Tree post-drought recovery: scenarios, regulatory mechanisms and ways to improve.

Efficient post-drought recovery of growth and assimilation enables a plant to return to its undisturbed state and functioning. Unlike annual plants, trees suffer not only from the current drought, but also from cumulative impacts of consecutive water stresses which cause adverse legacy effects on survival and performance. This review provides an integrated assessment of ecological, physiological and molecular evidence on the recovery of growth and photosynthesis in trees, with a view to informing the breeding of trees with a better ability to recover from water stress. Suppression of recovery processes can result not only from stress damage but also from a controlled downshift of recovery as part of tree acclimation to water-limited conditions. In the latter case, recovery processes could potentially be activated by turning off the controlling mechanisms, but several obstacles make this unlikely. Tree phenology, and specifically photoperiodic constraints, can limit post-drought recovery of growth and photosynthesis, and targeting these constraints may represent a promising way to breed trees with an enhanced ability to recover post-drought. The mechanisms of photoperiod-dependent regulation of shoot, secondary and root growth and of assimilation processes are reviewed. Finally, the limitations and trade-offs of altering the photoperiodic regulation of growth and assimilation processes are discussed.

Effects of drought stress memory on the accumulation of stress-protective compounds in naturally grown pine and spruce.

Forest trees are subjected to multiple stressors during their long lifetime and therefore require effective and finely regulated stress-protective systems. Stressors can induce protective systems either directly or with the involvement of stress memory mechanisms. Stress memory has only begun to be uncovered in model plants and is unexplored in coniferous species. Therefore, we studied the possible role of stress memory in the regulation of the accumulation of stress-protective compounds (heat shock proteins, dehydrins, proline) in the needles of naturally grown Scots pine and Norway spruce trees subjected to the subsequent action of long-term (multiyear) and short-term (seasonal) water shortages. Although the water deficit was relatively mild, it significantly influenced the pattern of expression of stress memory-related heat shock factor (HSF) and SWI/SNF genes, indicating the formation of stress memory in both species. In spruce, dehydrin accumulation was increased by water shortage in a manner compatible with Type II stress memory. The accumulation of HSP40 in spruce needles was positively influenced by long-term water shortage, but this increase was unlikely to be of biological importance due to the concomitant decrease in HSP70, HSP90 and HSP101 accumulation. Finally, proline accumulation was negatively influenced by short-term water deficit in spruce. In pine, no one protective compound accumulated in response to water stress. Taken together, the results indicate that the accumulation of stress-protective compounds was generally independent of stress memory effects both in pine and in spruce.

Priming Potato Plants with Melatonin Protects Stolon Formation under Delayed Salt Stress by Maintaining the Photochemical Function of Photosystem II, Ionic Homeostasis and Activating the Antioxidant System.

Melatonin is among one of the promising agents able to protect agricultural plants from the adverse action of different stressors, including salinity. We aimed to investigate the effects of melatonin priming (0.1, 1.0 and 10 µM) on salt-stressed potato plants (125 mM NaCl), by studying the growth parameters, photochemical activity of photosystem II, water status, ion content and antioxidant system activity. Melatonin as a pleiotropic signaling molecule was found to decrease the negative effect of salt stress on stolon formation, tissue water content and ion status without a significant effect on the expression of Na+/H+-antiporter genes localized on the vacuolar (NHX1 to NHX3) and plasma membrane (SOS1). Melatonin effectively decreases the accumulation of lipid peroxidation products in potato leaves in the whole range of concentrations studied. A melatonin-induced dose-dependent increase in Fv/Fm together with a decrease in uncontrolled non-photochemical dissipation Y(NO) also indicates decreased oxidative damage. The observed protective ability of melatonin was unlikely due to its influence on antioxidant enzymes, since neither SOD nor peroxidase were activated by melatonin. Melatonin exerted positive effects on the accumulation of water-soluble low-molecular-weight antioxidants, proline and flavonoids, which could aid in decreasing oxidative stress. The most consistent positive effect was observed on the accumulation of carotenoids, which are well-known lipophilic antioxidants playing an important role in the protection of photosynthesis from oxidative damage. Finally, it is possible that melatonin accumulated during pretreatment could exert direct antioxidative effects due to the ROS scavenging activity of melatonin molecules.

Structurally Different Exogenic Brassinosteroids Protect Plants under Polymetallic Pollution via Structure-Specific Changes in Metabolism and Balance of Cell-Protective Components.

Heavy metals and aluminum are among the most significant abiotic factors that reduce the productivity and quality of crops in acidic and contaminated soils. The protective effects of brassinosteroids containing lactone are relatively well-studied under heavy metal stress, but the effects of brassinosteroids containing ketone are almost unstudied. Moreover, there are almost no data in the literature on the protective role of these hormones under polymetallic stress. The aim of our study was to compare the stress-protective effects of lactone-containing (homobrassinolide) and ketone-containing (homocastasterone) brassinosteroids on the barley plant's resistance to polymetallic stress. Barley plants were grown under hydroponic conditions; brassinosteroids, increased concentrations of heavy metals (Mn, Ni, Cu, Zn, Cd, and Pb), and Al were added to the nutrient medium. It was found that homocastasterone was more effective than homobrassinolide in mitigating the negative effects of stress on plant growth. Both brassinosteroids had no significant effect on the antioxidant system of plants. Both homobrassinolide and homocastron equally reduced the accumulation of toxic metals (except for Cd) in plant biomass. Both hormones improved Mg nutrition of plants treated with metal stress, but the positive effect on the content of photosynthetic pigments was observed only for homocastasterone and not for homobrassinolide. In conclusion, the protective effect of homocastasterone was more prominent compared to homobrassinolide, but the biological mechanisms of this difference remain to be elucidated.

Abscisic Acid and Cytokinins Are Not Involved in the Regulation of Stomatal Conductance of Scots Pine Saplings during Post-Drought Recovery.

Delayed or incomplete recovery of gas exchange after water stress relief limits assimilation in the post-drought period and can thus negatively affect the processes of post-drought recovery. Abscisic acid (ABA) accumulation and antagonistic action between ABA and cytokinins (CKs) play an important role in regulation of stomatal conductance under water deficit. Specifically, in pine species, sustained ABA accumulation is thought to be the main cause of delayed post-drought gas exchange recovery, although the role of CKs is not yet known. Therefore, we aimed to study the effects of ABA and CKs on recovery of stomatal conductance in greenhouse-grown 3-year-old Scots pine saplings recovering from water stress. We analysed both changes in endogenous ABA and CK contents and the effects of treatment with exogenous CK on stomatal conductance. Drought stress suppressed stomatal conductance, and post-drought stomatal conductance remained suppressed for 2 weeks after plant rewatering. ABA accumulated during water stress, but ABA levels decreased rapidly after rewatering. Additionally, trans-zeatin/ABA and isopentenyladenine/ABA ratios, which were decreased in water-stressed plants, recovered rapidly in rewatered plants. Spraying plants with 6-benzylaminopurine (0.1-100 µM) did not influence recovery of either stomatal conductance or needle water status. It can be concluded that the delayed recovery of stomatal conductance in Scots pine needles was not due to sustained ABA accumulation or a sustained decrease in the CK/ABA ratio, and CK supplementation was unable to overcome this delayed recovery.

Stem notching decreases stem hydraulic conductance but does not influence drought impacts and post-drought recovery in Scots pine and Norway spruce.

The tight connection between the deterioration of xylem function and plant mortality under drought is well recognized. However, a lack of mechanistic understanding of how substantial conductivity loss influences plant performance under drought and during post-drought recovery hinders our ability to model tree responses to drought stress. We artificially induced a loss of 50% of xylem conducting area in Scots pine and Norway spruce saplings by stem notching and investigated plant performance under drought and during post-drought recovery. Plant mortality, xylem hydraulic conductivity, leaf water status and stomatal conductance were measured. We observed no preferential mortality of top plant parts (above the notches) compared to basal plant parts (below the notches), and no consistent trend in hydraulic conductivity loss was observed between top and basal parts of dying plants. Stem hydraulic conductivity, water status of the needles and stomatal conductance changed similarly between the top and basal parts during drought and post-drought recovery, which indicated the substantial hydraulic overcapacity of the stems. The recovery of stomatal conductance demonstrated prominent hysteresis due to non-hydraulic stomatal limitations. The results obtained are highly important for modelling the influence of plant hydraulic impairment on plant performance under drought and during post-drought recovery.

Mineral Nutrition of Naturally Growing Scots Pine and Norway Spruce under Limited Water Supply.

The deterioration of plant mineral nutrition during drought is a significant factor in the negative influence of drought on plant performance. We aimed to study the effects of seasonal and multiyear water shortages on nutrient supply and demand in Scots pine and Norway spruce. We studied pine and spruce trees naturally grown in the Bryansk region (Russia). The dynamics of several nutrients (K, Ca, Mg, P, Fe, Mn, Zn, and Ca) in wood, needles, and bark of current-year twigs and the dynamics of the available pools of these elements at different soil depths were analysed. To assess the physiological consequences of changes in element concentrations, lipid peroxidation products and photosynthetic pigments were measured in the needles. Water shortage increased the wood concentrations of all elements except for Mn. In pine, this increase was mainly due to seasonal water deficit, whereas in spruce, multiyear differences in water supply were more important. This increased availability of nutrients was not observed in soil-based analyses. In needles, quite similar patterns of changes were found between species, with Mg increasing almost twofold and Fe and Mn decreasing under water shortage, whereas the remainder of the elements did not change much under differing water supplies. Neither the concentrations of photosynthetic pigments nor the contents of lipid peroxidation products correlated with element dynamics in needles. In summary, water shortage increased the availability of all elements except Mn for the plant; however, needle element contents were regulated independently of element availability for plants.

Linking the growth patterns of coniferous species with their performance under climate aridization.

Tree growth is highly sensitive to water deficit. At the same time, growth processes substantially influence tree performance under water stress by changing the root-absorbing surface, leaf-transpiring surface, amount of conducting xylem, etc. Drought-induced growth suppression is often higher in conifers than in broadleaf species. This review is devoted to the relations between the growth of coniferous plants and their performance under increasing climate aridization in the temperate and boreal zones of the Northern Hemisphere. For adult trees, available evidence suggests that increasing the frequency and severity of water deficit would be more detrimental to those plants that have higher growth in favorable conditions but decrease growth more prominently under water shortage, compared to trees whose growth is less sensitive to moisture availability. Not only the overall sensitivity of growth processes to water supply but also the asymmetry in response to lower-than-average and higher-than-average moisture conditions can be important for the performance of coniferous trees under upcoming adverse climate change. To fully understand the tree response under future climate change, the responses to both drier and wetter years need to be analyzed separately. In coniferous seedlings, more active growth is usually linked with better drought survival, although physiological reasons for such a link can be different. Growth stability under exacerbating summer water deficit in coniferous plants can be maintained by more active spring growth and/or by a bimodal growth pattern; each strategy has specific advantages and drawbacks. The optimal choice of growth strategy would be critical for future reforestation programs.

Quantitative analysis of differential dehydrin regulation in pine and spruce seedlings under water deficit.

Dehydrins are well-known components of plant responses to different stresses that cause dehydration, including drought, freezing, salinity, etc. In conifers, the dehydrin gene family is very large, implying that the members of this family have important physiological functions in conifer stress tolerance. However, dehydrin gene expression displays a wide range of responses to stress, from thousand-fold increased expression to decreased expression, and it is generally unknown how regulatory systems are connected at the mRNA and protein levels. Therefore, we studied these aspects of dehydrin regulation in Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) H. Karst) seedlings under polyethylene glycol 6000-induced osmotic stress ranging from relatively low (culture medium water potential of -0.15 MPa) to very high (-1.0 MPa) intensities. In pine, the major dehydrin protein was Dhn1 in both the roots and needles, and in spruce, two isoforms of the Dhn4 protein were the major dehydrins; both of these proteins are AESK-type dehydrins. The genes encoding these major proteins were highly expressed even under control conditions; surprisingly, we also observed several highly expressed dehydrin genes that were not abundantly translated. Under osmotic stress, the most prominent expression changes were observed for the dehydrin genes with low basal expression levels, whereas highly expressed genes generally demonstrated rather modest changes in expression. We report proposed constitutive physiological functions of the AESK-type dehydrins in Pinaceae plants.

Current understanding of plant zinc homeostasis regulation mechanisms.

The essential nature of Zn and widespread Zn deficiency in plants under field conditions underlie the great interest of researchers in the regulation of plant Zn homeostasis. Here, the current knowledge of plant Zn homeostasis regulation, mainly in A. thaliana, is reviewed. The plant Zn homeostasis machinery is regulated largely at the transcriptional level. Local regulation in response to changes in cellular Zn status is based on the transcription factors bZIP19 and bZIP23, which sense changes in free Zn2+ concentrations in the cell. However, there are likely other unidentified ways to sense cellular free Zn2+ concentrations in addition to the well-known bZIP19 and bZIP23 factors. In recent years, the existence of a shoot-derived systemic Zn deficiency signal, which is involved in the upregulation of Zn transport from roots to shoots, was demonstrated. Additionally, rates of mRNA degradation of Zn homeostasis genes are likely regulated by changes in cellular Zn status. In addition to the regulation of Zn transport, other mechanisms for the regulation of plant Zn homeostasis exist. "Zn sparing" mechanisms could be involved in the decrease in plant Zn requirements under Zn deficiency. Additionally, autophagy is probably regulated by local Zn status and involved in Zn reutilization at the cellular level. Current issues related to studying Zn homeostasis regulation are discussed.

Profiles of endogenous phytohormones and expression of some hormone-related genes in Scots pine and Norway spruce seedlings under water deficit.

Different plant hormones are involved in plant adaptation to water deficit. In comparison to angiosperms, little is known about the impact of drought on the pool of phytohormones in gymnosperms. Therefore, we studied the effect of polyethylene glycol-induced water deficit on the changes in content of different phytohormones in Scots pine and Norway spruce seedlings, which are known for their different strategies of adaptation to water deficit. The following hormone classes were analysed: cytokinins, auxins, jasmonates, salicylic and benzoic acids, and 1-aminocyclopropane-1-carboxylic acid (an ethylene precursor). No consistent reaction to water stress was observed for the content of well-known stress-related hormones - salicylic acid and jasmonates. In contrast, drought induced a dose-dependent accumulation of cytokinins in pine needles, with less profound changes in spruce needles. The most prominent changes were observed for 1-aminocyclopropane-1-carboxylic acid content, which increased several-fold in spruce roots and pine needles under water deficit. Water-deficit-induced changes in the contents of cytokinins and 1-aminocyclopropane-1-carboxylic acid were accompanied by the differential regulation of genes involved in the metabolism of these hormones. Possible links between changes in hormone pools and the adaptation of seedlings to water deficit are discussed.

24-Epibrassinolide alleviates the toxic effects of NaCl on photosynthetic processes in potato plants.

Brassinosteroids are promising agents for alleviating the negative effects of salinity on plants, but the mechanism of their protective action is far from being understood. We investigated the effect of pretreatment with 24-epibrassinolide (24-EBL) on the photosynthetic and physiological parameters of potato plants under progressive salinity stress caused by root application of 100 mM NaCl. Salinity clearly inhibited primary photosynthetic processes in potato plants by reducing the contents of photosynthetic pigments, photosynthetic electron transport and photosystem II (PSII) maximal and effective quantum yields. These negative effects of salinity on primary photosynthetic processes were mainly due to toxic ionic effects on the plant's ability to oxidize the plastoquinone pool. Pretreatment with 24-EBL alleviated this stress effect and allowed the maintenance of plastoquinone pool oxidation and the efficiency of photosystem II photochemistry to be at the same levels as those in unstressed plants; however, the pretreatment did not affect the photosynthetic pigment content. 24-EBL pretreatment clearly alleviated the decrease in leaf osmotic potential under salinity stress. The stress-induced increases in lipid peroxidation and proline contents were not changed under brassinosteroid pretreatment. However, 24-EBL pretreatment increased the peroxidase activity and improved the K+/Na+ ratio in potato leaves, which were likely responsible for the protective 24-EBL action under salt stress.

Comparative photosynthetic responses of Norway spruce and Scots pine seedlings to prolonged water deficiency.

Stressors of different natures, including drought stress, substantially compromise the ability of plants to effectively and safely utilize light energy. We investigated the influence of water stress on the photosynthetic processes in Picea abies and Pinus sylvestris, two species with contrasting drought sensitivities. Spruce and pine seedlings were exposed to polyethylene glycol 6000-induced water deficits of different intensities and durations. The maintenance of photosystem I (PSI) oxidation in spruce required increased photosynthetic control and led to the increased reduction of the plastoquinone pool, which was not the case in pine seedlings. As a result of increased excitation pressure, photosystem II (PSII) inactivation was observed in spruce plants, whereas in pine, the decreased PSII photochemistry was likely due to sustained non-photochemical quenching. Downregulation of PSII photochemistry and maintenance of PSI in an oxidized state were linked with the prevention of oxidative stress, even under severe water deficit. The decreased photosynthetic pigment content and photosynthetic gene expression suggested the coordinated downregulation of photosynthetic apparatus components under water stress to reduce light energy absorption. In summary, the observed adaptative mechanisms of pine and spruce to water stress may be similar to the well-studied adaptative mechanisms to winter stress, which may indicate the universality of protective mechanisms under various stresses in conifers.

The labile zinc pool in plant cells.

Zinc is the most abundant and important transition metal in plants; however, the dynamic aspects of zinc homeostasis in plant cells are poorly understood. In this study we explored the pool of labile exchangeable zinc complexes in plant cells, and the potential influence of changes in intracellular zinc availability on cellular physiology. Work was performed on cultivated cell extracts of Arabidopsis thaliana (L.) Heynh. and Thellungiella salsuginea (Pall.) O.E. Schulz grown under control (3.48 µM Zn2+), 10-fold Zn excess or Zn starvation conditions. The free and labile Zn contents in the extracts were then determined by fluorimetric titration. We observed for the first time that plant cells contain micromolar concentrations of labile zinc complexes that account for a low percentage of the total zinc content. Labile zinc is mainly protein bound. Zn starvation inhibits cell proliferation and leads to the disappearance of the labile zinc pool, whereas Zn excess drastically increases the labile zinc pool. Free Zn2+ is buffered at picomolar concentrations in the intracellular milieu, and the increase in free Zn2+ concentrations to low nanomolar values clearly modulates enzyme activity by direct reversible binding. Such increases in free Zn2+ can be achieved by the substantial influx of additional zinc or by the oxidation of zinc-binding thiols. The observed features of the labile zinc pool in plant cells suggest it has a role in intracellular zinc trafficking and zinc signalling.

Comparative analysis of abscisic acid levels and expression of abscisic acid-related genes in Scots pine and Norway spruce seedlings under water deficit.

Abscisic acid (ABA) is one of the main participants in the regulation of plant responses to water deficiency. Knowledge of the ABA signal transduction pathways in gymnosperms is rather limited, especially in comparison with those in angiosperms. Seedlings of Scots pine and Norway spruce are known for their contrasting behaviour strategies under water deficit. To characterize the possible role of ABA in these differences, ABA dynamics were investigated under conditions of water deficit in seedlings of these two species. The content of ABA and its catabolites was followed in the roots and needles of seedlings of Pinus sylvestris and Picea abies under conditions of polyethylene glycol (PEG)-induced water deficiency (-0.15 and -0.5 MPa) for 10 days. The expression of the main genes for ABA-biosynthetic enzymes was also analysed. ABA showed more pronounced stress-dependent dynamics in pine roots than in spruce roots, whereas in needles, the response was greater for spruce than pine. The ABA increase during drought was mainly due to de novo synthesis and the shift in the balance between ABA synthesis and catabolism towards synthesis. The ABA-glucosyl ester did not serve as a reserve for the release of free ABA under water deficiency. The expression levels of the main ABA biosynthetic genes showed a weak or no correlation with changes in ABA content under water stress, i.e., the ABA content in the seedlings of both species was not directly linked to the transcript levels of the main ABA biosynthetic genes. Less-pronounced stress-induced changes in ABA in pine needles than in spruce needles may be related to pine seedlings having a less conservative strategy of growth and maintenance of water balance under water deficit.

Impact of weak water deficit on growth, photosynthetic primary processes and storage processes in pine and spruce seedlings.

We investigated the influence of 40 days of drought on growth, storage processes and primary photosynthetic processes in 3-month-old Scots pine and Norway spruce seedlings growing in perlite culture. Water stress significantly affected seedling water status, whereas absolute dry biomass growth was not substantially influenced. Water stress induced an increase in non-structural carbohydrate content (sugars, sugar alcohols, starch) in the aboveground part of pine seedlings in contrast to spruce seedlings. Due to the relatively low content of sugars and sugar alcohols in seedling organs, their expected contribution to osmotic potential changes was quite low. In contrast to biomass accumulation and storage, photosynthetic primary processes were substantially influenced by water shortage. In spruce seedlings, PSII was more sensitive to water stress than PSI. In particular, electron transport in PSI was stable under water stress despite the substantial decrease of electron transport in PSII. The increase in thermal energy dissipation due to enhancement of non-photochemical quenching (NPQ) was evident in both species under water stress. Simultaneously, the yields of non-regulated energy dissipation in PSII were decreased in pine seedlings under drought. A relationship between growth, photosynthetic activities and storage processes is analysed under weak water deficit.

Impact of drought stress induced by polyethylene glycol on growth, water relations and cell viability of Norway spruce seedlings.

We investigated physiological responses of 7-week-old Norway spruce seedlings to water deficits of different intensities. Hydroponically grown seedlings were subjected to mild (-0.15 MPa), strong (-0.5 and -1.0 MPa) and extreme (-1.5 MPa) water deficit induced by polyethylene glycol 6000, and their growth parameters, water status and physiological activity were analyzed. Seedlings effectively restricted water loss under drought, and even under extreme water deficit, shoot relative water content did not fall below 85%. Water stress induced substantial decreases in the osmotic potentials of root and needle cell sap, up to 0.3-0.4 MPa under extreme water deficit, though this did not result from water loss or accumulation of K+ and Na+ ions. Seedling growth was very susceptible to water stress because of poor capacity for cell wall adjustment. Water stress injured seedling roots, as evidenced by the loss of root cell physiological activity estimated by the ability to hydrolyse fluorescein diacetate and by increased root calcium content up to 8-10-fold under extreme water stress. At the same time, root hair growth was enhanced, especially under mild water deficit, which increased the root water-absorbing capacity. In summary, seedlings of Norway spruce were characterized by high susceptibility to water stress and concurrently by pronounced ability to maintain water status. These characteristics are fully consistent with spruce confinement to moist habitats.

Different roles of glutathione in copper and zinc chelation in Brassica napus roots.

We investigated the specific features of copper and zinc excess action on the roots of canola (Brassica napus L.) plants. Copper rapidly accumulated in canola root cells and reached saturation during several hours of treatment, whereas the root zinc content increased relatively slowly. Excessive copper and zinc entry inside the cell resulted in significant cell damage, as evidenced by alterations in plasmalemma permeability and decreases in cellular enzymatic activity. Zinc excess specifically damaged root hair cells, which correlated with a pronounced elevation of their labile zinc level. In vitro, we showed that reduced glutathione (GSH) readily reacted with copper ions to form complexes with blocked sulfhydryl groups. In contrast, zinc ions were ineffective as glutathione blockers, and glutathione molecules did not lose their specific chemical activity in the presence of Zn2+ ions. The effect of copper and zinc excess on the glutathione pool in canola root cells was analysed by a combination of biochemical determination of total and oxidized glutathione contents and fluorescent staining of free reduced glutathione with monochlorobimane dye. Excess copper led to dose-dependent diminution of free reduced glutathione contents in the root cells, which could not be explained by the loss of total cellular glutathione or its oxidation. In contrast, we observed little effect of much higher intracellular zinc concentrations on the free reduced glutathione content. We concluded that GSH plays an important role in copper excess, but not zinc excess chelation, in canola root cells.

Effects of 24-epibrassinolide and green light on plastid gene transcription and cytokinin content of barley leaves.

In order to evaluate whether brassinosteroids (BS) and green light regulate the transcription of plastid genes in a cross-talk with cytokinins (CKs), transcription rates of 12 plastid genes (ndhF, rrn23, rpoB, psaA, psaB, rrn16, psbA, psbD, psbK, rbcL, atpB, and trnE/trnY) as well as the accumulation of transcripts of some photoreceptors (PHYA, CRY2, CRY1A, and CRY1B) and signaling (SERK and CAS) genes were followed in detached etiolated barley leaves exposed to darkness, green or white light ±1µm 24-epibrassinolide (EBL). EBL in the dark was shown to up-regulate the transcription of 12 plastid genes, while green light activated 10 genes and the EBL combined with the green light affected the transcription of only two genes (psaB and rpoB). Green light inhibited the expression of photoreceptor genes, except for CRY1A. Under the green light, EBL practically did not affect the expression of CRY1A, CAS and SERK genes, but it reduced the influence of white light on the accumulation of CAS, CRY1A, CRY1B, and SERK gene transcripts. The total content of BS in the dark and under white light remained largely unchanged, while under green light the total content of BRs (brassinolide, castasterone, and 6-deoxocastasterone) and HBRs (28-homobrassinolide, 28-homocastasterone, and 6-deoxo-28-homocastasterone) increased. The EBL-dependent up-regulation of plastome transcription in the dark was accompanied by a significant decrease in CK deactivation by O-glucosylation. However, no significant effect on the content of active CKs was detected. EBL combined with green light moderately increased the contents of trans-zeatin and isopentenyladenine, but had a negative effect on cis-zeatin. The most significant promotive effect of EBL on active CK bases was observed in white light. The data obtained suggest the involvement of CKs in the BS- and light-dependent transcription regulation of plastid genes.

Blue light alters miR167 expression and microRNA-targeted auxin response factor genes in Arabidopsis thaliana plants.

The effect of blue LED (450 nm) on the photomorphogenesis of Arabidopsis thaliana Col-0 plants and the transcript levels of several genes, including miRNAs, photoreceptors and auxin response factors (ARF) was investigated. It was observed that blue light accelerated the generative development, reduced the rosette leaf number, significantly reduced the leaf area, dry biomass and led to the disruption of conductive tissue formation. The blue LED differentially influenced the transcript levels of several phytochromes (PHY a, b, c, d, and e), cryptochromes (CRY 1 and 2) and phototropins (PHOT 1 and 2). At the same time, the blue LED significantly increased miR167 expression compared to a fluorescent lamp or white LEDs. This increase likely resulted in the enhanced transcription of the auxin response factor genes ARF4 and ARF8, which are regulated by this miRNA. These findings support the hypothesis that the effects of blue light on A. thaliana are mediated by auxin signalling pathway involving miRNA-dependent regulation of ARF gene expression.