Éva Darkó

Photosynthesis under artificial light: the shift in primary and secondary metabolism

Providing an adequate quantity and quality of food for the escalating human population under changing climatic conditions is currently a great challenge. In outdoor cultures, sunlight provides energy (through photosynthesis) for photosynthetic organisms. They also use light quality to sense and respond to their environment. To increase the production capacity, controlled growing systems using artificial lighting have been taken into consideration. Recent development of light-emitting diode (LED) technologies presents an enormous potential for improving plant growth and making systems more sustainable. This review uses selected examples to show how LED can mimic natural light to ensure the growth and development of photosynthetic organisms, and how changes in intensity and wavelength can manipulate the plant metabolism with the aim to produce functionalized foods.

Synthesis and role of salicylic acid in wheat varieties with different levels of cadmium tolerance

Wheat genotypes with different endogenous SA contents were investigated, in order to reveal how cadmium influences salicylic acid (SA) synthesis, and to find possible relationships between SA and certain protective compounds (members of the antioxidants and the heavy metal detoxification system) and between the SA content and the level of cadmium tolerance. Cadmium exposure induced SA synthesis, especially in the leaves, and it is suggested that the phenyl-propanoid synthesis pathway is responsible for the accumulation of SA observed after cadmium stress. Cadmium influenced the synthesis and activation of protective compounds to varying extents in wheat genotypes with different levels of tolerance; the roots and leaves also responded differently to cadmium stress. Although a direct relationship was not found between the initial SA levels and the degree of cadmium tolerance, the results suggest that the increase in the root SA level during cadmium stress in the Mv varieties could be related with the enhancement of the internal glutathione cycle, thus inducing the antioxidant and metal detoxification systems, which promote Cd stress tolerance in wheat seedlings. The positive correlation between certain SA-related compounds and protective compounds suggests that SA-related signalling may also play a role in the acclimation to heavy metal stress.

Are either SOD and catalase or the polyamines involved in the paraquat resistance of Conyza canadensis

Paraquat/atrazine coresistant (PqAR) and paraquat resistant (PqR) horseweed (Conyza canadensis L. Cronq.) plants showed - in the first hour after 0.5 mM paraquat spraying - a decreased catalase activity followed by a slight increase. However, the enzyme activity remained always below the initial value Sensitive plants showed a significant increase of catalase activity in the first 4 hour after spraying The transient character of paraquat inhibition, the recovery of photosynthetic activity of the PqAR Conyza plants (characterized by variable fluorescence) after spraying remained unaffected by the superoxide dismutase (SOD) inhibitor, diethyldithiocarbamate This indicates that SOD is not involved in the resistance mechanism. Untreated resistant biotypes showed about 2.5 times higher total polyamine and putrescine level than the sensitive one. 100 μM of exogenously added putrescine was observed as having a protecting effect against paraquat in floated leaves of the sensitive biotype only. The resistant leaves were unaffected probably on account of their higher endogenous polyamine level. It is concluded that polyamines may play a role in the paraquat resistance of Coniza canadensis.

In vitro microspore selection in maize anther culture with oxidative-stress stimulators

Summary.In order to produce doubled-haploid maize plants tolerant of oxidative stress, in vitro microspore selection was carried out in anther culture with reactive oxygen species (ROS) progenitors such as paraquat, menadione, tert-butylhydroperoxide (t-BHP), and methionine combined with riboflavin. All the ROS progenitors reduced the anther induction, the formation of microspore-derived structures, and their regeneration potential. Abnormal cell divisions and progeny cell degradation could be observed during the development of microspores treated with ROS progenitors. Menadione and t-BHP influenced the microspore developmental pathway, as menadione induced the formation of embryoids, while t-BHP increased the proportion of calli in the microspore-derived structures. As the result of in vitro selection, 15, 10, 10, and 3 fertile doubled-haploid plants were obtained in cultures treated with paraquat, t-BHP, methionine combined with riboflavin, and menadione, respectively.

Defensive strategies against high light stress in wild and D1 protein mutant biotypes of Erigeron canadensis

The Ser264AEGly substitution on the D1 protein is accompanied by a higher photosensitivity of the mutant plant. This may be due to an increased D1 protein turnover and/or to a lower xanthophyll cycle activity in vivo. The relative importance of these two photoprotective mechanisms in wild and D1 protein mutant biotypes of Erigeron canadensis L. was established by using dithiothreitol and streptomycin. Moreover, the interconversion of violaxan-thin to zeaxanthin via antheraxanthin was studied in isolated thylakoids and in intact leaves treated with paraquat. Streptomycin caused a more severe decrease in the optimal quantum yield (Fv/Fm) of PS II and a large increase in the initial fluorescence yield (Fo) in the mutant compared to the wild biotype. In the fluorescence-quenching parameters of the wild-type leaves, dithiothreitol caused alterations similar to those observed in the mutant plant without dithiothreitol. A lowered activity of the xanthophyll cycle was detected in the mutant biotype compared to the wild-type in vivo. However, under in vitro, conditions which were optimal for violaxanthin de-epoxidation, or when paraquat was used on intact leaves to accelerate the electron transport, violaxanthin could readily be converted to zeaxanthin even in the mutant plants. This demonstrates that neither the decrease in the enzymatic activity of violaxanthin de-epoxidase nor the low availability of violaxanthin is responsible for the low zeaxanthin formation under in vivo conditions. It is presumed that, in vivo, the D1 protein mutation results in slower electron transport, a smaller DpH and lower zeaxanthin formation, and thereby in alterations in the defensive strategies against high light illumination.

Xanthophyll Cycle Patterns and in vivo Photoinhibition in Herbicide-Resistant Biotypes of Conyza canadensis

Summary The xanthophyll cycle and in vivo photoinhibition were investigated in the herbicide-susceptible (S), paraquat-resistant (PQ-R), atrazine-resistant (Atr-R) and paraquat-atrazine co-resistant (PQAtr-R) biotypes of Conyza canadensis using both low-(LLG) and high-light-grown (HLG) plants. HPLC revealed markedly reduced levels of zeaxanthin under photoinhibitory conditions in atrazine-resistant (Atr-R and PQAtr-R) biotypes as compared with the S and PQ-R biotypes. The variable chlorophyll fluorescence (F v ) decrease and constant fluorescence (F o ) increase revealed an enhanced susceptibility to in vivo photoinhibition in Atr-R and PQAtr-R biotypes. LLG plants were more susceptible than HLG plants to in vivo photoinhibitory treatment. It is suggested that the increased susceptibility to photoinhibition in Atr-R and PQAtr-R biotypes of C. canadensis is a consequence not only of D1 protein mutation, but also possibly of a lower rate of violaxanthin to zeaxanthin de-epoxidation.

Salt stress response of wheat–barley addition lines carrying chromosomes from the winter barley “Manas”

The salt stress responses of wheat–barley addition lines (2H, 3H, 3HS, 4H, 6H, 7H and 7HL) were compared to those of the parental genotypes wheat cv. Asakaze and barley cv. Manas and two other wheat genotypes [Chinese Spring (CS) and Mv9kr1] during germination and in young plants grown in hydroponic culture with or without salt treatment. Among the wheat genotypes frequently used for interspecific hybridization, Asakaze possesses relatively high salt tolerance, as indicated by the less pronounced reduction in germination % and in root and shoot growth and the retention of high leaf water content and photosynthetic activity, as compared to CS and Mv9kr1. The barley cv. Manas showed better salt tolerance than wheat cv. Asakaze, although Manas accumulated more Na in the root, but its transport to the shoots is restricted. Among the addition lines tested, the disomic addition line 7H and ditelosomic line 7HL exhibited higher salt tolerance both during germination and in the early developmental stages than the wheat parent, which may be related to the elevated osmotic adjustment capacity of these addition lines, similar to that found for barley cv. Manas. The paper also discusses the effects of other chromosomes on the salt stress response.

Salicylic Acid and Sodium Salicylate Alleviate Cadmium Toxicity to Different Extents in Maize (Zea mays L.).

The role of salicylic acid in Cd tolerance has attracted more attention recently but no information is available on the efficiency of different forms of salicylic acid. The aim was thus to investigate whether both the acid and salt forms of salicylic acid provide protection against Cd stress and to compare their mode of action. Young maize plants were grown under controlled environmental conditions. One group of 10-day-old seedlings were treated with 0.5 mM SA or NaSA for 1 day then half of the pants were treated with 0.5 mM Cd for 1 day. Another group of seedlings was treated with 0.5 mM CdSO4 for 1 day without pre-treatment with SA or NaSA, while a third group was treated simultaneously with Cd and either SA or NaSA. Both salicylic acid forms reduced the Cd accumulation in the roots. Treatment with the acidic form meliorated the Cd accumulation in the leaves, while Na-salicylate increased the phytochelatin level in the roots and the amount of salicylic acid in the leaves. Furthermore, increased antioxidant enzyme activity was mainly induced by the acid form, while glutathione-related redox changes were influenced mostly by the salt form. The acidic and salt forms of salicylic acid affected the two antioxidant systems in different ways, and the influence of these two forms on the distribution and detoxification of Cd also differed. The present results also draw attention to the fact that generalisations about the stress protective mechanisms induced by salicylic acid are misleading since different forms of SA may exert different effects on the plants via separate mechanisms.

Photosynthetic responses of a wheat (Asakaze)–barley (Manas) 7H addition line to salt stress

The photosynthetic responses to salt stress were examined in a wheat (Triticum aestivum L. cv. Asakaze)–barley (Hordeum vulgare L. cv. Manas) 7H addition line having elevated salt tolerance and compared to the parental wheat genotype. For this purpose, increasing NaCl concentrations up to 300 mM were applied and followed by a 7-day recovery period. Up to moderate salt stress (200 mM NaCl), forcible stomatal closure, parallel with a reduction in the net assimilation rate (PN), was only observed in wheat, but not in the 7H addition line or barley. Since the photosynthetic electron transport processes of wheat were not affected by NaCl, the impairment in PN could largely be accounted for the salt-induced decline in stomatal conductance (gs), accompanied by depressed intercellular CO2 concentration and carboxylation efficiency. Both, PN and nonstomatal limitation factors (Lns) were practically unaffected by moderate salt stress in barley and in the 7H addition line due to the sustained gs, which might be an efficient strategy to maintain the efficient photosynthetic activity and biomass production. At 300 mM NaCl, both PN and gs decreased significantly in all the genotypes, but the changes in PN and Lns in the 7H addition line were more favourable similar to those in wheat. The downregulation of photosynthetic electron transport processes around PSII, accompanied by increases in the quantum yield of regulated energy dissipation and of the donor side limitation of PSI without damage to PSII, was observed in the addition line and barley during severe stress. Incomplete recovery of PN was observed in the 7H addition line as a result of declined PSII activity probably caused by enhanced cyclic electron flow around PSI. These results suggest that the better photosynthetic tolerance to moderate salt stress of barley can be manifested in the 7H addition line which may be a suitable candidate for improving salt tolerance of wheat.

Differing metabolic responses to salt stress in wheat-barley addition lines containing different 7H chromosomal fragments

Salinity-induced osmotic, ionic and oxidative stress responses were investigated on Asakaze/Manas wheat/barley addition lines 7H, 7HL and 7HS, together with their barley (salt-tolerant) and wheat (relatively salt-sensitive) parents. Growth, photosynthetic activity, chlorophyll degradation, proline, glycine betaine accumulation, sugar metabolism, Na+ and K+ uptake and transport processes and the role of polyamines and antioxidants were studied in young plants grown in hydroponic culture with or without salt treatment. Changes in plant growth and photosynthetic activity of plants demonstrated that the salt tolerance of the addition lines 7H and 7HL was similar to that of barley parent cv. Manas, while the sensitivity of the addition line 7HS was similar to that of the wheat parent cv. Asakaze. The Na accumulation in the roots and shoots did not differ between the addition lines and wheat parent. The activation of various genes related to Na uptake and transport was not correlated with the salt tolerance of the genotypes. These results indicated that the direct regulation of Na transport processes is not the main reason for the salt tolerance of these genotypes. Salt treatment induced a complex metabolic rearrangement in both the roots and shoots of all the genotypes. Elevated proline accumulation in the roots and enhanced sugar metabolism in the shoots were found to be important for salt tolerance in the 7H and 7HL addition lines and in barley cv. Manas. In wheat cv. Asakaze and the 7HS addition line the polyamine metabolism was activated. It seems that osmotic adjustment is a more important process in the improvement of salt tolerance in 7H addition lines than the direct regulation of Na transport processes or antioxidant defence.