Search

Fruit crops provide various kinds of fruit commodities that are of significant nutritional benefit and economic value to humans [...]

The contribution of reactive oxygen species (ROS) and methylglyoxal (MG) formation and removal in high-pH-mediated alleviation of plant copper (Cu)-toxicity remains to be elucidated. Seedlings of sweet orange (Citrus sinensis) were treated with 0.5 (non-Cu-toxicity) or 300 (Cu-toxicity) μM CuCl2 × pH 4.8, 4.0, or 3.0 for 17 weeks. Thereafter, superoxide anion production rate; H2O2 production rate; the concentrations of MG, malondialdehyde (MDA), and antioxidant metabolites (reduced glutathione, ascorbate, phytochelatins, metallothioneins, total non-protein thiols); and the activities of enzymes (antioxidant enzymes, glyoxalases, and sulfur metabolism-related enzymes) in leaves and roots were determined. High pH mitigated oxidative damage in Cu-toxic leaves and roots, thereby conferring sweet orange Cu tolerance. The alleviation of oxidative damage involved enhanced ability to maintain the balance between ROS and MG formation and removal through the downregulation of ROS and MG formation and the coordinated actions of ROS and MG detoxification systems. Low pH (pH 3.0) impaired the balance between ROS and MG formation and removal, thereby causing oxidative damage in Cu-toxic leaves and roots but not in non-Cu-toxic ones. Cu toxicity and low pH had obvious synergistic impacts on ROS and MG generation and removal in leaves and roots. Additionally, 21 (4) parameters in leaves were positively (negatively) related to the corresponding root parameters, implying that there were some similarities and differences in the responses of ROS and MG metabolisms to Cu–pH interactions between leaves and roots.

Low pH-induced alterations in gene expression profiles and organic acids (OA) and free amino acid (FAA) abundances were investigated in sweet orange [Citrus sinensis (L.) Osbeck cv. Xuegan] leaves. We identified 503 downregulated and 349 upregulated genes in low pH-treated leaves. Further analysis indicated that low pH impaired light reaction and carbon fixation in photosynthetic organisms, thereby lowering photosynthesis in leaves. Low pH reduced carbon and carbohydrate metabolisms, OA biosynthesis and ATP production in leaves. Low pH downregulated the biosynthesis of nitrogen compounds, proteins, and FAAs in leaves, which might be conducive to maintaining energy homeostasis during ATP deprivation. Low pH-treated leaves displayed some adaptive responses to phosphate starvation, including phosphate recycling, lipid remodeling, and phosphate transport, thus enhancing leaf acid-tolerance. Low pH upregulated the expression of some reactive oxygen species (ROS) and aldehyde detoxifying enzyme (peroxidase and superoxidase) genes and the concentrations of some antioxidants (L-tryptophan, L-proline, nicotinic acid, pantothenic acid, and pyroglutamic acid), but it impaired the pentose phosphate pathway and VE and secondary metabolite biosynthesis and downregulated the expression of some ROS and aldehyde detoxifying enzyme (ascorbate peroxidase, aldo-keto reductase, and 2-alkenal reductase) genes and the concentrations of some antioxidants (pyridoxine and γ-aminobutyric acid), thus disturbing the balance between production and detoxification of ROS and aldehydes and causing oxidative damage to leaves.

Aluminum (Al) treatment significantly decreased the dry weight (DW) of stem, shoot and whole plant of both Citrus sinensis and C. grandis, but did not change that of root. Al significantly decreased leaf DW of C. grandis, increased the ratio of root to shoot and the lignin content in roots of both species. The higher content of Al in leaves and stems and lignin in roots of C. grandis than that of C. sinensis might be due to the over-expression of Al sensitive 3 (ALS3) and cinnamyl alcohol deaminase (CAD) in roots of C. grandis, respectively. By using yeast-two-hybridazation (Y2H) and bimolecular fluorescence complementation (BiFC) techniques, we obtained the results that glutathione S-transferase (GST), vacuolar-type proton ATPase (V-ATPase), aquaporin PIP2 (PIP2), ubiquitin carboxyl-terminal hydrolase 13 (UCT13), putative dicyanin blue copper protein (DCBC) and uncharacterized protein 2 (UP2) were interacted with ALS3 and GST, V-ATPase, Al sensitive 3 (ALS3), cytochrome P450 (CP450), PIP2, uncharacterized protein 1 (UP1) and UP2 were interacted with CAD. Annotation analysis revealed that these proteins were involved in detoxification, cellular transport, post-transcriptional modification and oxidation-reduction homeostasis or lignin biosynthesis in plants. Real-time quantitative PCR (RT-qPCR) analysis further revealed that the higher gene expression levels of most of these interacting proteins in C. grandis roots than that in C. sinensis ones were consistent with the higher contents of lignin in C. grandis roots and Al absorbed by C. grandis. In conclusion, our study identified some key interacting components of Al responsive proteins ALS3 and CAD, which could further help us to understand the molecular mechanism of Al tolerance in citrus plants and provide new information to the selection and breeding of tolerant cultivars, which are cultivated in acidic areas.

Magnesium (Mg) deficiency is one of the major constraining factors that limit the yield and quality of agricultural products. Uniform seedlings of the Citrus sinensis were irrigated with Mg deficient (0 mM MgSO4) and Mg sufficient (1 mM MgSO4) nutrient solutions for 16 weeks. CO2 assimilation, starch, soluble carbohydrates, TBARS content and H2O2 production were measured. Transcriptomic analysis of C. sinensis leaves was performed by Illumina sequencing. Our results showed that Mg deficiency decreased CO2 assimilation, but increased starch, sucrose, TBARS content and H2O2 production in C. sinensis leaves. A total of 4864 genes showed differential expression in response to Mg deficiency revealed by RNA-Seq and the transcriptomic data were further validated by real-time quantitative PCR (RT-qPCR). Gene ontology (GO) enrichment analysis indicated that the mechanisms underlying Mg deficiency tolerance in C. sinensis may be attributed to the following aspects: (a) enhanced microtubule-based movement and cell cycle regulation; (b) elevated signal transduction in response to biotic and abiotic stimuli; (c) alteration of biological processes by tightly controlling phosphorylation especially protein phosphorylation; (d) down-regulation of light harvesting and photosynthesis due to the accumulation of carbohydrates; (e) up-regulation of cell wall remodeling and antioxidant system. Our results provide a comprehensive insight into the transcriptomic profile of key components involved in the Mg deficiency tolerance in C. sinensis and enrich our understanding of the molecular mechanisms by which plants adapted to a Mg deficient condition.

Boron (B) toxicity in Citrus is a common physiological disorder leading to reductions in both productivity and quality. Studies on how Citrus roots evade B toxicity may provide new insight into plant tolerance to B toxicity. Here, using Illumina sequencing, differentially expressed microRNAs (miRNAs) were identified in B toxicity-treated Citrus sinensis (tolerant) and C. grandis (intolerant) roots. The results showed that 37 miRNAs in C. grandis and 11 miRNAs in C. sinensis were differentially expressed when exposed to B toxicity. Among them, miR319, miR171, and miR396g-5p were confirmed via 5′-RACE and qRT-PCR to target a myeloblastosis (MYB) transcription factor gene, a SCARECROW-like protein gene, and a cation transporting ATPase gene, respectively. Maintenance of SCARECROW expression in B treated Citrus roots might fulfill stem cell maintenance, quiescent center, and endodermis specification, thus allowing regular root elongation under B-toxic stress. Down-regulation of MYB due to up-regulation of miR319 in B toxicity-treated C. grandis roots might decrease the number of root tips, thereby dramatically changing root system architecture. Our findings suggested that miR319 and miR171 play a pivotal role in Citrus adaptation to long-term B toxicity by targeting MYB and SCARECROW, respectively, both of which are responsible for root growth and development.

Limited data are available on the sulfur (S)-mediated-alleviation of aluminum (Al)-toxicity in higher plants. Citrus grandis seedlings were irrigated for 18 weeks with 0.5 mM MgSO4 or 0.5 mM MgSO4 + 0.5 mM Na2SO4, and 0 (−Al) or 1 mM AlCl3·6H2O (+Al, Al-toxicity). Under Al-toxicity, S decreased the level of Al in leaves; increased the relative water content (RWC) of roots and leaves, the contents of phosphorus (P), calcium (Ca) and magnesium (Mg) per plant, the dry weights (DW) of roots and shoots, the ratios of root DW/shoot DW, and the Al-induced secretion of citrate from root; and alleviated the Al-induced inhibition of photosynthesis via mitigating the Al-induced decrease of electron transport capacity resulting from the impaired photosynthetic electron transport chain. In addition to decreasing the Al-stimulated H2O2 production, the S-induced upregulation of both S metabolism-related enzymes and antioxidant enzymes also contributed to the S-mediated-alleviation of oxidative damage in Al-treated roots and leaves. Decreased transport of Al from roots to shoots and relatively little accumulation of Al in leaves, and increased leaf and root RWC and P, Ca, and Mg contents per plant might also play a role in the S-mediated-alleviation of Al-toxicity.

Seedlings of aluminum-tolerant ‘Xuegan’ (Citrus sinensis) and Al-intolerant ‘sour pummelo’ (Citrus grandis) were fertigated for 18 weeks with nutrient solution containing 0 and 1.2 mM AlCl3·6H2O. Al toxicity-induced inhibition of photosynthesis and the decrease of total soluble protein only occurred in C. grandis leaves, demonstrating that C. sinensis had higher Al tolerance than C. grandis. Using isobaric tags for relative and absolute quantification (iTRAQ), we obtained more Al toxicity-responsive proteins from C. sinensis than from C. grandis leaves, which might be responsible for the higher Al tolerance of C. sinensis. The following aspects might contribute to the Al tolerance of C. sinensis: (a) better maintenance of photosynthesis and energy balance via inducing photosynthesis and energy-related proteins; (b) less increased requirement for the detoxification of reactive oxygen species and other toxic compounds, such as aldehydes, and great improvement of the total ability of detoxification; and (c) upregulation of low-phosphorus-responsive proteins. Al toxicity-responsive proteins related to RNA regulation, protein metabolism, cellular transport and signal transduction might also play key roles in the higher Al tolerance of C. sinensis. We present the global picture of Al toxicity-induced alterations of protein profiles in citrus leaves, and identify some new Al toxicity-responsive proteins related to various biological processes. Our results provide some novel clues about plant Al tolerance.