DNA methylation is a conserved epigenetic mark important for genome integrity, development, and environmental responses in plants and mammals. Active DNA demethylation in plants is initiated by a family of 5-mC DNA glycosylases/lyases (i.e., DNA demethylases). Recent reports suggested a role of active DNA demethylation in fruit ripening in tomato. In this study, we generated loss-of-function mutant alleles of a tomato gene, SlDML2, which is a close homolog of the Arabidopsis DNA demethylase gene ROS1. In the fruits of the tomato mutants, increased DNA methylation was found in thousands of genes. These genes included not only hundreds of ripening-induced genes but also many ripening-repressed genes. Our results show that SlDML2 is critical for tomato fruit ripening and suggest that active DNA demethylation is required for both the activation of ripeninginduced genes and the inhibition of ripening-repressed genes.DNA demethylase | 5-mC DNA glycosylase | DNA methylation | epigenetic regulation | gene silencing D NA methylation is a conserved epigenetic modification that is generally associated with inactive transcription in plants and mammals. As such, DNA methylation plays important roles in many biological processes, such as genome stability, gene imprinting, development, and response to the environment (1-3). In contrast to mammals, in which DNA methylation predominantly occurs at cytosines in the symmetric CG sequence context, plants commonly have methylation in the asymmetrical CHH sequence context (H = A, C, or T), as well as in the symmetrical CG and CHG contexts (1, 2). In plants, cytosines in all sequence contexts can be de novo methylated through the well-known RNA-directed DNA methylation pathway (RdDM), in which 24-nt siRNAs guide the DNA methyltransferase domains rearranged methyltransferase 2 (DRM2) to methylate target loci (4). DNA methylation can be maintained during replication; mCG and mCHG are maintained by the DNA methyltransferases DNA methyltransferase 1 (MET1) and chromomethylase 3 (CMT3), respectively, whereas mCHH is maintained by CMT2 and RdDM (4, 5).Cytosine methylation levels are dynamically regulated by DNA methylation and demethylation reactions (3, 6). DNA methylation can be lost either because of failure in maintaining methylation after replication (i.e., passive DNA demethylation) or because of active removal by enzymes (i.e., active DNA demethylation). Previous studies have identified and characterized several enzymes important for active DNA demethylation in Arabidopsis (7-11). The ROS1 family of bifunctional 5-methylcytosine DNA glycosylases/lyases, often referred to as DNA demethylases, initiate active DNA demethylation by removing the methylcytosine base from the DNA backbone, resulting in a single nucleotide gap that can be filled with an unmethylated cytosine through a base excision repair pathway (7,8,12,13). Several enzymes acting downstream of ROS1, such as the 3′ DNA phosphatase ZDP, AP endonuclease-like protein APE1L, and DNA ligase I (AtLIG1), have also been identified ...

The response of phytoplankton chlorophyll to Typhoon Damrey in the South China Sea (SCS) in September 2005 was studied by remote sensing. Chl a concentration increased in 2 areas after the typhoon: (1) An offshore bloom along Damrey's track exhibited a chl a peak (4 mg m -3 ) 5 d after the typhoon's passage. It was preceded by sea-surface cooling (-5°C), mainly on the right side of the typhoon track, and sea-level decrease (-25 cm) along the typhoon track 1 d post-typhoon. The offshore bloom was due to nutrient increase from mixing and upwelling. (2) A nearshore chl a increase succeeded typhoon rain (> 300 mm on 26 September) on Hainan Island in the northwest SCS. In the bloom region, the water was rich in suspended sediments, phytoplankton, and colored dissolved organic matter, and was entrained by an eddy. This nearshore feature may have resulted from rainwater discharge and seaward advection by a typhoon-induced current. By these 2 mechanisms, both typhoon winds and rain can enhance production of marine phytoplankton.

The occurrences of harmful algal blooms (HABs), in terms of frequency and area in the Chinese coastal waters, have been increasing since 1980s and caused considerable economic losses. In the present study, we have analyzed spatial and seasonal characteristics of HAB events in the southern Yellow Sea and East China Sea along Chinese coast from 1933 to 2004. With a total 435 HAB records, the most frequent HAB occurrence area (FHA) is off the Yangtze River mouth and another two FHA areas are located south of the Yangtze River estuary along about isobaths of 30-60 m coastal water in the East China Sea.

Two phytoplankton blooms in the South China Sea (SCS), triggered by 2 typhoons with different intensities and translation speeds, were compared using remotely sensed chlorophyll a (chl a), sea surface temperature (SST), vector wind field, and best-track typhoon data. Typhoon LingLing in 2001 was strong, with a maximum sustained surface wind speed of 59 m s ) and 36% are slow-moving. We conservatively estimate that typhoon periods may account for 3.5% of the annual primary production in the oligotrophic SCS.

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[1] The authors investigate two phytoplankton blooms near or off the Pearl River Estuary (PRE) triggered by Category 2 Typhoon Nuri with moderate wind intensity in the South China Sea using both remotely sensed and in situ data sets. An offshore triangular phytoplankton bloom (chlorophyll a (Chl a) > 0.5 mg m À3 ) was observed near Dongsha Archipelago along Nuri's track, prolongating southeastward to 18°N 1 week after the typhoon's passage; a stronger nearshore phytoplankton increase (Chl a > 1.5 mg m À3 ) with high total suspended sediment appeared southwest of the PRE from the coast toward the South China Sea. After the typhoon's passage, sea surface cooling ($3°C), strong wind (>20 m s À1 ), and heavy rainfall (>100 mm d À1 ) were also observed in the region, especially offshore. In addition, the freshwater discharge from the PRE during the typhoon passage increased fivefold in comparison with the August climatology in the nearshore bloom region. The nearshore bloom was probably due to the increased discharge from the PRE and favorable current, as well as mixing, upwelling, and near-inertial resonance driven by strong typhoon wind. The offshore bloom may be triggered by upwelling and entrainment induced by strong typhoon wind and the topography of islands and islets. The present study suggests that one typhoon may nourish phytoplankton biomass by inducing transport of nutrient-rich water from both the Pearl River Estuary to offshore and the sublayer to surface.

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