Boron deficiency increases expressions of asparagine synthetase, glutamate dehydrogenase and glutamine synthetase genes in tobacco roots irrespective of the nitrogen source

Abstract

Nitrate-ammonium (N-A) and ammonium-nitrate (A-N) transition experiments were performed with tobacco (Nicotiana tabacum, cv Gatersleben) plants to study whether the increased expression of ammonium assimilation-related genes found under short-term boron (B) deficiency is maintained when the nitrogen source changes. Asparagine synthetase (AS), glutamate dehydrogenase (GDH) and glutamine synthetase (GS) gene overexpressions in roots under B deprivation were detected in both nitrogen transitions, although GS and, especially, AS up-regulations were observed earlier in the ammonium-nitrate transition. Transcript levels were inversely correlated with hexose contents after 24 h of B deprivation. Our results suggest that tobacco roots respond to short-term B deficiency by increasing the expression of AS, GDH and GS genes irrespectively of the nitrogen source. A hypothetical model to explain these results is proposed and discussed. Thus, the combined action of these enzymes would avoid the accumulation of ammonia and help maintain the activity of the tricarboxylic acid cycle when tobacco roots are affected by B deficiency, the plant organ that first senses this stress. Therefore, under B deficiency, they would act as an ammonium detoxifying mechanism, being a complement to the glutamine synthetase/glutamate synthase (GS/GOGAT) pathway. We propose that the protective role of these enzymes could be extended to changing nitrogen-supply conditions.

The Sulphur Response in Wheat Grain and Its Implications for Acrylamide Formation and Food Safety

“…The importance of a balance between nitrogen and other nutrients on asparagine levels has also been discussed by Beato et al [38], in which the authors propose a model to explain asparagine accumulation in tobacco roots during boron deficiency. In this model, boron deficiency increases proteolysis and reduces cellular hexose, thereby creating a high ammonia to hexose ratio.…”

Stress, nutrients and genotype: understanding and managing asparagine accumulation in wheat grain

“…As a result, we recommend the application of sulphur at a rate of 20 kg per hectare to all wheat destined for human consumption (Raffan et al 2020). Sulphur deficiency may increase asparagine levels as a result of increased proteolysis and energy stress, in order to mobilise nitrogen and detoxify ammonia as outlined above (Beato et al 2014;Dong et al 2017), or to store nitrogen for synthesis of sulphur-rich proteins when sulphur availability increases (Zhao et al 1999). Similarly to the relationship between nitrogen and asparagine, the effect of sulphur on asparagine levels also varies greatly depending on the wheat genotype (Curtis et al 2018b).…”

“…Detoxification of ammonia may also be an important function of asparagine accumulation when nitrogen (N) is abundant and during stress when ammonia accumulates Oddy et al CABI Agric Biosci (2020) 1: 10Asparagine accumulation may also play a role in nitrogen remobilisation and ammonia detoxification during abiotic stress. Beato et al (2014) suggest that asparagine accumulation occurs for these reasons as a result of a high ammonia to hexose ratio in the cell. Many different abiotic stressors can induce general energy stress (Lastdrager et al 2014) and proteolysis (Hildebrandt et al 2015), causing decreases in cellular hexose levels and increases in ammonia.…”

Role of boron and its interaction with other elements in plants

“…Nevertheless, as new molecular biology techniques arise together with bioinformatics, it becomes increasingly interesting to try and elucidate the interaction of B with different elements at a molecular level. Thus, it has been reported that B deficiency affects the transcriptional level of genes related to nitrate assimilation ( Camacho-Cristóbal et al., 2011 ; Beato et al., 2014 ). For example, in root the mRNA concentration of NRT2 ( High Affinity Nitrate Transporter ) and leaf NIA ( Nitrate Reductase ) genes are low in tobacco ( Nicotina tabacum ) plants subjected to severe B deficiency, compared to control samples ( Camacho-Cristóbal and Gonzalez-Fontes, 1999 ; Camacho-Cristóbal and Gonzalez-Fontes, 2007 ) ( Table 2 ).…”