The aims of the study were to investigate whether hydrogen gas (H2) was involved in regulation of anthocyanin biosynthesis in two contrasting radish (Raphanus sativus L.) varieties (low [LA] and high [HA] level of anthocyanin) under UV irradiation. The results showed that hydrogen-rich water (HRW) significantly blocked the UV-A-induced increase of H2O2 and O2(•-) accumulation, and enhanced the UV-A-induced increase of superoxide dismutase (SOD) and ascorbate peroxidase (APX) activities in LA and HA. Furthermore, UV-A-induced increase of anthocyanin and total phenols was further enhanced only in HA sprouts cotreated with HRW. LC-MS/MS analysis showed that five anthocyanidins existed in HA sprouts, but only two in LA sprouts. Meanwhile, the cyanidin was the most abundant anthocyanidin in HA, and the cyanidin was 2-fold higher cotreated with HRW than UV-A. Molecular analyses showed that the anthocyanin biosynthesis-related genes were upregulated significantly in both HA (in particular) and LA sprouts treated with HRW plus UV-A. These data imply that HRW reestablishes reactive oxygen species homeostasis in both LA and HA, but exerts different effects on anthocyanin accumulation between them under UV-A.

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GABA has beneficial effects on salinity stress tolerance in Arabidopsis linked to increased activity of H+-ATPase, reduced ROS-induced K+ efflux from root epidermis, and increased SOS1 and NHX1 transcript levels in plant roots.

Summary Soil salinity is a major constraint for the global agricultural production. For many decades, Na+ exclusion from uptake has been the key trait targeted in breeding programs; yet, no major breakthrough in creating salt‐tolerant germplasm was achieved. In this work, we have combined the microelectrode ion flux estimation (MIFE) technique for non‐invasive ion flux measurements with confocal fluorescence dye imaging technique to screen 45 accessions of barley to reveal the relative contribution of Na+ exclusion from the cytosol to the apoplast and its vacuolar sequestration in the root apex, for the overall salinity stress tolerance. We show that Na+/H+ antiporter‐mediated Na+ extrusion from the root plays a minor role in the overall salt tolerance in barley. At the same time, a strong and positive correlation was found between root vacuolar Na+ sequestration ability and the overall salt tolerance. The inability of salt‐sensitive genotypes to sequester Na+ in root vacuoles was in contrast to significantly higher expression levels of both HvNHX1 tonoplast Na+/H+ antiporters and HvVP1 H+‐pumps compared with tolerant genotypes. These data are interpreted as a failure of sensitive varieties to prevent Na+ back‐leak into the cytosol and existence of a futile Na+ cycle at the tonoplast. Taken together, our results demonstrated that root vacuolar Na+ sequestration but not exclusion from uptake played the main role in barley salinity tolerance, and suggested that the focus of the breeding programs should be shifted from targeting genes mediating Na+ exclusion from uptake by roots to more efficient root vacuolar Na+ sequestration.

Stronger Na+ extrusion and vacuolar sequestration are essential to confer better salt tolerance in bread wheat than in durum wheat. Removal of the root meristems increased salt sensitivity in wheat.

The cross talk among hydrogen peroxide (H2O2), nitric oxide (NO) and UV RESISTANCE LOCUS8 (UVR8) in UV-B-induced anthocyanin accumulation in the hypocotyls of radish sprouts was investigated. The results showed that UV-B irradiation significantly increased the anthocyanin accumulation and the expression of UVR8, and a similar trend appeared in radish sprouts subjected to cadmium, chilling and salt stresses regardless of light source. However, these responses disappeared under dark exposure. These results suggest that abiotic stress-induced anthocyanin accumulation and UVR8 expression were light-dependent. Moreover, abiotic stresses all enhanced the production of H2O2 and exogenous H2O2 addition significantly increased the anthocyanin concentration and UVR8 transcription, while these increases were severely inhibited by addition of dimethylthiourea (DMTU, a chemical trap for H2O2). It seems to suggest that H2O2 played an important role in the anthocyanin biosynthesis. Furthermore, addition of 0.5 mM sodium nitroprusside (SNP, a NO-releasing compound) substantially induced the anthocyanin accumulation, and H2O2-induced anthocyanin accumulation and UVR8 expression were significantly suppressed by co-treatment with 2-phenyl-4,4,5,5-tetramethylimidazoline-3-oxide-1-oxyl (PTIO, a NO scavenger), which was parallel with the expression of anthocyanin biosynthesis-related transcription factors and structural genes. All these results demonstrate that both H2O2 and NO are involved in UV-B-induced anthocyanin accumulation, and there is a crosstalk between them as well as a classical UVR8 pathway.

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