• BBX24
• CAT.
• HY5
• POD
• PSII
• salinity stress
• SOD
• tomato plants
• UV-B radiation
• WRKY36

Plants are generally exposed to multiple abiotic and biotic stresses. Among abiotic stresses, soil and water salinization is a growing problem in the Mediterranean basin, due to seawater intrusion and bad agricultural practices. Salinity affects approximately 20% of the world's territory, which influences not only agricultural production but also various aspects of plant physiology and biochemistry. High levels of ions in the soil solution rapidly induce osmotic effects, reducing water uptake by the roots (physiological drought), and excess ion uptake produces cytotoxicity and pH/nutrient imbalance, interfering with physiological and metabolic processes. To improve plant resistance to stresses, researchers have adopted numerous techniques. Among these strategies, seed and seedling priming is one of the most flexible and effective methods to increase abiotic stress tolerance in crop plants, helping them to improve agricultural production in stressful environments. Ultraviolet-B (UV-B) radiation has been identified as a potential priming agent for improving productivity, crop quality, yield, and plant tolerance to both biotic and abiotic stressors. UV-B priming consists of exposing seeds or seedlings to low doses of UV-B radiation to improve tolerance to a subsequent stress through activation of the expression of genes involved in pathways that regulate plant growth and metabolism.
Based on these premises, this work aimed at testing whether UV-B radiation can act as a priming factor to improve the resistance of tomato (Solanum Lycopersicum, cv. Moneymaker) plants to salinity stress, thus limiting the negative effect of the stress itself. The influence of UV-B pre-treatments on plant reactions to salt stress was carried out at the leaf and root level using young plants to check the effectiveness of UV-B radiation in conferring resistance against abiotic stress.
Several UV-B radiation doses (1 h, 3 h, 6 h, 8 h and 16h) and NaCl doses (75 mM, 150 mM, and 300 mM) have been screened, separately, to be employed as preconditioning treatments of tomato plantlets cultivated in a controlled environment. Biometric parameters and antioxidant compounds were quantified in leaves and roots. The results indicate that mild salt treatments, particularly a concentration of 75 mM sodium chloride (NaCl) for a duration of 14 days, resulted in a decrease only in the leaf area. Conversely, a concentration of 150 mM induced a reduction in all biometric parameters, and a concentration of 300 mM led to complete inhibition of the plant's growth. The exposure of tomato plants to NaCl caused a decrease in the phenolic content within the roots while simultaneously promoting an increase in flavonoid levels within the leaves. It was observed that 150 mM NaCl increased anthocyanin levels in the leaves and, additionally, the leaf antioxidant activity exhibited an improvement in response to the salinity conditions.
Considering the response to UV-B, 8 h of irradiation (0.4 W m2) per day for 7 days (18.43 kJ m-2), were successful in increasing the content of foliar total phenolics and flavonoids without inducing strong impairments in growth and development of hydroponically grown tomato plantlets. In addition, exposure to UV-B radiation for 16 hours per day for a total of 7 days (36.86 kJ m-2) resulted in a notable increase in both leaf area and stem length, as well as in alteration of the shoot-to-root length ratio. Nevertheless, the plantlets have enhanced the levels of foliar anthocyanins and antioxidant activity in both leaves and roots.
Based on these preliminary tests, the subsequent research focused on verifying the effectiveness of UV-B pre-treatments to alleviate the impact of salt stress in tomato plants. Tomato seedlings were pretreated with UV-B radiation (0.4 W m2, 16 h/day for 7 days) provided by LEDs (peak at 310 nm) before being subjected to salt stress (150 mM NaCl). Attention has been specifically addressed to the main biometric parameters, the concentration of mineral nutrients, the photosynthetic apparatus, and the enzymatic and non-enzymatic antioxidant systems.
An increase in stem length and weight, and in leaf area was observed in UV-B pre-treated plants, indicating an alteration of the plant architecture carried on by UV-B radiation. The results on phenols, flavonoids and anthocyanins, together with those on the activity of superoxide dismutases, peroxidases and catalases indicate the potential of UV-B pre-treatment to boost non-enzymatic and enzymatic systems, giving the seedlings an advantage against saline stress.
Foliar and root ionimic profile was mainly affected by salinity, that, apart an obvious increase in Na concentration, generally led to a decrease of both macro and micronutrients, more evident in the leaves. The pretreatment with UV-B radiation was able to prevent the increase in Cd accumulation induced by salinity in tomato leaves.
The results of the chlorophyll a fluorescence evidenced that UV-B pre-treatment reduced the negative impact that salinity induced on yield, photochemical quenching coefficient (qP), “lake” model (qL) and non-regulated Y(NO) non-photochemical energy dissipation at PSII, thus playing an important role in inducing resistance to salt stress. Interestingly, compared to salt stress group there was an increase in Chl a in the primed (P-BN) treated leaves. Based on these results, an analysis of the protein composition of the photosystem II (PSII) was carried out by determining the level of D1, D2, LHCII, cp43, and cp47 proteins by western blotting.
A gene expression analysis of the genes involved in acclimation and photomorphogenic responses to UV-B radiation was performed. HY5 was tested as an indicator of the activation of the specific UVR8 signaling pathway under the UV-B radiation applied. Furthermore, other transcription factors were analyzed such as WRKY36, a transcriptional repressor of HY5 in response to UVB radiation, and BBX24 that negatively regulates HY5 expression, and is induced by high salinity and UV-B radiation.
Based on the overall results, UV-B pre-treatments were able to induce changes in the growth parameters of leaves and roots of tomato plants, causing modifications in Chl a fuorescence, in proteins of PSII, acting also at the gene level and modification on the resistance of plants against salinity stresses.