Indigenous Proline is a Two-Dimensional Safety-Relief Valve in Balancing Specific Amino Acids in Rice under Hexavalent Chromium Stress

Abstract

This study highlights the roles of different N sources (N starvation [−N], nitrate [+NO 3 − ], and ammonium [+NH 4 + ]) in specific amino acids involved in rice proline (Pro) metabolism under hexavalent chromium [Cr(VI)] stress. In treatment with Cr(VI) − N, the content of glutamate (Glu) in shoots was comparable to that of Pro, while the content of Glu in the roots was significantly higher than that of Pro. In case of treatment with Cr(VI) + NO 3 − /NH 4 + , the content of Glu in both tissues was significantly higher than that of Pro, suggesting that the synthesis of γ-aminobutyric acid and glutathione was most likely involved. Pro synthesis in rice is mainly derived from the Glu pathway rather than the ornithine (Orn) pathway, which is strongly affected by different N sources. The genetic regulation network of Pro metabolism demonstrated a good signal for describing amino acid balance in rice under Cr(VI) stress.

Proline-mediated regulation on jasmonate signals repressed anthocyanin accumulation through the MYB-bHLH-WDR complex in rice under chromium exposure

“…The seeds of rice ( Oryza sativa L. XZX 45) were obtained from the Hunan Academy of Agricultural Sciences, which were prepared as described in our previous work ( Zhang et al, 2021 ). In brief, rice seeds were soaked for 12 h before cultivation, then placed in clean river sand inside plastic cups, and deployed in an artificial climate chamber to maintain stable running conditions (light: 20,000 lux, temperature: 25°C ± 0.5°C and humidity: 60% ± 2%).…”

“…Two different treatments were conducted: (1) Cr(VI) treatment: 16-days old seedlings were grown in nutrient solution containing 0, 2.0, 8.0, and 16.0 mg Cr/L for 2-days exposure; (2) “Pro + Cr(VI)” treatment: 16-days old seedlings were first pretreated in Pro solution (1 mM) after soaking for 12 h ( Yang et al, 2021 ), and then transferred into the nutrient solution containing 0, 2.0, 8.0, and 16.0 mg Cr/L for 2-days exposure. Three different effective concentrations of Cr(VI; i.e., EC 20 , EC 50 , and EC 75 ) were used based on the inhibition rate with 20%, 50%, and 75% of relative growth rate during 2-days exposure ( Zhang et al, 2021 ). The pH of solution was approximately 6.7 to 6.9 during the entire period of exposure.…”

“…To lessen water loss and repress the growth of algae, each flask was wrapped with aluminum foil. Four independent replicates were deployed in each experiment, and 10 seedlings of similar size were taken for each replicate ( Zhang et al, 2021 ). The grade of all chemicals used was analytical purity (AR).…”

“…Numerous researches reported that soil Cr contamination results in a remarkable decrease in the yield and quality of rice ( Ma et al, 2016 ; Infante et al, 2021 ; Xiao et al, 2021 ). One of the most frequently detected modes of Cr(VI) phytotoxicity is to stimulate over-production of reactive oxygen species (ROS) in plant tissues that destroy the plant’s internal defense system and cause oxidative damage, whereas irreversible modification of proteins, lipid peroxidation, and DNA lesion is detectable, and eventually retards plant growth and even causes cell death ( Gomes et al, 2017 ; Fan et al, 2020 ; Nie et al, 2021 ; Zhang et al, 2021 ).…”

“…Sadak et al (2012) observed that exogenous arginine can enhance the photosynthesis system and regulate the physiological performance of sunflowers under salt stress. It is intriguing to notice that the function of proline (Pro) in plants is manifold compared with other amino acids, such as osmolyte balancer, free radical scavenger, and macromolecules stabilizer ( Zhang et al, 2015 , 2021 ). Previous studies suggested that exogenous Pro positively regulated the enzyme activities of the ascorbate-glutathione cycle in tobacco under salt stress, and minimized the negative effect ( Hoque et al, 2007 ).…”