CHARACTERIZATION OF THE PLASTIDS AFFECTED BY CLASS-II ALBINO MUTATIONS OF MAIZE AT THE MORPHOLOGICAL AND TRANSCRIPT LEVELS

Class II albino mutations have been defined to be ones that do not aff ect pigmentation in endosperm but show a very pale yellow ('off-white' ) color in seedlings (ROBERTSON, 1975). While class I albinos with whi te-endosperm and white-seedlings have been extensively studied with re spect to carotenoid biosynthesis, little study has been done on class II albino mutants. To obtain molecular insights into the possible role s of the class II nuclear genes in plastid biogenesis during leaf deve lopment: we screened and characterized new class II albino mutants der ived from transposon Mu and Ac lines. Based on Northern analysis with 61 class II albinos, are selected 14 mutants that had the most substan tial alterations in levels and sizes of plastid transcripts. The 14 mu tants had almost identical plastid transcript patterns to those of w1 (white 1), a know class II albino, but were distinctly different from those of ho (lemon white), a class I albino. By using electron microsc opy, we examined the morphology of plastids affected by three of the c lass II mutable albinos. The plastids of the mutant tissues were devoi d of most of the internal membranes. The most prominent internal struc tures found in the affected plastids were either vesicle-like membrane s or a few lamellae of various sizes. The lamellae were either in para llel groups or scattered. The plastids of older cells contained less l amellar structures than those of younger cells. The plastid morphologi es of the class II mutants were different from that of w3 (white 3), a class I albino, grown under the same conditions. All the plastids in revertant cells showed completely normal membrane structures while adj acent mutant cells contained typical mutant plastids. This observation indicates that the genes responsible for class II albinism act on pla stid development in a cell-autonomous fashion. Our study shows that a subset of the class II albino mutants impair the expression of plastid genomes and the development of plastids in a unique and distinct way. New mutable class II albinos reported here may provide molecular acce ss to the nuclear genes which play significant roles in plastid biogen esis during leaf development.