THE CELLULAR PATHWAY OF PHOTOSYNTHATE TRANSFER IN THE DEVELOPING WHEAT-GRAIN .3. A STRUCTURAL-ANALYSIS AND PHYSIOLOGICAL-STUDIES OF THE PATHWAY FROM THE ENDOSPERM CAVITY TO THE STARCHY ENDOSPERM

The cellular pathway of sucrose transfer from the endosperm cavity to the starchy endosperm of developing grains of wheat (Triticum turgidum ) has been elucidated, The modified aleurone and sub-aleurone cells ex hibit a dense cytoplasm enriched in mitochondria and endoplasmic retic ulum, Significantly, the sub-aleurone cells are characterized by secon dary wall ingrowths. Numerous plasmodesmata interconnect all cells bet ween the modified aleurone and starchy endosperm, The protonophore car bonylcyanide-m-chlorophenyl hydrazone (CCCP) slowed [C-14]sucrose upta ke by grain tissue slices enriched in modified aleurone and sub-aleuro ne cells but had no effect on uptake by the starchy endosperm, The flu orescent weak acid sulphorhodamine G (SRG) was preferentially accumula ted by the modified aleurone and sub-aleurone cells, and this uptake w as sensitive to CCCP, The combined plasma membrane surface areas of th e modified aleurone and sub-aleurone cells appeared to be sufficient t o support the ill vivo rates of sucrose transfer to the starchy endosp erm, Plasmolysis of intact excised grain inhibited [C-14]sucrose trans fer from the endosperm cavity to the starchy endosperm, The sulphydryl group modifier p-chloromercuribenzene-sulphonic acid (PCMBS) decrease d [C-14]sucrose uptake hy the modified aleurone and sub-aleurone cells but had little effect on uptake by the starchy endosperm. In, contras t, when PCMBS and [C-14]sucrose were supplied to the endosperm cavity of intact excised grain, PCMBS slowed accumulation by all tissues equa lly, Estimates of potential sucrose fluxes through the interconnecting plasmodesmata were found to be within the published range, It is conc luded that the bulk of sucrose is accumulated from the endosperm cavit y by the modified aleurone and sub-aleurone cells and subsequently tra nsferred through the symplast to the starchy endosperm.