Wheat α-gliadin and high-molecular-weight glutenin subunit accumulate in different storage compartments of transgenic soybean seed.

Abstract

Wheat seed storage proteins (prolamins) are important for the grain quality because they provide a characteristic texture to wheat flour products. In wheat endosperm cells, prolamins are transported from the Endoplasmic reticulum to Protein storage vacuoles through two distinct pathways-a conventional pathway passing through the Golgi apparatus and an unconventional Golgi-bypassing pathway during which prolamins accumulate in the ER lumen, forming Protein bodies. Unfortunately, transport studies conducted previously achieved limited success because of the seed-specificity of the latter pathway and the multigene architecture of prolamins. To overcome this difficulty, we expressed either of the two families of wheat prolamins, namely α-gliadin or High-molecular-weight subunit of glutenin, in soybean seed, which naturally lacks prolamin-like proteins. SDS-PAGE analysis indicated the successful expression of recombinant wheat prolamins in transgenic soybean seeds. Their accumulation states were quite different-α-gliadin accumulated with partial fragmentation whereas the HMW-glutenin subunit formed disulfide-crosslinked polymers without fragmentation. Immunoelectron microscopy of seed sections revealed that α-gliadin was transported to PSVs whereas HMW-glutenin was deposited in novel ER-derived compartments distinct from PSVs. Observation of a developmental stage of seed cells showed the involvement of post-Golgi Prevacuolar compartments in the transport of α-gliadin. In a similar stage of cells, deposits of HMW-glutenin surrounded by membranes studded with ribosomes were observed confirming the accumulation of this prolamin as ER-derived PBs. Subcellular fractionation analysis supported the electron microscopy observations. Our results should help in better understanding of molecular events during the transport of prolamins in wheat.