| Popis: |
In the developing chloroplast, many cytoplasmic-made components must cross the plastid envelope to reach functional sites within the organelle. Data from cell-free systems have led to several hypotheses to explain the selection and the passage of proteins through the envelope (A. Waksman et al, 1980, R.J. Ellis, 1981). In in vitro systems, the role of structural membrane relationships which might intervene in the movement of material from one site to another cannot be assessed. We have analysed developing cells in vivo in order to determine if and how structural modifications within or between plastids and other cytoplasmic organelles might be involved in a transport mechanism. We present evidence of junctions between nuclear envelope, endoplasmic reticulum and plastid outer envelope membranes and their subsequent fusion leading to a continuous channelled pathway through the cytoplasm. Within the plastid, an “elasticity” of the envelope membranes leads to envaginations and invaginations ensuring continuity to the stroma or the developing thylakoids. The high frequency of ER/plastid conjoints at very early developmental stages suggests that intra-cellular membrane relations may be a vehicule for some protein “traffic” between the site of synthesis and the site of use. |
