Mehmet Tufan Öz

Signal Transduction of Phytohormones Under Abiotic Stresses

Growth and productivity of higher plants are adversely affected by various environmental stresses which are of two main types, biotic and abiotic, depending on the source of stress. Broad range of abiotic stresses includes osmotic stress caused by drought, salinity, high or low temperatures, freezing, or flooding, as well as ionic, nutrient, or metal stresses, and others caused by mechanical factors, light, or radiation. Plants contrary to animals cannot escape from these environmental constraints, and over the course of evolution, they have developed some physiological, biochemical, or molecular mechanisms to overcome effects of stress. Phytohormones such as auxin, cytokinin, abscisic acid, jasmonic acid, ethylene, salicylic acid, gibberellic acid, and few others, besides their functions during germination, growth, development, and flowering, play key roles and coordinate various signal transduction pathways in plants during responses to environmental stresses. Complex networks of gene regulation by these phytohormones under abiotic stresses involve various cis- or trans-acting elements. Some of the transcription factors regulated by phytohormones include ARF, AREB/ABF, DREB, MYC/MYB, NAC, and others. Changes in gene expression, protein synthesis, modification, or degradation initiated by or coupled to these transcription factors and their corresponding cis-acting elements are briefly summarized in this work. Moreover, crosstalk between signal transduction pathways involving phytohormones is explained in regard to transcriptional or translational regulation under abiotic stresses.

Antioxidant Responses of Lentil to Cold and Drought Stress

The effects of cold and drought stress on antioxidant responses and growth parameters in shoots and roots of lentil (Lens culinaris M cv Sultan 1) seedlings were investigated. Ten-day-old hydroponically grown seedlings were subjected to drought and cold (4°C) stress for 5 days. The length and fresh weight of shoots decreased significantly under both stress conditions, contrary to the increase in these growth parameters for roots under the same conditions. The oxidative damage as generation of malondialdehyde and hydrogen peroxide, was markedly higher in shoots under cold. Both stress conditions caused a significant increase in malondialdehyde levels in root tissues. The increase in proline levels was more pronounced under cold stress in shoots and roots. The tested stress conditions had no significant effect on chlorophyll contents. Superoxide dismutase activity was differentially altered in shoot and root tissues under drought and cold stress. The catalase activity was higher in roots under drought stress. On the other hand, ascorbate peroxidase activity increased in root tissues under cold stress. The results indicate that improved tolerance to cold and drought stress in root and shoot tissues of lentil might be correlated to the increased capacity of antioxidative defense system.

Diversity in boron toxicity tolerance of Australian barley (Hordeum vulgare L.) genotypes

BackgroundBoron (B) is an important micronutrient for plant growth, but is toxic when levels are too high. This commonly occurs in environments with alkaline soils and relatively low rainfall, including many of the cereal growing regions of southern Australia. Four major genetic loci controlling tolerance to high soil B have been identified in the landrace barley, Sahara 3771. Genes underlying two of the loci encode the B transporters HvBot1 and HvNIP2;1.ResultsWe investigated sequence and expression level diversity in HvBot1 and HvNIP2;1 across barley germplasm, and identified five novel coding sequence alleles for HvBot1. Lines were identified containing either single or multiple copies of the Sahara HvBot1 allele. We established that only the tandemly duplicated Sahara allele conferred B tolerance, and this duplicated allele was found only in a set of nine lines accessioned in Australian collections as Sahara 3763–3771. HvNIP2;1 coding sequences were highly conserved across barley germplasm. We identified the likely causative SNP in the 5’UTR of Sahara HvNIP2;1, and propose that the creation of a small upstream open reading frame interferes with HvNIP2;1 translation in Sahara 3771. Similar to HvBot1, the tolerant HvNIP2;1 allele was unique to the Sahara barley accessions. We identified a new source of the 2H B tolerance allele controlling leaf symptom development, in the landrace Ethiopia 756.ConclusionsEthiopia 756, as well as the cultivar Sloop Vic which carries both the 2H and HvBot1 B tolerance alleles derived from Sahara 3771, may be valuable as alternative parents in breeding programs targeted to high soil B environments. There is significant diversity in B toxicity tolerance among contemporary Australian barley varieties but this is not related to variation at any of the four known B tolerance loci, indicating that novel, as yet undiscovered, sources of tolerance exist.

Physiological, Biochemical, and Transcriptomic Responses to Boron Toxicity in Leaf and Root Tissues of Contrasting Wheat Cultivars

In this study, we examined physiological, biochemical, and transcriptomic responses to toxic boron (B) treatment in leaves and roots of two wheat cultivars in order to gain better insight into B response or tolerance mechanisms. Delayed development and reduced vigor caused by high B were not observed in leaves and roots of both cultivars. Length, wet weight, and dry weight were not markedly changed under B toxicity. In leaves, when compared to control, 995 and 892 genes were significantly expressed at least twofold under B toxicity in Atay and Bolal, respectively. In roots, expressions of 1248 and 957 genes were responsive to B toxicity in Atay and Bolal, respectively. In leaf and root tissues, B toxicity induced more genes related to protein degradation in Atay than those in Bolal. These differences in transcriptome were attributed to higher B accumulation in the sensitive cultivar which required high level of metabolic adjustment. B toxicity stress did not cause any significant change in photosynthetic activity and contents of proline and glycine betaine in both cultivars. Coordinately, we did not find any differentially expressed genes required for proline and glycine betaine metabolisms. Genes related to hormone signaling, kinases, transcription factors such as WRKY and MYB, and key enzymes in reactive oxygen species (ROS) scavenging mechanisms were differentially affected by B toxicity in both cultivars. Among commonly regulated genes in Atay and Bolal, glutathione S-transferase (GST) and NIP4;1 (nodulin-26-like intrinsic proteins) genes stand out as prominent actors in B stress response.

Functional Role of Nitric Oxide Under Abiotic Stress Conditions

Nitric oxide (NO), a free radical in living organisms, is considered a phytohormone and a key signalling molecule functioning in various physiological processes of plants. These physiological processes include germination, growth, senescence, and photosynthesis as well as response mechanisms to specific environmental stresses. Plants under abiotic stress conditions experience oxidative and nitrosative stress ; the latter mainly elicited by regulation of NO production. Nitrosative stress describes the molecular or cellular damage promoted by imbalance in NO homeostasis and other reactive nitrogen species . Additionally, depending on its concentration and location in plant cells or tissues, NO might function as an antioxidant and scavenge some other reactive intermediates. Direct or indirect involvement of NO in response mechanisms under water stress, drought, salinity, heavy metal stress, high or low temperature extremities, and ultraviolet radiation has been reported. In this work, the recent findings and current knowledge on the function of NO in plants under abiotic stress conditions are reviewed briefly.

Cloning and heterologous expression of chlorophyll a synthase in Rhodobacter sphaeroides

Rhodobacter sphaeroides is a purple non‐sulfur bacterium which photoheterotrophically produces hydrogen from organic acids under anaerobic conditions. A gene coding for putative chlorophyll a synthase (chlG) from cyanobacterium Prochlorococcus marinus was amplified by nested polymerase chain reaction and cloned into an inducible‐expression plasmid which was subsequently transferred to R. sphaeroides for heterologous expression. Induced expression of chlG in R. sphaeroides led to changes in light absorption spectrum within 400–700 nm. The hydrogen production capacity of the mutant strain was evaluated on hydrogen production medium with 15 mM malate and 2 mM glutamate. Hydrogen yield and productivity were increased by 13.6 and 22.6%, respectively, compared to the wild type strain. The results demonstrated the feasibility of genetic engineering to combine chlorophyll and bacteriochlorophyll biosynthetic pathways which utilize common intermediates. Heterologous expression of key enzymes from biosynthetic pathways of various pigments is proposed here as a general strategy to improve absorption spectra and yield of photosynthesis and hydrogen gas production in bacteria.