Plastid-dividing Rings in Ferns

Plastid-dividing rings (PDs) are described for the first timein the ferns Ophioglossum, Gleichenia, Hymenophyllum, Trichomanes,Athyrium, Dryopteris, Ceratopteris and Pteridium. They are foundin the constricted isthmuses of undifferentiated plastids, amyloplasts,amylochloroplasts and chloroplasts in gametophytes and in arange of sporophytic tissues including apices, differentiatingleaf mesophyll and vascular parenchyma from roots, stems andleaves. Fern PDs comprise two concentric structures; a densegranular ring 60-100 nm wide adhering to the stromal face ofthe constriction and a smaller cytoplasmic annulus only 20-40nm wide. The diameters of the PD-containing constrictions, particularlythose in vascular parenchyma, where the outer component is usuallylacking, are extremely uniform (means of 177-188 nm in differentspecies and tissues) and closely in line with those in angiosperms.PDs in ferns differ from those previously described in othergroups of lands plants in that they are far more persistentand in multilobed plastids one is present at each constriction.The occurrence of multiple PDs in the giant plastids found inup to 30% of vascular parenchyma cells may be associated withorganelle fusion rather than division.Copyright 1993, 1999 AcademicPress Chloroplast division, division cycle, ferns, plastid-dividing rings, pteridophytes, transmission electron microscopy