Downregulation of Polyamine and Diamine Oxidases in Silicon-Treated Cucumber.

Silicon (Si) is a ubiquitous element in soil with well-known beneficial effects under certain conditions, in several plant species, if supplied in available form for uptake. It may alleviate damage in various stress situations and may also promote growth when no obvious stressors are applied. Effects of Si are often linked to mitigation of oxidative stress, in particular to the induction of antioxidant defense mechanisms. In the work presented, the impact of silicon provision on pro-oxidant systems was investigated in cucumber. Plants of the F1 cultivar hybrid 'Joker' were grown under in vitro conditions in the absence of any applied external stressor. Silicon provision decreased H2O2 content and lowered lipid peroxidation in the leaves of the treated plants. This was paralleled by declining polyamine oxidase (PAO) and diamine oxidase (DAO) activities. Several PAO as well as lipoxygenase (LOX) genes were coordinately downregulated in Si-treated plants. Unlike in similar systems studied earlier, the Si effect was not associated with an increased transcript level of gene coding for antioxidant enzymes. These results suggest an inhibitory effect of Si provision on pro-oxidant amine oxidases, which may decrease the level of reactive oxygen species by retarding their production. This extends the molecular mechanisms linked to silicon effects onto redox balance in plants.

Oxidative stress level and dehydrin gene expression pattern differentiate two contrasting cucumber F1 hybrids under high fertigation treatment.

Two cucumber F1 cultivar hybrids were investigated for stress tolerance markers upon application of different strength of Hoagland fertigation solutions (HG). 'Joker' and 'Oitol' cultivar hybrids were studied, representing typically field grown and greenhouse cultivated genotypes, respectively. At standard fertigation level (0.5 × HG) in controlled environment young 'Joker' plants displayed slower growth than 'Oitol' based on total leaf area. At this basal nutrient concentration leaves of 'Joker' plants had significantly lower antioxidant capacity and higher malondialdehyde (MDA, an indicator of lipid peroxidation) level than 'Oitol'. According to RT-qPCR transcript levels of several antioxidant enzymes' genes (ascorbate peroxidase, glutathione reductase and glutathione peroxidase) were significantly higher in 'Joker' compared to 'Oitol'. At increased HG concentrations (1.0, 1.5, 2.0, and 2.5 × HG) growth didn't change significantly in either hybrid. Osmotic potential declined at higher fertigation levels. Antioxidant capacity increased in both hybrids with strong characteristic differences favouring 'Oitol' plants. Higher MDA content of leaves testified more oxidative burden in 'Joker' plants at all and especially at the more concentrated HG treatments. This trend was also approved by results of bio photon emission imaging, which is a powerful method to quantify stress level in living tissues with autoluminescence detection technology. Gene expression for antioxidant enzymes followed HG concentration-dependent increase in both hybrids, at a substantially higher level in 'Joker'. Expression of the dehydrin gene DHN3 was preferentially induced at elevated fertigation levels in 'Oitol' plants, which could contribute to the lower oxidative stress detected in this hybrid. Results presented in this report demonstrate differences in shoot growth, antioxidant capacity, level of oxidative stress and antioxidant gene expression in two contrasting cucumber hybrids at basal fertigation. Furthermore, excessive HG fertigation was found to increase oxidative stress in a genotype-specific way. This effect may be due to different antioxidant capacity and differential expression of stress protective genes, such as the DHN3 dehydrin.