Vol 64, No 6 (2017)

Diversity of proteins and enzymes engaged in carbohydrate metabolism is vast. This is related to the fact that plants contain the greater part of biospheric carbohydrates, whose structures are extremely diverse as well. In plant genomes, proteins involved in carbohydrate metabolism are grouped into numerous families, and each of them may include tens of sequences. This is especially typical of enzymes modifying polysaccharides of the cell wall. Expression of genes encoding such proteins is finely tuned. It may differ in different tissues and organs and depends on stages of development of the entire plant and its particular cells. However, certain genes, including highly expressive ones, encode enzymes with “limping” catalytic centers, which may be unable to conduct reactions characteristic of the particular enzymatic family. The review surveys examples of such proteins and discusses causes of their origin and possible functions.

Optimization of nitrogen (N) use by grasses is a central issue of the current work. The effects of different N concentrations (0, 0.25, 0.5, 1.0, 2.5, and 5.0 mM NH₄NO₃) on growth of Brachypodium distachyon were assessed on controlled hydroponic culture. Maximal growth (132% of control) was obtained at 0.5 mM NH₄NO₃, critical N level, and was maintained at higher N concentrations. The highest N level (5.0 mM) has a similar effect on growth as 0.5 mM NH₄NO₃. It has no significant effects on water status, and total and reduced N contents in shoots while, increased those in roots, compared to plants receiving 0.5 mM NH₄NO₃. The high N availability, however, increased nitrate contents in shoots and roots by 3- and 20-folds, respectively, compared to those of plant receiving 0.5 mM NH₄NO₃. In addition, high N availability reduced the nitrogen use efficiency (NUE) by 18% compared to that of plant receiving only 0.5 NH₄NO₃. In view of B. distachyon productivity and environmental concerns, it is concluded that the critical level of N application should be 0.5 mM NH₄NO₃ and the excess fertilization led to a high nitrate accumulation.

Aeluropus littoralis (Gouan) Parlatore is a rhizomatous perennial monocotyledonous halophyte that withstands environmental stresses. The role of the glyoxalase system, which plays an important role in carbohydrate metabolic process and compatible solutes production, in salt tolerance of A. littoralis was proved to be extremely momentous. Thus in the present study, a GLY I gene was isolated and sequenced from this plant (revealing the partial sequence of AlGLY I), and its expression profiling has been performed in response to salinity and recovery conditions, by fluorescent real-time PCR (qPCR). Experimental samples were prepared separately from shoot and root tissues after 600 mM NaCl treatment, as well as after stress removing. Maximum mRNA expression of GLY I, which was observed after 6 h salt stress in shoot tissue, was 5.9-fold higher compared to the control. Characterization of the partial sequence of AlGLY I gene, containing 896 bp, using publicly available databases demonstrated that the deduced transcripts, encoding 297 amino acids with a 32.5869 kD molecular mass including 5.19 isoelectric points, shared a high homology (~90%) to Oryza sativa GLY I protein. Setaria italica, Sorghum bicolor, Brachypodium distachyon, Triticum aestivum, and Hordeum vulgare with 86, 85, 84, 83 and 78%, respectively, also revealed high homology. The promoter analysis also showed the presence of various stress related CREs, which probably activate the AlGLY I gene transcription under abiotic stress conditions. These results suggested that AlGLY I may be a potentially useful candidate gene for engineering salinity tolerance in cultivated plants.

Despite the demonstration that proline accumulation and gene expression of Δ¹-pyrroline-5-carboxylate synthase (p5cS) increased under osmotic stress, the impact of excess boron on proline metabolism is not well known. Therefore, we investigated the effect of different boron concentrations (10, 50, 70, 140 and 200 ppm) on seedlings root growth, lipid peroxidation rate, antioxidant enzyme activity (glutathione reductase (GR), ascorbate peroxidase (APX), catalase (CAT)), proline accumulation and transcription level of p5cS gene in Triticum aestivum L. AK-702. It was observed that seed germination and root growth in T. aestivum decreased depending on the concentration of boron. Our results indicated that boron toxicity induced lipid peroxidation and decreased GR activity under a high concentration of boron. However, the APX activity did not significantly change under high concentrations of boron (70, 140 and 200 ppm), while it increased under the lower levels of boron (10 and 50 ppm). In addition, excess boron enhanced CAT activity in the 200 ppm boron treated groups. Proline accumulation increased 2.25 and 1.45 fold in the 140 and 200 ppm boron applications. In addition, analyses of the mRNA transcription level using the semi-quantitative RTPCR results showed that excess boron increased the p5cS mRNA transcript levels and showed a positive correlation of these levels with proline accumulation in T. aestivum roots.

Parsley (Petroselinum hortense L.) plants cultivated under controlled conditions were exposed to different doses of cadmium to investigate the antioxidant capacity and cadmium accumulation in the samples. Two-months-old parsley seedlings were treated with four different concentrations of CdCl₂ (0, 75, 150, and 300 μM) for fifteen days. Cadmium level in leaves increased significantly by increasing the Cd contamination in the soil. Total chlorophyll and carotenoid content declined considerably with Cd concentration. Cd treatment caused a significant increase lipid peroxidation in tissue of leaf. Superoxide dismutase activity (SOD, EC 1.15.1.1) increased partially at 75 and 150 μM CdCl₂ concentrations whereas the activity decreased at 300 μM CdCl₂. Catalase (CAT, EC 1.11.1.6) and ascorbate peroxidase (APX, EC 1.11.1.11) activities were reduced by Cd application. Total phenolic compound amount increased significantly with increasing Cd concentration, as ferric reduction power, superoxide anion radical, and DPPH˙ free radical scavenging activities elevated slightly by the concentration. These results suggest that antioxidant enzymes activities were repressed depending on accumulation of cadmium in leaves of parsley while the non-enzymatic antioxidant activities slightly increased.

Genetic engineering for heat stress tolerance can promote crop growth and improve yield. One wheat (Triticum aestivum L.) line Y16 (wild type) and two transgenic plants (Y16-3 and Y16-46) that express Hpa1_10-42, a functional fragment of harpin protein, were used in this study to investigate their possible abiotic stress tolerance under heat stress. Results showed that enhanced thermotolerance was observed in the Y16-3 and Y16-46 lines over the control wheat under stress conditions. However, this increased stress tolerance was significantly abolished by specific inhibitors such as fluridone or sodium tungstate (i.e., arrests abscisic acid (ABA) biosynthesis) and EGTA or La³⁺ (i.e., arrests Ca²⁺ signaling pathway) under heat exposure. By contrast, high activities of antioxidant enzymes such as superoxide dismutase, catalase, and ascorbate peroxidase (but not peroxidase) and low levels of oxidative damage (thiobarbituric acid reactive substance (TBARS) and chlorophyll) were detected in transgenic wheat lines compared with the control plant under stress exposure. However, this significant difference diminished after the addition of these specific inhibitors. Furthermore, a slight increase of H₂O₂ was observed in the transgenic plant, instead of the control, without the addition of chemicals under heat stress. These results suggested that antioxidant enzymes, calcium, and ABA signaling pathways were involved in this Hpa1_10–42-mediated thermotolerance of transgenic wheat plants under stress exposure. Finally, a hypothetical model based on H₂O₂ signaling was proposed to illustrate the possible mechanism of this enhanced heat stress tolerance.

Effects were investigated of kinetin replacement on 24-epibrassinolide (24-EB) in the in vitro cultured embryonic explants that were obtained from two varieties of spring soft wheat (Triticum aestivum L.)— Bashkirskaya 26 (drought resistant) and Salavat Yulaev (weakly resistant), differing in drought resistance, on the calli formation, its growth indices, contents of ABA and cytokinins, morphological and histological parameters, as well as their regenerative capacity. The resistant Bashkirskaya 26 variety, in contrast to the Salavat Yulaev variety, was characterized by a significantly higher frequency of calli formation in the culture of immature embryos on the 24-EB induction medium, higher increase in fresh and dry weights, and a large number of morphogenetic centers. On the medium containing kinetin, the Salavat Yulaev calli were characterized by an increased level of ABA throughout the experiment with a maximum of 15–25 days, whereas, in the Bashkirskaya 26 calli, the maximum ABA accumulation occurred on the seventh to 11th day of cultivation, after which a decrease in the hormone content was observed. It was found that calli of both varieties cultivated on the 24-EB medium against the background of absence in the ABA content changes were characterized by an increased content of endogenous cytokinins, especially significant in Bashkirskaya 26 calli. Calli of both varieties were characterized by a high regenerative capacity in all the studied variants of the regeneration medium. At the same time, the maximum capacity for regeneration and formation of regenerants with a single callus were revealed by replacing kinetin with 24-EB, especially pronounced in a resistant variety. The combination of the results obtained demonstrates the efficacy of 24-EB introducing instead of kinetin into the in vitro culture medium for explants of two varieties of spring soft wheat that differ in drought resistance, as evidenced by an increase in the frequency of callus formation from immature embryos, as well as the number of morphogenetic centers in the resulting calli.

RNA interference (RNAi) is one of the key defense mechanisms directed against virus infections in plants and other organisms. In this case in plants infected with viruses, short interfering RNAs (siRNAs) are formed from two-chain replicated forms of virus molecules of RNA. These siRNAs program one of the RNAi basic components, RNA-induced complex of genes silencing (RISC, RNA induced silencing complex) associated with sequence-specific removing virus RNA. Virus protein P19 is a suppressor of RNAi and is capable of trapping the siRNAs being formed before their binding with RISC. Here, it was shown that preliminary entering leaves of plants Nicotiana benthamiana Domin (before virus infecting) of siRNAs eluted from the complex P19/siRNA from the infected plant lowers development of infection symptoms induced by tomato bushy stunt virus (TBSV) in inoculated plants. Exogenous addition of suppressor-associated siRNAs to plants leads to not only lowering virus accumulation but also to survival of infected plants. Thus, it has been established that preliminary addition of virus siRNAs elevates plant tolerance to the virus infection by means of early programming RISC and activation of the defense action of RNAi.