Transcriptome profiling and in silico docking analysis of phosphine resistance in rice weevil, Sitophilus oryzae (Coleoptera: Curculionidae).

The rice weevil, Sitophilus oryzae (Linnaeus, Coleoptera: Curculionidae), is a serious cosmopolitan pest that affects grain in storage and has developed high levels of resistance toward phosphine. In this study, RNA-seq data was used to study the phosphine resistance mechanisms in S. oryzae. Resistant and susceptible populations of S. oryzae were identified based on phosphine bioassays conducted in 32 populations collected across Tamil Nadu, India. Differential expression of mitochondrial (COX1, COX2, COX3, ND2, ND3, ATP6, and ATP8) and detoxification genes (Cyps, Gsts, and Cbe) were observed in the resistant and susceptible populations of S. oryzae. The previously characterized phosphine resistant gene, dld (dihydrolipoamide dehydrogenase) linked to the rph2 locus, was found to be up-regulated in resistant S. oryzae population (ISO-TNAU-RT) treated with phosphine. Also, the genes involved in Tricarboxylic acid (TCA) cycle were significantly down-regulated. In addition, a significant up-regulation in the expression of the antioxidant enzymes superoxide dismutase (2.5×) and catalase (2.1×) in ISO-TNAU-RT populations was recorded. Furthermore, a distinct amino acid substitution, Lysine > Glutamic acid (K141E) was identified in resistant phenotypes. In silico docking studies of both resistant and susceptible DLD protein with phosphine molecule revealed that the amino acid residues involved in the interaction were different. This suggested that the amino acid substitution might lead to structural modifications which reduces the affinity of the target (phosphine). This study provides insight on the various genes, pathways, and functional mechanisms having a significant role in phosphine resistance in S. oryzae.

Glutathione plays an essential role in nitric oxide-mediated iron-deficiency signaling and iron-deficiency tolerance in Arabidopsis.

Iron (Fe) deficiency is a common agricultural problem that affects both the productivity and nutritional quality of plants. Thus, identifying the key factors involved in the tolerance of Fe deficiency is important. In the present study, the zir1 mutant, which is glutathione deficient, was found to be more sensitive to Fe deficiency than the wild type, and grew poorly in alkaline soil. Other glutathione-deficient mutants also showed various degrees of sensitivity to Fe-limited conditions. Interestingly, we found that the glutathione level was increased under Fe deficiency in the wild type. By contrast, blocking glutathione biosynthesis led to increased physiological sensitivity to Fe deficiency. On the other hand, overexpressing glutathione enhanced the tolerance to Fe deficiency. Under Fe-limited conditions, glutathione-deficient mutants, zir1, pad2 and cad2 accumulated lower levels of Fe than the wild type. The key genes involved in Fe uptake, including IRT1, FRO2 and FIT, are expressed at low levels in zir1; however, a split-root experiment suggested that the systemic signals that govern the expression of Fe uptake-related genes are still active in zir1. Furthermore, we found that zir1 had a lower accumulation of nitric oxide (NO) and NO reservoir S-nitrosoglutathione (GSNO). Although NO is a signaling molecule involved in the induction of Fe uptake-related genes during Fe deficiency, the NO-mediated induction of Fe-uptake genes is dependent on glutathione supply in the zir1 mutant. These results provide direct evidence that glutathione plays an essential role in Fe-deficiency tolerance and NO-mediated Fe-deficiency signaling in Arabidopsis.

Control of Zn uptake in Arabidopsis halleri: a balance between Zn and Fe.

Zinc (Zn) is an essential plant micronutrient but is toxic in excess. To cope with excess Zn, plant species possess a strict metal homeostasis mechanism. The Zn hyperaccumulator Arabidopsis halleri has developed various adaptive mechanisms involving uptake, chelation, translocation and sequestration of Zn. In this mini review, we broadly discuss the different Zn tolerance mechanisms and then focus on controlled Zn uptake in A. halleri. Members of the ZRT/IRT-like protein (ZIP) family of metal transporters are mainly regulated by Zn and are involved in Zn uptake. A few members of the ZIP family, such as IRT1 and IRT2, are regulated by iron (Fe) and can transport multi-metals, including Zn, Fe, Mn, Cd, and Co. This mini-review also discusses the differential expression of multiple metal ZIP transporters in A. halleri and A. thaliana, a non-hyperaccumulator, with Zn exposure as well as Fe deficiency and their role in controlled Zn uptake and tolerance.

Iron is involved in the maintenance of circadian period length in Arabidopsis.

The homeostasis of iron (Fe) in plants is strictly regulated to maintain an optimal level for plant growth and development but not cause oxidative stress. About 30% of arable land is considered Fe deficient because of calcareous soil that renders Fe unavailable to plants. Under Fe-deficient conditions, Arabidopsis (Arabidopsis thaliana) shows retarded growth, disordered chloroplast development, and delayed flowering time. In this study, we explored the possible connection between Fe availability and the circadian clock in growth and development. Circadian period length in Arabidopsis was longer under Fe-deficient conditions, but the lengthened period was not regulated by the canonical Fe-deficiency signaling pathway involving nitric oxide. However, plants with impaired chloroplast function showed long circadian periods. Fe deficiency and impaired chloroplast function combined did not show additive effects on the circadian period, which suggests that plastid-to-nucleus retrograde signaling is involved in the lengthening of circadian period under Fe deficiency. Expression pattern analyses of the central oscillator genes in mutants defective in CIRCADIAN CLOCK ASSOCIATED1/LATE ELONGATED HYPOCOTYL or GIGANTEA demonstrated their requirement for Fe deficiency-induced long circadian period. In conclusion, Fe is involved in maintaining the period length of circadian rhythm, possibly by acting on specific central oscillators through a retrograde signaling pathway.

ZINC TOLERANCE INDUCED BY IRON 1 reveals the importance of glutathione in the cross-homeostasis between zinc and iron in Arabidopsis thaliana.

Zinc is an essential micronutrient for plants, but it is toxic in excess concentrations. In Arabidopsis, additional iron (Fe) can increase Zn tolerance. We isolated a mutant, zinc tolerance induced by iron 1, designated zir1, with a defect in Fe-mediated Zn tolerance. Using map-based cloning and genetic complementation, we identified that zir1 has a mutation of glutamate to lysine at position 385 on γ-glutamylcysteine synthetase (GSH1), the enzyme involved in glutathione biosynthesis. The zir1 mutant contains only 15% of the wild-type glutathione level. Blocking glutathione biosynthesis in wild-type plants by a specific inhibitor of GSH1, buthionine sulfoximine, resulted in loss of Fe-mediated Zn tolerance, which provides further evidence that glutathione plays an essential role in Fe-mediated Zn tolerance. Two glutathione-deficient mutant alleles of GSH1, pad2-1 and cad2-1, which contain 22% and 39%, respectively, of the wild-type glutathione level, revealed that a minimal glutathione level between 22 and 39% of the wild-type level is required for Fe-mediated Zn tolerance. Under excess Zn and Fe, the recovery of shoot Fe contents in pad2-1 and cad2-1 was lower than that of the wild type. However, the phytochelatin-deficient mutant cad1-3 showed normal Fe-mediated Zn tolerance. These results indicate a specific role of glutathione in Fe-mediated Zn tolerance. The induced accumulation of glutathione in response to excess Zn and Fe suggests that glutathione plays a specific role in Fe-mediated Zn tolerance in Arabidopsis. We conclude that glutathione is required for the cross-homeostasis between Zn and Fe in Arabidopsis.

Differential expression and regulation of iron-regulated metal transporters in Arabidopsis halleri and Arabidopsis thaliana--the role in zinc tolerance.

To avoid zinc (Zn) toxicity, plants have developed a Zn homeostasis mechanism to cope with Zn excess in the surrounding soil. In this report, we uncovered the difference of a cross-homeostasis system between iron (Fe) and Zn in dealing with Zn excess in the Zn hyperaccumulator Arabidopsis halleri ssp. gemmifera and nonhyperaccumulator Arabidopsis thaliana. Arabidopsis halleri shows low expression of the Fe acquisition and deficiency response-related genes IRT1 and IRT2 compared with A. thaliana. In A. thaliana, lowering the expression of IRT1 and IRT2 through the addition of excess Fe to the medium increases Zn tolerance. Excess Zn induces significant Fe deficiency in A. thaliana and reduces Fe accumulation in shoots. By contrast, the accumulation of Fe in shoots of A. halleri was stable under various Zn treatments. Root ferric chelate reductase (FRO) activity and expression of FIT are low in A. halleri compared with A. thaliana. Overexpressing a ZIP family member IRT3 in irt1-1, rescues the Fe-deficient phenotype. A fine-tuned Fe homeostasis mechanism in A. halleri maintains optimum Fe level by Zn-regulated ZIP transporters and prevents high Zn uptake through Fe-regulated metal transporters, and in part be responsible for Zn tolerance.