Desiccoplast–etioplast–chloroplast transformation under rehydration of desiccated poikilochlorophyllous Xerophyta humilis leaves in the dark and upon subsequent illumination

The transformation of desiccoplasts into etioplasts and the parallel appearance of protochlorophyllide (Pchlide) forms were observed with transmission electron microscopy and 77 K fluorescence spectroscopy, when air-dried detached leaves of the poikilochlorophyllous desiccation tolerant plant Xerophyta humilis were floated in water in the dark. After 1 week of rehydration, pregranal plastids with newly synthesized prothylakoid (PT) lamellae and mainly non-photoactive Pchlide forms developed, while etioplasts with prolamellar bodies (PLBs) and photoactive, 655 nm emitting Pchlide form accumulated primarily in the basal leaf regions after 2 weeks of regeneration. When these latter leaves were illuminated with continuous light for 3 days, the etioplasts transformed into regular chloroplasts and the fluorescence emission bands characteristic of green leaves appeared. These results show that, upon rehydration, the dehydrated chlorenchyma cells are able to regenerate pregranal plastids and etioplasts from desiccoplasts in the dark, which can transform into regular chloroplasts when they are illuminated. This means that the differentiation of pregranal plastids and etioplasts and their greening process is a basic property of fully differentiated cells of X. humilis. Consequently, these processes are not merely characteristic for seedlings with meristematic and differentiating young tissues.