The responses of normal (al+/al+) and albino mutant (aljal) seedlings of Plantago insularis to light and thiamine pyrophosphate (TPP) were studied by transmission electron microscopy.
Plastids in seedlings of normal and albino genotypes grown in darkness had the structure of etioplasts with a thylakoid plexus (prolamellar body) from which extend thylakoids. Some of the thylakoids were often associated into parallel or concentric sheets. In the mutant, the thylakoid sheets were generally shorter, and some were vesiculated. Exposure of normal plants to light resulted in the transformation of the etioplasts to chloroplasts, with the loss of the thylakoid plexus and the formation of typical grana and an intergranal thylakoid system, plus a few globules in the stroma. Exposure of the albino mutant to light caused the loss of the thylakoid plexus and the formation of highly ordered, concentrically associated thylakoid sheet systems, but no grana. After 24 h of illumination, the plastids of the albino mutant showed evidence of thylakoid degeneration and accumulation of globules and starch. These changes are interpreted as a consequence of very low TPP in the mutant and as evidence for the role of TPP in the degradation of glucose through the Krebs cycle in the normal genotype.
The application of TPP to the albino mutant resulted in the formation of essentially normal chloroplasts. The ultrastructural observations support the view that TPP deficiency does not handicap the mutant in protein synthesis until after exposure to light. TPP and light are essential for the formation of normal grana and TPP is in some way involved in the pathways for pigment and protein synthesis in chloroplasts.