Practice question

Plastids constitute a family of interrelated organelles arising from undifferentiated proplastids, with interconversion regulated by light, developmental cues and nuclear-encoded transcription factors like GLK. Leucoplasts are non-pigmented plastids predominantly found in roots, seeds, tubers and non-green tissues, serving as biosynthetic hubs rather than photosynthetic factories. Unlike chloroplasts and chromoplasts, they lack extensive thylakoid grana and photosynthetic apparatus but retain double envelope and stromal enzymes for fatty acid, amino acid, starch and secondary metabolite synthesis. Subclasses include amyloplasts for starch, elaioplasts for lipids and proteinoplasts for proteins. Importantly, colorless leucoplasts synthesize monoterpenes, diterpenes and isoprenoid precursors through plastidial methylerythritol phosphate, MEP, pathway providing geranylgeranyl diphosphate substrates for carotenoid and chlorophyll pigment production that later accumulate after differentiation into chromoplasts and chloroplasts. Chromoplasts then actively synthesize and store carotenoids such as lycopene, beta-carotene and lutein for fruit and flower coloration via phytoene synthase. This division highlights metabolic channeling among plastid types with leucoplasts supplying upstream steps.