTL-STAS interaction was investigated using yeast mutant cells devoid of endogenous SO₄²⁻ uptake activity but co-expressing SULTR1;2 and OASTL. The analysis of SO₄²⁻ transport in these cells suggests that the binding of OASTL to the STAS domain in this heterologous system negatively impacts transporter activity. In contrast, the activity of purified OASTL measured in vitro was enhanced by co-incubation with the STAS domain of SULTR1;2 but not with the analogous domain of the SO₄²⁻ transporter isoform SULTR1;1, even though the SULTR1;1 STAS peptide also interacts with OASTL based on the yeast two-hybrid system and in vitro binding assays. These observations suggest a regulatory model in which interactions between SULTR1;2 and OASTL coordinate internalization of SO₄²⁻ with the energetic/metabolic state of plant root cells.     

Single Amino Acid Alteration between Valine and Isoleucine Determines the Distinct Pyrabactin Selectivity by PYL1 and PYL2

Abscisic acid (ABA) is one of the most important phytohormones in plant. PYL proteins were identified to be ABA receptors in Arabidopsis thaliana. Despite the remarkably high degree of sequence similarity, PYL1 and PYL2 exhibit distinct responses toward pyrabactin, an ABA agonist. PYL1 inhibits protein phosphatase type 2C upon binding of pyrabactin. In contrast, PYL2 appears relatively insensitive to this compound. The crystal structure of pyrabactin-bound PYL1 revealed that most of the PYL1 residues involved in pyrabactin binding are conserved, hence failing to explain the selectivity of pyrabactin for PYL1 over PYL2. To understand the molecular basis of pyrabactin selectivity, we determined the crystal structure of PYL2 in complex with pyrabactin at 1.64 Å resolution. Structural comparison and biochemical analyses demonstrated that one single amino acid alteration between a corresponding valine and isoleucine determines the distinct pyrabactin selectivity by PYL1 and PYL2. These characterizations provide an important clue to dissecting the redundancy of PYL proteins.     

Host specificity of a Brazilian mistletoe, Struthanthus aff. polyanthus (Loranthaceae), in cerrado tropical savanna

The aim of this study was to determine if the mistletoe Struthanthus aff. polyanthus shows host specificity, and if host abundance and twig architecture influence this specificity in Cerrado (senso strictu). An area of 1.3 ha of natural savanna was sampled for the occurrence of the mistletoe. The twigs of the hosts were classified as vertical or horizontal, and the bark as smooth or rough. We sampled a total of 666 trees in the study site and 118 individuals (17.7%) hosted S. aff. polyanthus. The abundance of mistletoe was significantly affected by the bark type, but not by the twigs inclination. In the cerrado, S. aff. polyanthus seems to be a generalist, with a tendency to be more common on some hosts (Kielmeyera coriacea, Pouteria ramiflora and Styrax ferrugineus), prefering that with rough bark type.     

Savanna trees in Namibia—Factors controlling their distribution at the arid end of the spectrum

This study reviewed the distribution of ten common savanna trees in Namibia. Tree distributions were investigated in relation to bioclimatic, topographic and edaphic variables at a national scale. The factors of importance at these broad geographic scales appeared to be rainfall, substrate and, likely, the incidence of frost. Baikiaea plurijuga, Burkea africana, Guibourtia coleosperma and Pterocarpus angolensis seem to reach their bioclimatic limits in Namibia.At the local level, plant traits become important and contribute to explaining distribution patterns. High water and/or nitrogen use efficiency (Acacia erioloba, Colophospermum mopane), dual water obtaining strategies (Faidherbia albida), fire tolerance (e.g. Acacia species, Burkea africana and Pterocarpus angolensis) and drought tolerance (Boscia albitrunca) are some key attributes providing additional explanations for current distributions.Amongst the selected trees and at broad geographic scales, below-ground adaptations are governed by rainfall regime in combination with coarse-textured soils, whereby shallow-rooted trees prevail in the Kalahari sandveld. Deep-rooted species are found largely on non-sandy soils. Physiological performance of many trees appears to be directly linked to rainfall regime and trees may hence show varying performance throughout their distribution range. Insight into plant functional attributes of trees in Namibia is required to develop appropriate management strategies in the light of climate change. Modelling climate change impacts should consider the relative contribution of bioclimatic versus local environmental factors that explain the current distribution patterns of the selected trees.     

Site fidelity and homing in tropical coral reef cardinalfish: are they using olfactory cues?

A number of tropical coral reef fish hold station and display restricted home ranges. If artificially displaced, they will return to their home site. We questioned if marine fish are using the same mechanisms for home site detection as many freshwater fish, that is, by olfactory sensing of chemical signals deposited on the substrate by conspecific fish. Behavioral experiments were conducted on Lizard Island Research Station, Queensland, Australia, in 2001 and 2002. Five-lined cardinalfish (Cheilodipterus quinquelineatus) were tested in groups with split-branded cardinalfish (Apogon compressus) as a reference species and individually against Apogon leptacanthus as well as conspecifics of another reef site. The group tests showed that both species preferred artificial reef sites that had previously been occupied by conspecifics. Individual C. quinquelineatus preferred scent of conspecifics from their own reef site to that from another site. They also preferred the scent released by artificial reefs previously occupied by conspecifics of their reef site to that of similar reefs previously occupied by conspecifics of another reef site. No discrimination between species from the same reef site was obtained in experiments with individual fish. Our data suggest that cardinalfish are keeping station and are homing by use of conspecific olfactory signals.     

温度、抗坏血酸、柠檬酸和氯化钠对采后红毛丹果实几种生理指标与品质的影响

采后红毛丹（品种‘粉红’）果实贮于10℃下时呼吸速率下降、细胞膜透性减小、多酚氧化酶活性（PPO）下降、病害少;4℃下贮藏至第4天时即发生冷害;采后用3％抗坏血酸溶液、2％柠檬酸溶液和1％NaCl处理的红毛丹果实中含糖量和果皮花色素苷含量均不减小,其中以10℃下用化学药品处理的效果最显著。     

Effect of ABA upon anthocyanin synthesis in regenerated torenia shoots

To elucidate the mechanism of anthocyanin synthesis induction concomitant with chlorophyll degradation, we established a system in which anthocyanin synthesis and degradation of chlorophyll in regenerated torenia (Torenia fournieri) shoots was induced on medium containing 7% sucrose. Here, we studied the effect of several plant-growth regulators on anthocyanin synthesis and the degradation of chlorophyll in the torenia shoot regenerating system. Exogenous abscisic acid (ABA) could induce anthocyanin synthesis and chlorophyll senescence in regenerating torenia shoots on the medium containing a low concentration of sucrose (1.5%). We determined the changes in the amount of endogenous ABA in the regenerated shoots during the process of anthocyanin synthesis on the medium containing 7% sucrose. It was revealed that the 7% sucrose treatment elevated endogenous ABA levels before the induction of anthocyanin synthesis and chlorophyll degradation. However, while retransfer to the 1.5% sucrose medium resulted in a gradual decrease in the ABA level and a failure of induction of anthocyanin synthesis, normal shoot regeneration. These results suggest that changes in the amount of endogenous ABA may play an important role in the induction of anthocyanin synthesis and chlorophyll degradation in regenerated torenia shoots.     

Spontaneous plant regeneration in transformed roots and calli from Tylophora indica: changes in morphological phenotype and tylophorine accumulation associated with transformation by Agrobacterium rhizogenes

We examined the effects of genetic transformation by Agrobacterium rhizogenes on the production of tylophorine, a phenanthroindolizidine alkaloid, in the Indian medicinal plant, Tylophora indica. Transformed roots induced by the bacterium grew in axenic culture and produced shoots or embryogenic calli in the absence of hormone treatments. However, hormonal treatment was required to regenerate shoots in root explants of wild type control plants. Transformed plants showed morphological features typically seen in transgenic plants produced by A. rhizogenes, which include, short internodes, small and wrinkled leaves, more branches and numerous plagiotropic roots. Plants regenerated from transformed roots showed increased biomass accumulation (350–510% in the roots and 200–320% in the whole plants) and augmented tylophorine content (20–60%) in the shoots, resulting in a 160–280% increase in tylophorine production in different clones grown in vitro.