The molecular basis of cereal grain proteostasis

Abstract

Storage proteins deposited in the endosperm of cereal grains are both a nitrogen reserve for seed germination and seedling growth and a primary protein source for human nutrition. Detailed surveys of the patterns of storage protein accumulation in cereal grains during grain development have been undertaken, but an in-depth understanding of the molecular mechanisms that regulate these patterns is still lacking. Accumulation of storage proteins in cereal grains involves a series of subcellular compartments, a set of energy-dependent events that compete with other cellular processes, and a balance of protein synthesis and protein degradation rates at different times during the developmental process. In this review, we focus on the importance of rates in cereal grain storage protein accumulation during grain development and outline the potential implications and applications of this information to accelerate modern agriculture breeding programmes and optimize energy use efficiency in proteostasis.

Keywords: cereal grain development; proteases; proteostasis; storage protein.

Figure 1. Cereal grain synthesis, degradation, and deposition of storage proteins

(A) The synthesis and degradation of cereal grain storage proteins as typically observed in maize, rice, and wheat. Storage proteins are generally synthesized on the rER and then translocated into the rER lumen by N-terminal signal peptides. Once signal peptides are cleaved off, a maturation process transforms nascent polypeptides into fully mature, stable, and storage proteins. Major events that participate in the maturation process include polypeptide folding [46,78], inter- and intra-disulfide bond formation [25,79–81], post-translational modifications [82–85], and protein sorting [28,32,86]. Mis-folded polypeptides are tagged by ubiquitin ligases and delivered to the 26S proteasome of the UPS for degradation and subsequent amino acid recycling. Properly folded mature protein aggregates are temporarily placed in PB before being transported into PSV for permanent storage, while irreversible aggregates are delivered to lysosomes for degradation and amino acid recycling [46,87]. (B) The major trafficking routes of storage proteins in cereal grains. There are three main trafficking routes in cereal grains to allow transport of storage proteins from rER to PSV [36]. The classic route 1, also known as the Golgi-dependent route, involves mature storage proteins, such as albumins and globulins, being transported into the Golgi, followed by the formation of the DV in the TGN and the merger of DV into the PSV. Prolamin aggregates are often deposited via route 2 and 3 that are categorized as Golgi-independent routes [29]. In route 2, ER-derived protein bodies containing storage protein aggregates are formed, which are then delivered to and fused with the PSV. Some storage proteins are deposited through route 3, in which protein aggregates are firstly gathered in the ER-derived PCA and then directly delivered into the PSV. Protein aggregates containing temporary protein vesicles like DV, PB, and PAC are released into the PSV via an autophagy-like process. Only the commonly shared mechanisms among major cereal crops are presented to give an overview of the process. Further detail of the sorting and trafficking mechanisms and the discussion of differences in these processes between cereal crop species are reviewed elsewhere [33–36]. Abbreviations: DV, dense vesicle; PB, protein body; PAC, precursor-accumulating vesicles; PSV, protein storage vacuole; rER, rough endoplasmic reticulum; SignalP, signal peptide; TGN, trans-Golgi network; Ub, ubiquitin ligases.

Figure 2. The accumulation profile of the four major protease and two major protease inhibitor classes in wheat grains during grain development

(A) The relative protease activities of four major protease families in wheat grain during grain development. Relative protease activities were summarized from previously reported enzyme assays [65] and are broadly consistent with recent quantitative proteome data [14]. (B) The fold change in protein abundance, total mRNA abundance (transcripts per million) and protein synthesis and degradation rates (in % change per day) of the four major protease families and the two protease inhibitor families during wheat grain development. The fold change in protein abundance and protein turnover rate data were originally reported by Cao et al. in 2022 [14], while the transcript data were originally reported by Pfeifer et al. in 2014 [67]. The averaged fold change in protein and the total mRNA abundance are presented for each family of proteins or transcripts. The median protein synthesis and degradation rates of each protein family during early- and middle grain filling stages from 7 to 17 DPA are also presented.

Figure 3. Role of protein turnover analysis in sustainable agricultural advancement through analysis of protein accumulation profiles

The abundance of proteins are directly determined by the relative rates of their synthesis and degradation. Adopting breeding strategies that include optimizing energy use efficiency of protein production (PEUE) in crop plants provides a valuable focus for breeding high quality crops in a more sustainable way. Targeting proteins with a specific stability status, selected from protein turnover data and/or protein accumulation profiles integrated by multi-omics data, could enhance PEUE and can be altered using either conventional cross breeding approaches or advanced gene editing technologies.