The influence of nitrogen on the development and accumulation of protein bodies in the developing endosperm of wheat caryopses

New insights into the mechanism of storage protein biosynthesis in wheat caryopsis under different nitrogen levels

Effect of different nitrogen levels (0, 150, and 300 kg hm^-2) at booting stage on storage protein biosynthesis and processing quality of wheat was investigated using microstructural and ultrastructural observation, RNA sequencing, and quality analysis in this study. The results showed that the storage protein genes encoding ω- and γ-gliadin and low molecular weight glutenin subunit were upregulated at N150, and the genes encoding α- or β-gliadin and avenin-like protein were upregulated at N300. Two nitrogen levels induced expression of some interesting regulating genes, such as USE1, STX1B_2_3, SEC23, SEC24, SEC61A, HSP A1_8, HSP20, and HSP90B/TRA1. These regulatory genes were enriched in the KEGG pathway protein export, SNARE interactions in vesicular transport, and protein processing in endoplasmic reticulum. The amount, morphology, and accumulation pattern of protein body in four different endosperm regions in developing caryopsis show different response to N150 and N300, of which N300 had greater influence than N150. N150 and N300 both enhanced the contents of protein components, endosperm fullness, grain hardness, and parameters of processing quality, with the latter showing a greater degree of influence. Contrary to the accumulation pattern of protein body, N300 reduced the ratio of the amount of starch granules to the area ratio of protein body to starch granule. Results suggested that the difference of different nitrogen levels affecting storage protein biosynthesis might be through affecting the expression of the encoding and regulating gene of storage protein.

Changes in spatiotemporal protein and amino acid gradients in wheat caryopsis after N-topdressing

Wheat grain nitrogen content displays large variations within different pearling fractions of grains because of radial gradients in the protein content. We identified how spatiotemporal mechanisms regulate this. The protein gradients emerged clearly at 19 days after anthesis, with the highest N content in aleurone and seed coat, followed by outer endosperm, whereas the lowest was in middle and inner endosperm. Laser microdissection, qRT-PCR and LC-MS were used to dissect tissue from aleurone, outer endosperm, middle endosperm, inner endosperm and transfer cells, measure gene expression and levels of free and protein-bound amino acids, respectively. The results showed that different FAA transportation pathways worked in parallel during grain filling stage while the grain protein gradient did not follow spatial expression of storage proteins. Additionally, two nitrogen (N) topdressing timings were conducted, either at the emergence of top third leaf (standard timing) or top first leaf (delayed timing), finding that delayed N topdressing enhanced both amino acids supply and protein synthesis capacity. The results provide insight into protein synthesis and amino acid transport pathways in endosperm and suggest targets for the enhancement of specialty pearled wheat with higher quality.

Novel insights into the effect of nitrogen on storage protein biosynthesis and protein body development in wheat caryopsis

Molecular and cytological mechanisms concerning the effects of nitrogen on wheat (Triticum aestivum L.) storage protein biosynthesis and protein body development remain largely elusive. We used transcriptome sequencing, proteomics techniques, and light microscopy to investigate these issues. In total, 2585 differentially expressed genes (DEGs) and 57 differentially expressed proteins (DEPs) were found 7 days after anthesis (DAA), and 2456 DEGs and 64 DEPs were detected 18 DAA after nitrogen treatment. Gene ontology terms related to protein biosynthesis processes enriched these numbers by 678 and 582 DEGs at 7 and 18 DAA, respectively. Further, 25 Kyoto Encyclopedia of Genes and Genomes pathways were involved in protein biosynthesis at both 7 and 18 DAA. DEPs related to storage protein biosynthesis contained gliadin and glutenin subunits, most of which were up-regulated after nitrogen treatment. Quantitative real-time PCR analysis indicated that some gliadin and glutenin subunit encoding genes were differentially expressed at 18 DAA. Structural observation revealed that wheat endosperm accumulated more and larger protein bodies after nitrogen treatment. Collectively, our findings suggest that nitrogen treatment enhances storage protein content, endosperm protein body quantity, and partial processing quality by altering the expression levels of certain genes involved in protein biosynthesis pathways and storage protein expression at the proteomics level.