Bio-immobilization and recovery of chromium using a denitrifying biofilm system: Identification of reaction zone, binding forms and end products.

Chromium is an important resource in strategic metals. Different from most studies focusing on the bio-reduction of hexavalent chromium [Cr(VI)], this study aims to achieve the immobilization and recovery of chromium using a sequencing batch biofilm reactor. Results showed that Cr(VI) removal efficiency remained more than 99%, and 97% of reduced Cr(III) was immobilized in the biofilm. Immobilization zone, chromium forms and extracellular polymeric substances composition changes were combined to reveal the mechanism of Cr(VI) reduction and immobilization. The chromium distribution in biofilm demonstrated that intercellular layer was the main active zone with an immobilization amount of 891.70±126.32 mg/g-VSS. The reduced products analysis confirmed that trivalent chromium [Cr(III)] chelated with carboxyl, amino and other functional groups and immobilized in the form of organic Cr(III). The digestion method realized a chromium recovery efficiency of 74.59%. This study provides an alternative method for the bioremediation and resources recovery in chromium polluted wastewater.

The ß-amylase PbrBAM3 from pear (Pyrus betulaefolia) regulates soluble sugar accumulation and ROS homeostasis in response to cold stress.

ß-Amylase (BAM) is involved in sugar metabolism, but the role of BAM genes in cold tolerance remains poorly understood. Here, we report the identification and functional characterization of the chloroplast-localized BAM-encoding gene PbrBAM3 isolated from Pyrus betulaefolia. The transcript levels of PbrBAM3 were up-regulated under cold, dehydration and ABA, but repressed by maltose. Overexpression of PbrBAM3 in tobacco (Nicotiana tabacum) and pear (P. ussuriensis) conferred increased BAM activity, promoted starch degradation after chilling treatments and enhanced tolerance to cold. Under the chilling stress, the transgenic tobacco and P. ussuriensis exhibited lessened reactive oxygen species (ROS) generation, higher levels of antioxidant enzymes activity, and greater accumulation of soluble sugars (specially maltose) than the corresponding wild type plants. Taken together, these results demonstrate that PbrBAM3 plays an important role in cold tolerance, at least in part, by raising the levels of soluble sugars capable of acting as osmolytes or antioxidants.

Overexpression of PbrNHX2 gene, a Na+/H+ antiporter gene isolated from Pyrus betulaefolia, confers enhanced tolerance to salt stress via modulating ROS levels.

Intracellular Na+/H+ antiporters (NHXs) play important roles in plant tolerance to salt stress. However, plant NHXs functioning in salt tolerance and the underlying physiological mechanisms remain poorly understood. In this report, we report the identification and functional characterization of PbrNHX2 isolated from Pyrus betulaefolia. PbrNHX2 expression levels were induced by salt, and dehydration, but was unaffected by cold. PbrNHX2 was localized in the tonoplast. Overexpression of PbrNHX2 in tobacco and Pyrus ussuriensis conferred enhanced tolerance to salt tolerance, whereas down-regulation of PbrNHX2 in Pyrus betulaefolia by virus-induced gene silencing (VIGS) resulted in elevated salt sensitivity. The transgenic lines contained lower levels of Na+, higher levels of K+, and higher K/Na ratio, whereas they were changed in an opposite way when PbrNHX2 was silenced. In addition, the transgenic plants accumulated lower levels of reactive oxygen species compared with wild type, accompanied by higher activities of three antioxidant enzymes. Taken together, the data demonstrate that PbrNHX2 plays a positive role in salt tolerance and that it holds a great potential for engineering salt tolerance in crops.

A WRKY transcription factor PbrWRKY53 from Pyrus betulaefolia is involved in drought tolerance and AsA accumulation.

WRKY comprises a large family of transcription factors in plants, but most WRKY members are still poorly understood. In this study, we report the identification and functional characterization of PbrWRKY53 isolated from Pyrus betulaefolia. PbrWRKY53 was greatly up-regulated by drought and abscisic acid, but slightly induced by salt and cold. Subcellar localization analyses showed that PbrWRKY53 was located in the nucleus. Ectopic expression of PbrWRKY53 in tobacco and Pyrus ussuriensis conferred enhanced tolerance to drought stress. The transgenic plants exhibited better water status, less reactive oxygen species generation and higher levels of antioxidant enzyme activities and metabolites than the wild type. In addition, overexpression of PbrWRKY53 in transgenic tobacco resulted in enhanced expression level of PbrNCED1, and led to the increase in larger amount of vitamin C accumulation in comparison to WT. Knock-down of PbrWRKY53 in P. ussuriensis down-regulated PbrNCED1 abundance, accompanied by compromised drought tolerance. Yeast one-hybrid assay, EMSA and transient expression analysis demonstrated that PbrWRKY53 could bind to the W-box element in the promoter region of PbrNCED1. Taken together, these results demonstrated that PbrWRKY53 plays a positive role in drought tolerance, which might be, at least in part, promoting production of vitamin C via regulating PbrNCED1 expression.

Transcriptomic and evolutionary analyses of white pear (Pyrus bretschneideri) ß-amylase genes reveals their importance for cold and drought stress responses.

ß-amylase (BAM) genes play essential roles in plant abiotic stress responses. Although the genome of Chinese white pear (Pyrus bretschneideri) has recently been made available, knowledge regarding the BAM family in pear, including gene function, evolutionary history and patterns of gene expression remains limited. In this study, we identified 17 PbBAMs in the pear genome. Of these, 12 PbBAM members were mapped onto 9 chromosomes and 5 PbBAM genes were located on scaffold contigs. Based on gene structure, protein motif analysis, and the topology of the phylogenetic tree of the PbBAM family, we classified member genes into 4 groups. All PbBAM genes were found to contain typical glycosyl hydrolysis 14 domain motifs. Interfamilial comparisons revealed that the phylogenetic relationships of BAM genes in other Rosaceae species were similar those found in pear. We also found that whole-genome duplication (WGD)/segmental duplication events played critical roles in the expansion of the BAM family. Next, we used transcriptomic data to study gene expression during the response of drought and low temperate responses, and found that genes in Group B were related to drought and cold stress. We identified four PbBAM genes associated with abiotic stress in Pear. Finally, by analyzing co-expression networks and co-regulatory genes, we found that PbBAM1a and PbBAM1b were associated with the pear abiotic stress response.