Expression of phytoglobin affects nitric oxide metabolism and energy state of barley plants exposed to anoxia

Class 1 plant hemoglobins (phytoglobins) are upregulated during low-oxygen stress and participate in metabolism and cell signaling via modulation of the levels of nitric oxide (NO). We studied the effects of over expression and knockdown of the class 1 phytoglobin gene in barley (Hordeum vulgare L.) under low-oxygen stress. The overexpression of phytoglobin reduced the amount of NO released, while knockdown significantly stimulated NO emission. It has previously been shown that NO inhibits aconitase activity, so decreased aconitase activity in knockdown plants acts as a biomarker for high internal NO levels. The overexpression of phytoglobin corresponded to higher ATP/ADP ratios, pyrophosphate levels and aconitase activity under anoxia, while knockdown of phytoglobin resulted in the increased level of protein nitrosylation, elevation of alcohol dehydrogenase and nitrosoglutathione reductase activities. The overexpressing plants showed various signs of stunted growth under normoxia, but were the only type to germinate and survive under hypoxia. The results show that overexpression of phytoglobin protects plant cells via NO scavenging and improves their low-oxygen stress survival. However, it may not be useful for cereal crop improvement since it comes with a significant interference with normoxic NO signalling pathways.