Heavy metal contamination of soil and water causing toxicity/stress has become one important constraint to crop productivity and quality. This situation has further worsened by the increasing population growth and inherent food demand. It have been reported in several studies that counterbalancing toxicity, due to heavy metal requires complex mechanisms at molecular, biochemical, physiological, cellular, tissue and whole plant level, which might manifest in terms of improved crop productivity. Recent advances in various disciplines of biological sciences such as metabolomics, transcriptomics, proteomics etc. have assisted in the characterization of metabolites, transcription factors, stress-inducible proteins involved in heavy metal tolerance, which in turn can be utilized for generating heavy metal tolerant crops. This review summarizes various tolerance strategies of plants under heavy metal toxicity, covering the role of metabolites (metabolomics), trace elements (ionomics), transcription factors (transcriptomics), various stress-inducible proteins (proteomics) as well as the role of plant hormones. We also provide a glance at strategies adopted by metal accumulating plants also known as “metallophytes”.

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Heavy Metal Tolerance in Plants: Role of Transcriptomics, Proteomics, Metabolomics, and Ionomics.

In mammals, cadmium is widely considered as a non-genotoxic carcinogen acting through a methylation-dependent epigenetic mechanism. Here, the effects of Cd treatment on the DNA methylation patten are examined together with its effect on chromatin reconfiguration in Posidonia oceanica. DNA methylation level and pattern were analysed in actively growing organs, under short- (6 h) and long- (2 d or 4 d) term and low (10 mM) and high (50 mM) doses of Cd, through a Methylation-Sensitive Amplification Polymorphism technique and an immunocytological approach, respectively. The expression of one member of the CHROMOMETHYLASE (CMT) family, a DNA methyltransferase, was also assessed by qRT-PCR. Nuclear chromatin ultrastructure was investigated by transmission electron microscopy. Cd treatment induced a DNA hypermethylation, as well as an up-regulation of CMT, indicating that de novo methylation did indeed occur. Moreover, a high dose of Cd led to a progressive heterochromatinization of interphase nuclei and apoptotic figures were also observed after long-term treatment. The data demonstrate that Cd perturbs the DNA methylation status through the involvement of a specific methyltransferase. Such changes are linked to nuclear chromatin reconfiguration likely to establish a new balance of expressed/repressed chromatin. Overall, the data show an epigenetic basis to the mechanism underlying Cd toxicity in plants.

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In Posidonia oceanica cadmium induces changes in DNA methylation and chromatin patterning

The development of linkage maps with large numbers of molecular markers has stimulated the search for methods to map genes involved in quantitative traits (QTLs). A promising method, proposed by Lander and Botstein (1989), employs pairs of neighbouring markers to obtain maximum linkage information about the presence of a QTL within the enclosed chromosomal segment. In this paper the accuracy of this method was investigated by computer simulation. The results show that there is a reasonable probability of detecting QTLs that explain at least 5% of the total variance. For this purpose a minimum population of 200 backcross or F2 individuals is necessary. Both the number of individuals and the relative size of the genotypic effect of the QTL are important factors determining the mapping precision. On the average, a QTL with 5% or 10% explained variance is mapped on an interval of 40 or 20 centiMorgans, respectively. Of course, QTLs with a larger genotypic effect will be located more precisely. It must be noted, however, that the interval length is rather variable.

Accuracy of mapping quantitative trait loci in autogamous species.

Chromium-induced physio-chemical and ultrastructural changes in four cultivars of Brassica napus L.

Citric acid assisted phytoremediation of copper by Brassica napus L.

Because of its prolific growth, oilseed rape (Brassica napus L.) can be grown advantageously for phytoremediation of the lands contaminated by industrial wastes. Therefore, toxic effect of cadmium on the germination of oilseed rape, the capability of plants for cadmium phytoextraction, and the effect of exogenous application of plant growth regulators to mitigate phytotoxicity of cadmium were investigated. For the lab study of seedlings at early stage, seeds were grown on filter papers soaked in different solutions of Cd2+ (0, 10, 50, 100, 200 and 400 μM). In greenhouse study, seedlings were grown in soil for 8 weeks, transferred to hydroponic pots for another 6 weeks growth, and then treated with plant growth regulators and cadmium. Four plant growth regulators viz. jasmonic acid (12.5 μM), abscisic acid (10 μM), gibberellin (50 μM) and salicylic acid (50 μM); and three levels of Cd2+ (0, 50 and 100 μM) were applied. Data indicated that lower concentration of Cd2+ (10 μM) promoted the root growth, whereas the severe stresses (200 or 400 μM) had negative effect on the establishment of germinating seedlings. Plants treated with any of the tested plant growth regulators alleviated cadmium toxicity symptoms, which were reflected by more fresh weight, less malondialdehyde concentration in leaves and lower antioxidant enzyme activities. The application of abscisic acid to the plants cultivated in the medium containing 100 μM Cd2+ resulted in significantly lower plant internal cadmium accumulation.

Cadmium-induced stress on the seed germination and seedling growth of Brassica napus L., and its alleviation through exogenous plant growth regulators.

The oilseed rape plant’s transition from the vegetative to the reproductive stage is important to its yield. This transition is controlled by a large group of flowering time genes that respond to environmental and endogenous cues. The role of jasmonates in flowering is almost unknown in Brassicaceae, even in the genus Arabidopsis. In this paper, the clear effect of exogenous methyl jasmonate (MeJA) on the flowering time, floral organ morphology, and transcript levels of a group of genes implicated in floral development is shown. In controlled greenhouse experiments, we found that the effect of MeJA depended on both plant genotype and jasmonate dosage. MeJA promoted maximum flowering when it was applied to the cultivars of early flowering types of oilseed rape, such as cultivars Mei-Jian and Fu-You 4. In addition, a concentration of 100 μM resulted in the most number of early open flowers, in comparison with the results obtained for concentrations of 50 and 80 μM. Furthermore, the application of high concentrations of MeJA (100 μM) also produced various kinds of abnormal flowers. Our results demonstrated that the combined actions of the floral identity genes, specifically BnAP1, BnAP2, BnAP3, BnAG1, and BnPI3, as reflected by their respective relative transcript levels, were responsible for causing the different kinds of flower abnormalities previously undescribed in oilseed rape. We expect our assay to be an enriching addition to the body of work that attempts to understand the signaling function of jasmonates in the floral inductive pathway.

The effect of exogenous methyl jasmonate on the flowering time, floral organ morphology, and transcript levels of a group of genes implicated in the development of oilseed rape flowers ( Brassica napus L.)

To date, little is known about the correlations among the tocopherol (T) and fatty acid (FA) components in rapeseed oils. In the current study, a germplasm collection of landraces from the species Brassica juncea, Brassica rapa, and Brassica napus and a collection of low erucic acid (EA) breeding lines from B. napus were analyzed for FA and T contents. In the groups comprising landraces, the most notable correlation was the significantly positive one between α-T and the sum of C18:1 and C18:2, whereas neither positive correlations were found between α-T and C18:3 nor were positive correlations observed between α-T and very long chain FAs (VLCFA). Hardly any association between γ-T and FA components was observed, indicating the possible function of α-T beyond its antioxidant property. The complexity of correlation between T and FA components in Brassica oils may arise from the role of α-T in the FA metabolism of endoplasmic reticulum (ER).

Correlations between tocopherol and fatty acid components in germplasm collections of Brassica oilseeds.

Efficient molecular markers for the selection of rapeseed genetic materials with high seed oil content and ideal fatty acid (FA) composition are preferred by rapeseed breeders. Recently, we reported the molecular mechanism of TRANSPARENT TESTA 2 (TT2) in inhibiting seed FA biosynthesis in Arabidopsis. However, evidence showing the association of rapeseed TT2 homologs and seed FA production are still insufficient. In this study, we collected 83 rapeseed (Brassica napus L.) landraces from different geographical backgrounds to conduct association mapping of BnaC.TT2.a in relation to seed coat color and FA biosynthesis. Population background was corrected by 84 pairs of SSR markers that were uniformly distributed among the linkage groups of the Tapidor-Ningyou-7 DH population. A single copy of BnaC.TT2.a for single nucleotide polymorphism (SNP) assay was cloned by a pair of previously reported specific primers. From the analysis of BnaC.TT2.a allelic variations using GLM+Q model, four SNPs on intron 1 of BnaC.TT2.a that were associated with seed FA were discovered. Moreover, an InDel at position 738 on exon 3 of BnaC.TT2.a indicated a change of protein function that was significantly associated with seed coat color, linoleic acid (C18:2), and total FA content. These findings revealed the role of BnaC.TT2.a in regulating the seed color formation and seed FA biosynthesis in rapeseed, thereby suggesting effective molecular markers for rapeseed breeding.

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Allelic Variation of BnaC.TT2.a and Its Association with Seed Coat Color and Fatty Acids in Rapeseed (Brassica napus L.).

Knowledge about the genes implicated in lipid biosynthesis acquired from the model plant Arabidopsis is useful in understanding the formation of seed oil in Brassica oilseeds. In this paper, we report the screening of polymorphic markers at the loci putative for the seed oil formation between two geographically different genotypes: the Chinese cultivar Ningyou-7 and the European cultivar Tapidor. These primer pairs (150) were designed based on 75 Brassica genes that were Arabidopsis orthologues implicated in the oil formation. A total of 52 out of the 150 primer pairs associated with 47 of the 75 genes showed polymorphisms between the two genotypes. The type of polymorphisms that could be detected on capillary electrophoresis images and their respective visual futures are described. Further, we selected 34 polymorphic markers to scan allelic variations and found rich DNA polymorphisms among the 54 Brassica oilseed cultivars. On the average, each primer pair resulted in 5.6 alleles at the region that was covered. The correlation between the alleles and seed quality traits revealed that the alleles of BnFAD7 were related to the variation of linolenic acid (C18:3) contents among the cultivars. The allele FAD7-ics11170 (3/4)-b that was significantly correlated with high linolenic acid content can be used as an efficient marker for the selection of breeding materials with high linolenic acid content.

Yuanlong Li

Papers

Cadmium-induced stress on the seed germination and seedling growth of Brassica napus L., and its alleviation through exogenous plant growth regulators.

The effect of exogenous methyl jasmonate on the flowering time, floral organ morphology, and transcript levels of a group of genes implicated in the development of oilseed rape flowers ( Brassica napus L.)

Correlations between tocopherol and fatty acid components in germplasm collections of Brassica oilseeds.

Allelic Variation of BnaC.TT2.a and Its Association with Seed Coat Color and Fatty Acids in Rapeseed (Brassica napus L.).

DNA allelic variations at the loci putatively implicated in seed oil formation among Brassica oilseed cultivars