Bojana Živanović

Xanthomonas campestris pv. campestris (Xcc), the causal agent of black rot, has a significant impact on cabbage production worldwide. The goal of this research was to evaluate the effect of preventive foliar treatments with Bacillus velezensis strain RD-FC 88 on the primary and secondary metabolism of Xcc-infected cabbage cv. Futoški plants. Special attention was given to measuring metabolites’ changes, aiming to determine the influence of the applied biocontrol treatment on the development of plant immune response and resistance to pathogen. This study reports the first comprehensive biochemical and physiological analysis of the interaction between host plant, biocontrol strain and pathogen, thus providing novel insight into black rot management. Pathogen inoculation caused a significant decrease in the majority of measured metabolites, including most free amino acids (Gln, Ala, BCAA), phenolics, and glucosinolates. Preventive application of B. velezensis strain in Xcc-infected plants restored the levels of aromatic amino acids, Asp, Glu, Leu, Val, and Ala to control values. A similar pattern was observed in aliphatic glucosinolates sinigrin and glucoiberin, as well as for the indolic glucosinolate 4-methoxy-glucobrassicin. Additionally, increased accumulation of hydroxybenzoic acids, hydroxycinnamic acids, and kaempferol derivatives was also observed in the plants treated with the biocontrol strain and subsequently infected with Xcc, compared to plants solely infected with Xcc. The obtained results imply that the RD-FC 88 strain holds potential as an efficient priming agent, capable of stimulating cabbage cv. Futoški defense responses and enhancing its resistance to Xcc.

Three basil plant varieties (Ocimum basilicum var. Genovese, Ocimum × citriodorum, and Ocimum basilicum var. purpurascens) were grown under moderate light (about 300 µmol photons m⁻² s⁻¹) in a glasshouse or growth chamber and then either transferred to an open field (average daily dose: 29.2 kJ m⁻² d⁻¹) or additionally exposed to UV-B irradiation in a growth chamber (29.16 kJ m⁻² d⁻¹), to reveal the variety-specific and light-specific acclimation responses. Total antioxidant capacity (TAC), phenolic profile, ascorbate content, and class III peroxidase (POD) activity were used to determine the antioxidant status of leaves under all four light regimes. Exposure to high solar irradiation at the open field resulted in an increase in TAC, total hydroxycinnamic acids (HCAs, especially caffeic acid), flavonoids, and epidermal UV-absorbing substances in all three varieties, as well as a two-fold increase in the leaf dry/fresh weight ratio. The supplemental UV-B irradiation induced preferential accumulation of HCAs (rosmarinic acid) over flavonoids, increased TAC and POD activity, but decreased the ascorbate content in the leaves, and inhibited the accumulation of epidermal flavonoids in all basil varieties. Furthermore, characteristic leaf curling and UV-B-induced inhibition of plant growth were observed in all basil varieties, while a pro-oxidant effect of UV-B was indicated with H₂O₂ accumulation in the leaves and spotty leaf browning. The extent of these morphological changes, and oxidative damage depended on the basil cultivar, implies a genotype-specific tolerance mechanism to high doses of UV-B irradiation.

Plants are inevitably exposed to extreme climatic conditions that lead to a disturbed balance between the amount of absorbed energy and their ability to process it. Variegated leaves with photosynthetically active green leaf tissue (GL) and photosynthetically inactive white leaf tissue (WL) are an excellent model system to study source–sink interactions within the same leaf under the same microenvironmental conditions. We demonstrated that under excess excitation energy (EEE) conditions (high irradiance and lower temperature), regulated metabolic reprogramming in both leaf tissues allowed an increased consumption of reducing equivalents, as evidenced by preserved maximum efficiency of photosystem II (Ф_PSII) at the end of the experiment. GL of the EEE-treated plants employed two strategies: (i) the accumulation of flavonoid glycosides, especially cyanidin glycosides, as an alternative electron sink, and (ii) cell wall stiffening by cellulose, pectin, and lignin accumulation. On the other hand, WL increased the amount of free amino acids, mainly arginine, asparagine, branched-chain and aromatic amino acids, as well as kaempferol and quercetin glycosides. Thus, WL acts as an important energy escape valve that is required in order to maintain the successful performance of the GL sectors under EEE conditions. Finally, this role could be an adaptive value of variegation, as no consistent conclusions about its ecological benefits have been proposed so far.

Two tomato genotypes with constitutively different ABA level, flacca mutant and wild type of Ailsa Craig cv. (WT), were subjected to three repeated drought cycles, with the aim to reveal the role of the abscisic acid (ABA) threshold in developing drought tolerance. Differential responses to drought of two genotypes were obtained: more pronounced stomatal closure, ABA biosynthesis and proline accumulation in WT compared to the mutant were compensated by dry weight accumulation accompanied by transient redox disbalance in flacca. Fourier-transform infrared (FTIR) spectra analysis of isolated cell wall material and morphological parameter measurements on tomato leaves indicated changes in dry weight accumulation and carbon re-allocation to cell wall constituents in flacca, but not in WT. A higher proportion of cellulose, pectin and lignin in isolated cell walls from flacca leaves further increased with repeated drought cycles. Different ABA-dependent stomatal closure between drought cycles implies that acquisition of stomatal sensitivity may be a part of stress memory mechanism developed under given conditions. The regulatory role of ABA in the cell wall restructuring and growth regulation under low leaf potential was discussed with emphasis on the beneficial effects of drought priming in developing differential defense strategies against drought.

Water deficit has a global impact on plant growth and crop yield. Climate changes are going to increase the intensity, duration and frequency of severe droughts, particularly in southern and south-eastern Europe, elevating the water scarcity issues. We aimed to assess the contribution of endogenous abscisic acid (ABA) in the protective mechanisms against water deficit, including stomatal conductance, relative water potential and the accumulation of osmoprotectants, as well as on growth parameters. To achieve that, we used a suitable model system, ABA-deficient tomato mutant, flacca and its parental line. Flacca mutant exhibited constitutively higher levels of soluble sugars (e.g., galactose, arabinose, sorbitol) and free amino acids (AAs) compared with the wild type (WT). Water deficit provoked the strong accumulation of proline in both genotypes, and total soluble sugars only in flacca. Upon re-watering, these osmolytes returned to the initial levels in both genotypes. Our results indicate that flacca compensated higher stomatal conductance with a higher constitutive level of free sugars and AAs. Additionally, we suggest that the accumulation of AAs, particularly proline and its precursors and specific branched-chain AAs in both, glucose and sucrose in flacca, and sorbitol in WT, could contribute to maintaining growth rate during water deficit and recovery in both tomato genotypes.