Comparative study of root growth and sucrose-cleaving enzymes in metallicolous and non-metallicolous populations of Rumex dentatus under copper stress

Comparison and Characterization of a Cell Wall Invertase Promoter from Cu-Tolerant and Non-Tolerant Populations of Elsholtzia haichowensis

Cell wall invertase (CWIN) activity and the expression of the corresponding gene were previously observed to be significantly elevated in a Cu-tolerant population of Elsholtzia haichowensis relative to a non-tolerant population under copper stress. To understand the differences in CWIN gene regulation between the two populations, their CWIN promoter β-glucuronidase (GUS) reporter vectors were constructed. GUS activity was measured in transgenic Arabidopsis in response to copper, sugar, and phytohormone treatments. Under the copper treatment, only the activity of the CWIN promoter from the Cu-tolerant population was slightly increased. Glucose and fructose significantly induced the activity of CWIN promoters from both populations. Among the phytohormone treatments, only salicylic acid induced significantly higher (p < 0.05) activity of the Cu-tolerant CWIN promoter relative to the non-tolerant promoters. Analysis of 5′-deletion constructs revealed that a 270-bp promoter fragment was required for SA induction of the promoter from the Cu-tolerant population. Comparison of this region in the two CWIN promoters revealed that it had 10 mutation sites and contained CAAT-box and W-box cis-elements in the Cu-tolerant promoter only. This work provides insights into the regulatory role of SA in CWIN gene expression and offers an explanation for differences in CWIN expression between E. haichowensis populations.

Regulation Network of Sucrose Metabolism in Response to Trivalent and Hexavalent Chromium in Oryza sativa

The presence of chromium (Cr) in cultivated fields affects carbohydrate metabolism of rice (Oryza sativa L.) and weakens its productivity. Little is known about the molecular mechanism of sucrose metabolism underlying Cr stress response in rice plants. In the present study, the transcriptome map of sucrose metabolism in rice seedlings exposed to both trivalent and hexavalent chromium was investigated using Agilent 4 × 44K rice microarray analysis. Results indicated that Cr exposure (3 days) significantly (p < 0.05) improved sucrose accumulation, and altered the activities of sucrose synthetase, sucrose phosphate phosphatase, and amylosynthease in rice tissues. We identified 119 differentially regulated genes involved in 17 sucrose metabolizing enzymes and found that gene responses in roots were significantly (p < 0.05) stronger than in shoots under both Cr(III) and Cr(VI) treatment. The network maps of gene regulation responsible for sucrose metabolism in rice plants provide a theoretical basis for further cultivating Cr-resistant rice cultivars through molecular genetic improvement.

Differential expression of vacuolar and defective cell wall invertase genes in roots and seeds of metalliferous and non-metalliferous populations of Rumex dentatus under copper stress

Acid invertase activities in roots and young seeds of a metalliferous population (MP) of Rumex dentatus were previously observed to be significantly higher than those of a non-metalliferous population (NMP) under Cu stress. To date, no acid invertase gene has been cloned from R. dentatus. Here, we isolated four full-length cDNAs from the two populations of R. dentatus, presumably encoding cell wall (RdnCIN1 and RdmCIN1 from the NMP and MP, respectively) and vacuolar invertases (RdnVIN1 and RdmVIN1 from the NMP and MP, respectively). Unexpectedly, RdnCIN1 and RdmCIN1 most likely encode special defective invertases with highly attenuated sucrose-hydrolyzing capacity. The transcript levels of RdmCIN1 were significantly higher than those of RdnCIN1 in roots and young seeds under Cu stress, whereas under control conditions, the former was initially lower than the latter. Unexpected high correlations were observed between the transcript levels of RdnCIN1 and RdmCIN1 and the activity of cell wall invertase, even though RdnCIN1 and RdmCIN1 do not encode catalytically active invertases. Similarly, the transcript levels of RdmVIN1 in roots and young seeds were increased under Cu stress, whereas those of RdnVIN1 were decreased. The high correlations between the transcript levels of RdnVIN1 and RdmVIN1 and the activity of vacuolar invertase indicate that RdnVIN1 and RdmVIN1 might control distinct vacuolar invertase activities in the two populations. Moreover, a possible indirect role for acid invertases in Cu tolerance, mediated by generating a range of sugars used as nutrients and signaling molecules, is discussed.

Heterologous expression and comparative characterization of vacuolar invertases from Cu-tolerant and non-tolerant populations of Elsholtzia haichowensis

Vacuolar invertases (VINs) from Cu-tolerant and non-tolerant populations of Elsholtzia haichowensis have similar enzyme properties, and the enzyme protein divergences contribute little to the varied VIN activities between the contrasting populations. In our previous studies of Elsholtzia haichowensis, vacuolar invertase (VIN) activity in roots of a Cu-tolerant population was found to be significantly higher than that of a non-tolerant population under Cu stress. Divergences of amino acid residues in a sucrose-binding box and other regions of the VINs were detected. To test whether the amino acid divergences influence the enzyme properties of VINs, and thus are relevant to the differences in enzyme activities between the contrasting populations of E. haichowensis, two VIN genes from the Cu-tolerant population (EhCvINV) and non-tolerant population (EhNvINV) were heterologously expressed in Pichia pastoris, and the enzyme properties of the recombinants were characterized and compared. Both of the recombinant enzymes showed temperature optima of 70 °C and pH optima of 4.5-5.5. Copper as well as other heavy metals caused almost the same inhibition to EhNvINV and EhCvINV. No statistically significant differences were observed between EhNvINV and EhCvINV in K m and k cat values for sucrose. The results provided evidence that the observed residue divergences had little influence on the enzyme properties of VIN in E. haichowensis, and the varied VIN activities between the contrasting populations under Cu stress were not relevant to the amino acid divergences in the proteins. Also, some other possible reasons accounting for this difference in invertase activities were discussed, such as up-regulation of expression of the EhCvINV gene under Cu stress, as Cu tolerance mechanisms in Cu-mine plants.