Maize calreticulin localizes preferentially to plasmodesmata in root apex

Using a polyclonal antibody raised against calreticulin purified and sequen ced from maize, we performed an immunocytological study to characterize put ative domain-specific subcellular distributions of endoplasmic reticulum (E R)-resident calreticulin in meristematic cells of maize root tip. At the li ght microscopy level, calreticulin was immunolocalized preferentially at ce llular peripheries, in addition to nuclear envelopes and cytoplasmic struct ures. Punctate labelling at the longitudinal walls and continuous labelling at the transverse walls was characteristic. Immunogold electron microscopy revealed plasmodesmata as the most prominently labelled cell periphery str ucture. In order to further probe the ER-domain-specific distribution of ma ize calreticulin at plasmodesmata, root apices were exposed to mannitol-ind uced osmotic stress. Plasmolysis was associated with prominent accumulation s of calreticulin at callose-enriched plasmodesmata and pit fields while th e contracting protoplasts were depleted of calreticulin. In contrast, other ER-resident proteins recognized by HDEL peptide and Sip antibodies localiz ed exclusively to contracted protoplasts. This finding reveals that, in pla smolysed cells, calreticulin enriched ER domains at plasmodesmata and pit f ields are depleted of other ER-resident proteins containing the HDEL retent ion peptide.