Plastid development and floridean starch grain formation during carposporogenesis in the coralline red alga Lithothrix aspergillum Gray

The development of the chloroplasts and the formation of floridean starch grains during carposporegenesis in the coralline algaLithothrix aspergillum Gray, was studied by electron microscopy. Proplastids are first recognized next to the plasmalemma in the youngest cell of the carposporangial filament. The peripheral thylakoid is the first internal membrane system observed in the proplastid. As the pro-plastid increases in size an irregular branched tubular membrane system, made up of membranous tubules 30–35 nm in diameter, forms in the DNA region of the plastid. This membrane system appears to function as a “pro-lamellar body” as the internal thylakoids are connected to it and appear ot form from it. As thylakoid development continues, the plastids enlarge and may divide, leaving one of the daughter plastids with the “pro-lamellar body”. Thylakoids develop either from the “pro-lamellar body”, or by extension and possibly duplication of existing thylakoids. At about the time that starch grain formation commences the thylakoids have arranged themselves into parallel groups and the single DNA containing region of the plastid has separated into a number of small areas of DNA fibrils destributed throughout the plastid. The earliest signs of phycobilisomes on the outside of the thylakoids are also seen at this stage. Starch grains develop in the cytoplasm, but in intimate association with the endoplasmic reticulum (ER) which passes on both sides of the developing starch grain, and which appears to be involved in starch grain formation. This close ER-starch grain association, can persist into the mature carpospore. At no stage has any close association between the ER and the chloroplasts been observed