Reducing Rice Seed Storage Protein Accumulation Leads to Changes in Nutrient Quality and Storage Organelle Formation    

Abstract

Rice (Oryza sativa) seed storage proteins (SSPs) are synthesized and deposited in storage organelles in the endosperm during seed maturation as a nitrogen source for germinating seedlings. We have generated glutelin, globulin, and prolamin knockdown lines and have examined their effects on seed quality. A reduction of one or a few SSP(s) was compensated for by increases in other SSPs at both the mRNA and protein levels. Especially, reduction of glutelins or sulfur-rich 10-kD prolamin levels was preferentially compensated by sulfur-poor or other sulfur-rich prolamins, respectively, indicating that sulfurcontaining amino acids are involved in regulating SSP composition. Furthermore, a reduction in the levels of 13-kD prolamin resulted in enhancement of the total lysine content by 56% when compared with the wild type. This observation can be mainly accounted for by the increase in lysine-rich proteins. Although reducing the level of glutelins slightly decreased protein storage vacuoles (PSVs), the simultaneous reduction of glutelin and globulin levels altered the inner structure of PSVs, implicating globulin in framing PSV formation. Knock down of 13-kD prolamins not only reduced the size of endoplasmic reticulumderived protein bodies (PBs) but also altered the rugged peripheral structure. In contrast, PBs became slightly smaller or unchanged by severe suppression of 10-or 16-kD prolamins, respectively, indicating that individual prolamins have distinct functions in the formation of PBs. Extreme increases or decreases in sulfur-poor prolamins resulted in the production of small PBs, suggesting that the ratio of individual prolamins is crucial for proper aggregation and folding of prolamins.

Glutelin is partially degraded in globulin-less mutants of rice (Oryza sativaL.)

“…Iida et al (1993) and Ashida et al (2006) found a new glutelin band at ~32 kDa in the globulin-less mutant. No new polypeptide was evident, however, in the globulin RNAi knock-down transformants (Kawakatsu et al, 2010).…”

“…Analysis of the ultrastructure by TEM showed that the globulin-less mutant had deformed PB-II (Ashida et al, 2011), while a globulin RNAi knock-down transformants had cracked PB-IIs (Kawakatsu et al, 2010). Iida et al (1993) and Ashida et al (2006) found a new glutelin band at ~32 kDa in the globulin-less mutant.…”

“…Glutelin, globulin, and prolamin knock-down lines produced by Kawakatsu et al (2010) demonstrated the reduction in one or several storage proteins was compensated by other storage proteins preferentially in terms of sulfur-rich or sulfur-poor characteristics of the proteins. In this study, however, globulin-less mutation was unique to induce partial degradation of glutelin rather than compensation.…”

Increasing the production yield of recombinant protein in transgenic seeds by expanding the deposition space within the intracellular compartment

“…23 Furthermore, in transgenic rice, the suppression of genes encoding individual SSPs or a combination of several SSPs such as glutelins, prolamins and globulin significantly affects the expression of other seed protein genes, as well as PB formation, as rebalancing the proteome occurs to maintain the protein content of rice seeds. 19 Such a compensatory effect is regulated at both the transcriptional and translational levels. A similar situation was also reported in maize grains.…”

“…In particular, when seeds are deficient in PSV proteins such as glutelin and globulin, the production of sulfur-poor 13 kD prolamins is preferentially increased in rice. 19 Such preferential compensation may depend on the sulfur levels of the seed proteins. 20 Therefore, when the production of some SSPs is depressed using a knock-down approach (RNA interference and anti-sense RNA) or by mutation, the production of other types of endogenous seed proteins increases due to the compensatory or rebalancing mechanisms inherent in seeds, which maintain constant levels of total seed protein.…”