title: Gamma-zeins are essential for endosperm modification in quality protein maize.
abstract: Essential amino acids like lysine and tryptophan are deficient in corn meal because of the abundance of zein storage proteins that lack these amino acids. A natural mutant, opaque 2 (o2) causes reduction of zeins, an increase of nonzein proteins, and as a consequence, a doubling of lysine levels. However, o2's soft inferior kernels precluded its commercial use. Breeders subsequently overcame kernel softness, selecting several quantitative loci (QTLs), called o2 modifiers, without losing the high-lysine trait. These maize lines are known as "quality protein maize" (QPM). One of the QTLs is linked to the 27-kDa gamma-zein locus on chromosome 7S. Moreover, QPM lines have 2- to 3-fold higher levels of the 27-kDa gamma-zein, but the physiological significance of this increase is not known. Because the 27- and 16-kDa gamma-zein genes are highly conserved in DNA sequence, we introduced a dominant RNAi transgene into a QPM line (CM105Mo2) to eliminate expression of them both. Elimination of gamma-zeins disrupts endosperm modification by o2 modifiers, indicating their hypostatic action to gamma-zeins. Abnormalities in protein body structure and their interaction with starch granules in the F1 with Mo2/+; o2/o2; gammaRNAi/+ genotype suggests that gamma-zeins are essential for restoring protein body density and starch grain interaction in QPM. To eliminate pleiotropic effects caused by o2, the 22-kDa alpha-zein, gamma-zein, and beta-zein RNAis were stacked, resulting in protein bodies forming as honeycomb-like structures. We are unique in presenting clear demonstration that gamma-zeins play a mechanistic role in QPM, providing a previously unexplored rationale for molecular breeding.
authors.affiliation: Waksman Institute of Microbiology, Rutgers University, Piscataway, NJ 08854, USA.
authors.forename: Yongrui
authors.initials: Y
authors.lastname: Wu
authors.forename: David R
authors.initials: DR
authors.lastname: Holding
authors.forename: Joachim
authors.initials: J
authors.lastname: Messing
mesh_terms: D001947:Breeding; D020090:Chromosome Segregation; D056625:Endosperm / Q000378:metabolism* / Q000648:ultrastructure; D055785:Gene Knockdown Techniques; D005838:Genotype; D046529:Microscopy, Electron, Transmission; D008954:Models, Biological; D010641:Phenotype; D020836:Protein Structure, Quaternary; D034622:RNA Interference; D019076:Transgenes / Q000235:genetics; D003313:Zea mays / Q000235:genetics / Q000378:metabolism* / Q000648:ultrastructure; D015028:Zein / Q000737:chemistry / Q000378:metabolism*
chemical_list: D015028:Zein