Fifteen mutant strains of. Chlamydomonas reinhardi were isolated which showed defects in some aspect oi the process of cell-wall formation. Genetic analyses indicated that most of the mutations were due to single gene changes; two were anomalous in that non-Mendelian segregations were obtained on crossing with other genotypes, and on selfing they frequently gave rise to wild-type phenotypes.Occasional somatic revertants were also obtained from these two strains. On the basis of these analyses it is suggested that there are two levels of control operating in the process of cell wall biogenesis -one concerned with subunit production at the nuclear level and another, possibly concerned with three-dimensional organization, at another level. Electronmicroscope analyses of the different mutants showed the mutants to be divided into three main categories: those in which the wall was formed but was not attached to the plasma membrane, those in which the wall was attached to the membrane, and those in which very little wall was formed. In the last class in particular, vesicles containing wall precursors were clearly visible, and were shed through the plasma membrane into the medium.
A spiroplasma isolated from citrus with little-leaf disease was grown in a cell-free medium and injected into leafhoppers (Euscelis plebejus). Injected leafhoppers, but not those fed on infected plants, transmitted the spiroplasma to white clover (Trifolium repens cv. S100) and sweet orange (Citrus sinensis cv. Valencia). Infected clover plants were severely stunted; infected sweet orange plants showed typical symptoms of citrus little-leaf disease. The spiroplasma was detected in clover and sweet orange plants by electron microscopy; the helical morphology of the organisms was most easily recognizable in sections 150-200 nm thick. The organism was re-isolated in cell-free media both from infected plants and from injected E. plebejus. The original isolate and those re-isolated from experimentally infected clover and sweet orange appeared by morphological, cultural, biochemical and serological criteria to be identical to each other and to the R8-A2 (type) and C-189 strains of Spiroplasma citri. Serological tests and electrophoretic analysis of protein preparations indicated no relationship to Acholeplasma laidlawii, although this organism survived for at least 10 wk after injection into E. plebejus. Our results show that the causal agent of little-leaf disease is related to S. citri.
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