Expression and transport of cellulase in avocado mesocarp during ripening

Abstract

Cellulase localization in the mesocarp of ripening avocado fruits (Persea americana Mill. cv. Hass) was studied by a variety of immunological methods. As localized by immunodetection on whole fruit tissue blots, cellulase first appeared near the stylar end of the fruit during the late portion of the rise in climacteric respiration. Cellulase appearance subsequently expanded outward and upward, reaching the peduncle end of the fruit by the day after the climacteric peak. Cellulase expression in cells surrounding vascular bundles was delayed relative to expression in the adjacent mesocarp. Immuno-labeled frozen sections, viewed with the light microscope, showed that cellulase appeared in both parenchyma and oil cells concom-mitantly with wall breakdown. Immunogold detection of cellulase by electron microscopy revealed labeling associated with the endoplasmic reticulum, plasmodesmata, and cell wall during the period between the late portion of the climacteric rise and the day after the climacteric peak. Cellulase appeared in the nucleus during all stages of ripening after the early portion of the climacteric rise. Immunoblot analysis of organelle fractions, isolated from avocado fruit at the climacteric peak of respiration, revealed three molecular weight forms of cellulase; a light and a heavy form found in endoplasmic reticulum-enriched fractions, and an intermediate form found in Golgi and plasma membrane-enriched fractions.