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

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.