Micronutrients in plant tissue culture are required:
Micronutrients, also called trace elements, are required in quantities less than 0.5 millimolar, typically micromolar to nanomolar range, yet indispensable for viability and morphogenesis in vitro. Group includes iron 0.1 mM as Fe-EDTA for chlorophyll synthesis, cytochromes and Fe-S proteins of electron transport; manganese activating oxygen evolving complex of photosystem II and Mn superoxide dismutase; zinc structural component of zinc-finger transcription factors carbonic anhydrase alcohol dehydrogenase; boron crosslinking rhamnogalacturonan II in pectin maintaining wall integrity and membrane stability; copper in plastocyanin for photosynthesis and cytochrome oxidase for respiration; molybdenum essential for nitrate reductase and aldehyde oxidase cofactor; nickel for urease; chlorine for PSII water splitting; iodine and cobalt in minute amounts improving growth in some species. They function mainly as enzyme cofactors or prosthetic groups not as bulk structural mass. Despite low requirement, omission causes severe abnormalities stunted growth chlorosis and failure of embryo maturation, while excess induces toxicity, hence careful chelation and balanced formulation are critical for successful culture performance.
Ref: Taiz & Zeiger Chap. 5 mineral nutrition; Epstein & Bloom mineral nutrition.