Membranous modifications in sieve element plastids of spinach affected by the aster yellows disease

Abstract

The sieve element plastids of spinach plants ( Spinacia oleracea L.), affected by the aster yellows disease and containing the associated mycoplasma-like organism (MLO), show a variety of structural modifications expressed mainly in the amount and the organization of the internal membranes. Some plastids do not deviate from the normal, others contain extensive convoluted membranes, sometimes assuming spheroidal confirmations. A peculiar but apparently rare membranous modification consists of numerous double-membraned cisternae stacked parallel with the periphery of the plastid and surrounding an aggregation of vesicles. A developmental relation seems to exist between the peripheral membranes and the vesicles. All these modified membranous complexes deviate from those of the closely packed small dilated thylakoids in normal plastids. Some plastids degenerate without developing abnormal membrane conformations. Variable amounts of plastoglobules (osmiophilic globules) accumulate in the plastids. The modified plastids are larger than the normal ones. MLO may or may not be present in the sieve elements with modified plastids. Some forms of the degenerative modifications resemble those observed in sieve element plastids of spinach infected with the curly top virus.