Vol 70, No 1 (2023)

Photosystem II (PSII) of the photosynthetic apparatus in oxygenic organisms contains a catalytic center that performs one of the most important reactions in bioenergetics: light-dependent water oxidation to molecular oxygen. The catalytic center is a Mn4CaO5 cluster consisting of four cations of manganese and one calcium cation linked by oxygen bridges. The authors reported earlier that a structural transition occurs at pH 5.7 in the cluster resulting in changes in manganese cation(s) redox potential and elevation of the Mn‑clus-ter resistance to reducing agents. The discovered effect was examined in a series of investigations that are reviewed in this work. It was found that, at pH 5.7, Fe(II) cations replace not two manganese cations as it happens at pH 6.5 but only one cation; as a result, a chimeric Mn3Fe1 cluster is produced. In the presence of exogenous calcium ions, membrane preparations of PSII with such a chimeric cluster are capable of evolving oxygen in the light (at a rate of approximately 25% of the rate in native PSII). It was found that photoinhibition that greatly depends on the processes of oxidation or reduction at pH 5.7 slows down as compared with pH 6.5. PSII preparations were also more resistant to thermal inactivation at pH 5.7 than at pH 6.5. However, in PSII preparations lacking manganese cations in the oxygen-evolving complex, the rates of photoinhibition at pH 6.5 and 5.7 did not differ. In thylakoid membranes, protonophores that abolish the proton gradient and increase pH in the lumen (where the manganese cluster is located) from 5.7 to 7.0 considerably elevated the rate of PSII photoinhibition. It is assumed that the structural transition in the Mn-cluster at pH 5.7 is involved in the mechanisms of PSII defense against photoinhibition.

The approach we developed earlier to describe the chains of conjugate enzymatic reactions of the photosynthesis process for the first time made it possible to propose a strict mathematical model of non-rectangular hyperbola, which describes the dependence of the speed of photosynthesis on the intensity of light (I), the concentration of CO₂ and the interaction of these two factors. The fundamental parameters of this model are light (I_K) and carbon dioxide (C_K) substrate constants, as well as hyperbola curvature (γ), conjugation parameters (r_IP and r_CP) and maximum photosynthesis rate (P^m). Special straightening coordinates are proposed, in which the entire hyperbole family has the form of a straight line. The main parameters can only be found when constructing experimental data in straightening coordinates. The proposed non-rectangular hyperbola model is applicable both for describing the process of photosynthesis in seaweed and higher plants, and for absorbing vitamin B12 by algae, and the dependence of the rate of food consumption by zooplankton and fish.

In fruits of cultivars of pepper species Capsicum annuum (Sirenevyi Cub), C. frutescens (Samotsvet) and C. chinense (Pimenta da Neyde), which differ in the pigmentation profile during maturation, the content of anthocyanins and the expression pattern of the structural and regulatory genes of anthocyanin biosynthesis were determined. Anthocyanins were found in the fruit of the Sirenevyi Cub cultivar as well as in the peel and pulp of Samotsvet and Pimenta da Neyde cultivars. In the peel of the fruits of all three analyzed cultivars, the levels of expression of structural genes for the biosynthesis of anthocyanins are two or more times higher than in the pulp. In cv. Samotsvet and Pimenta da Neyde, the expression in the pulp of CHS, F3´5´H, DFR, ANS, and UFGT are higher than that of cv. Sirenevyi Cub. The cv. Pimenta da Neyde is characterized by a high expression of structural genes in the pulp and peel of ripe fruit. Expression of transcription factor genes anthocyanin2 (R2R3-MYB) and MYC (bHLH) was detected in the peel of fruits of cv. Sirenevyi Cub as well as in the peel and pulp of fruits of cv. Samotsvet and Pimenta da Neyde at all the analyzed stages of ripening. For all structural genes of the anthocyanin biosynthesis pathway, a high correlation was found (r = 0.54‒0.93) between the expression levels of all anthocyanin biosynthesis structural genes and anthocyanin content in fruits. High correlation (r = 0.88) is also shown for MYC. For the anthocyanin2, correlation (r = 0.85) is present only for cv. Sirenevyi Cub and Pimenta da Neyde but not for cv. Samotsvet.

Exposure of plants to biotic and abiotic stress agents causes changes in the composition and content of metabolites of different chemical nature, including lipophilic compounds. One of the ways to simulate a stress situation is plant treatment with exogenous phytohormones. This work deals with investigation of organ specificity of composition of lipophilic compounds and changes in their content in wheat Triticum aestivum L. seedlings treated with exogenous stress hormones: abscisic acid (ABA), salicylic acid (SA), and methyl jasmonate (MeJA). It was found that roots and leaves of wheat seedlings have identical composition of lipophilic compounds but their content considerably differed. In the leaves, the quantity of hydrocarbons, including squalene, as well as triterpenes, sterols, and phosphatidyl choline, was much greater than in the roots. In the leaves, glycoceramides of type 1 containing a FA residue with α-hydroxyl group predominated; on the contrary, glycoceramides of type 2 whose FA residues lack α-hydroxyl group prevailed in the roots. Moreover, lipid extracts from the leaves contain lipophilic pigments (chlorophylls a and b and carotenoids) and hydrophobic phenolic compounds in the form of hydroxycinnamic acids. Treatment with stress phytohormones brings about considerable changes in growth characteristics, the rate of photosynthesis, and the profile of lipophilic compounds in wheat seedlings depending on the plant organ and the chemical nature of the phytohormone. In the case of ABA and MeJA, the growth of roots and leaves was suppressed, the level of nonphotochemical quenching rose, and the content of photosynthetic pigments changed. An unexpected effect was observed upon treatment with MeJA that raised the level of cholesterol and phosphatidyl serine. SA was notable for organ-specific changes in the content of products of mevalonate pathway, triterpenes, and sterols. Thus, the simulation of stress conditions by means of treatment of wheat seedlings with exogenous phytohormones strongly affected the composition of lipophilic compounds. Specific changes in lipid composition induced by hormones may contribute to adaptive structural transformations of cellular membranes, whereas changes in the content of hydrophobic phenolic metabolites and photosynthetic pigments may reinforce antioxidant defense of plants under stress conditions.

The existing data on the role of PTF1/TCP13 belonging to the TCP family of transcription factors in regulating expression of a psbD plastid gene encoding a D2 protein of PSII are controversial. To analyze biological functions of PTF1/TCP13, transformed plants expressing PTF1/TCP13 under a β-estradiolinducible promoter were used. PTF1/TCP13 overexpression did not provide the expected increase in the accumulation of psbD transcripts transcribed from BLRP (Blue Light Responsive Promoter), though their level significantly increased under exposure to light or abscisic acid (ABA). PTF1/TCP13 was up-regulated by ABA; moreover, genes of the canonic pathway of the ABA signal transduction were involved in the regulation of PTF1/TCP13 expression. In addition, PTF1/TCP13 was induced in response to salt stress However, in the overexpressing line, salt tolerance and expression of salt stress markers, as well as a number of genes for the synthesis and signaling of ABA, were reduced compared to plants with the normal level of expression of this transcription factor, that is, PTF1/TCP13 acted as a negative regulator of salt stress Thus, PTF1 does not belong to plastid transcription factors. Nevertheless, it represents one of the components of the ABA-dependent regulatory chain capable of modifying expression of nuclear and chloroplast genes in response to changes in homeostasis.

A comprehensive assessment was carried out of the changes occurring during the darkening of the callus obtained from vegetative buds of 40-year-old Scots pine trees Pinus sylvestris L. Based on biometric assessment of callus (intensity of callus formation, proportion of light callus, callus mass) from 32 analyzed trees (16 genotypes represented by two clones), two genotypes with high callus-forming ability were singled out. Analysis of mitosis showed that, although the proportion of aberrant cells in the callus does not exceed the rate of spontaneous mutation for P. sylvestris, the range of violations at the stage of meta-, ana-, and telophase in the callus culture was wider compared to that in the seed progeny of the same pine trees. Darkening of the callus was accompanied by a decrease in sucrose metabolism in the cell (decrease in cytoplasmic, vacuolar invertase and sucrose synthase) and a significant decrease in peroxidase activity. At the same time, the activity of apoplast invertase was maintained at a constant level. The activity of superoxide dismutase, catalase, polyphenol oxidase, and phenylalanine ammonia lyase, on the contrary, was higher in dark callus. The possible use of the studied enzymes as biochemical markers of the transition to darkening callus pine crops is discussed.