Exogenous riboflavin (vitamin B2) application enhances salinity tolerance through the activation of its biosynthesis in rice seedlings under salinity stress.

Salinity stress triggers the accumulation of reactive oxygen species (ROS), leading to impaired plant growth. Riboflavin (RIB; vitamin B2) is synthesized by plants, fungi, and microorganisms and is a precursor of the coenzymes, flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN), which are important for cellular metabolism. In this study, we aimed to elucidate the mechanistic basis of the RIB-mediated alleviation of salinity stress in rice. We observed higher biomass accumulation and lower concentrations of malondialdehyde (MDA) and hydrogen peroxide (H2O2) in RIB-pretreated seedlings under salinity stress. In vitro assays showed that H2O2 was scavenged as the RIB concentration increased, implying that RIB may function as a non-enzymatic antioxidant in ROS detoxification. RIB-pretreated seedlings accumulated more Na+ in the roots than in the leaf blades because of the contributions of OsHKT2;1, OsNHX1, and OsHKT1;4 in the roots and leaf sheaths, respectively. Liquid chromatography-mass spectrometry (LC-MS/MS) analysis revealed increased RIB concentration in roots and shoots and upregulation of key genes (OsRIBA1, OsGCHI, OsLS, and OsRS) involved in RIB biosynthesis in the roots of RIB-pretreated seedlings. Taken together, our findings suggest that RIB pretreatment ameliorates salinity stress in rice by improving (1) oxidative stress tolerance, as increased RIB concentration may function as a non-enzymatic antioxidant, and (2) ionic stress tolerance, as RIB pretreatment limits Na+ accumulation in the leaf blades and maintains a favorable Na+/K+ balance.

Different Rhizospheric pH Conditions Affect Nutrient Accumulations in Rice under Salinity Stress.

This study was conducted to determine the responses to saline-alkaline (SA) stress with regard to nutrient accumulation in two rice varieties having different tolerances to salt-stress. A salinity-tolerant landrace, Pokkali, and a salinity-sensitive variety, PTT1, were exposed to three levels of SA conditions, pH 7.0 (mild), pH 8.0 (moderate), and pH 9.0 (severe), under 50 mM Na stress. The results indicated that Pokkali had comparably greater SA tolerance than PTT1 owing to its higher biomass production. The maintenance of the lower Na/K ratio in Pokkali shoots was achieved by the higher expression of OsHKT1;5 encoding a Na+ transporter in the shoots, OsNHX1 encoding a tonoplast-localized Na+/H+ antiporter in the roots, and OsHAK16 encoding a K+ transporter in the roots under SA conditions. We propose that the high expression of Fe deficiency-responsive genes, OsIRT1, OsIRO2, OsYSL15, OsNAS1, and OsNAS2, in both rice varieties under all SA conditions should contribute to Fe homeostasis in the shoots. In addition, SA treatment increased the concentrations of Ca, Mn, Zn, and Cu in the roots but decreased their concentrations in the shoots of both varieties. Overall, the results indicated that high rhizospheric pH influenced nutrient uptake and translocation from the roots to the shoots in rice.