The rice crop is considered as a strategic commodity in most Asian countries due to its importance in providing food security and introducing employment opportunities. Water and nitrogen accessibility are two of the most important environmental factors in determining the productivity of rice. Of all required nutrients, plants need nitrogen in the largest quantities, as nitrogen is a crucial component of proteins, enzymes, nucleotides, and plant hormones. Even though it is a prominent component of food security, rice production is projected to deteriorate due to climate change and natural resource depletion. Achieving food security along with the efficient use of resources are the main challenges in rice production.
In this study, Oryza sativa cv. Nipponbare was exposed to a prolonged water stress (25 days) in the presence of two levels of nitrogen supplementation. Leaves from water-stressed (40% field capacity) and well-watered (100% field capacity) plants grown in 500 and 1500 mg nitrogen supplementation per kilogram of soil were harvested. Proteins were extracted and peptides and phosphopeptides enriched using TiO2 chromatography were analysed by nanoLC-MS/MS analysis coupled with label-free quantitation.
We hypothesized that phosphorylation, as one of the important post-translational modifications (PTMs), would play significant roles in addition to the proteome rearrangements observed in response to prolonged environmental changes. Extensive changes in phosphorylation were seen in transporter proteins, such as nitrogen-dependent phosphorylation of aquaporin PIP2-6, suggesting the regulatory role of PTM in leaf cell water maintenance. Dephosphorylation of channel proteins was detected in the droughted rice plants, which is mostly in correlation with inhibition of their activity for those well-characterized phosphoproteins such as plasma membrane ATPase and aquaporin PIP2-6.
Our study also revealed that phosphorylation in RNA processing proteins and those involved in carbohydrate metabolism may regulate the signalling cascades required for plant response to drought and nitrogen resources. In the comparison of well-watered and droughted plants grown in 500mg nitrogen, two isomers of sucrose-phosphate synthase, and two of GAPC (Glyceraldehyde-3-phosphate dehydrogenase C), were found to be differentially phosphorylated. Phosphorylation changes have been proposed as the main initial factor for stress-dependent translocation of GAPC to the nucleus. The results of this study show that phosphorylation plays an important signalling role in rice acclimation to drought.