BIOCHEMICAL AUTONOMY OF HIGHER PLANT CHLOROPLASTS AND THEIR SYNTHESIS OF SMALL MOLECULES

Summary 1. Mature chloroplasts are able to synthesize a wide variety of compounds of low molecular weight in addition to carbohydrates. 2. Mature chloroplasts from higher plants can synthesize fatty acids from acetate, and galactolipids from UDP‐galactose; but, thus far, there is no direct evidence that chloroplasts can produce their principal fatty acid, α‐linolenate, independently of the rest of the cell. 3. Chloroplasts possess the enzymic machinery necessary to generate most of the common amino acids from inorganic nitrogen plus appropriate a‐keto analogs of amino acids. However, the plastids do not appear able to synthesize many α‐keto carbon compounds from the initial products of photosynthetic carbon dioxide fixation. 4. Whether chloroplasts can generate their own supply of acetate remains in doubt. 5. There is little evidence for or against the existence of chloroplastic enzymes catalysing synthesis of purines and pyrimidines. 6. Recent evidence confirms that immature plastids possess the complement of enzymes required for synthesis of protochlorophyllide from 8‐aminolaevulinic acid but leaves open the possibility that extrachloroplastic cofactors may be involved in protochlorophyllide biosynthesis. 7. The weight of the available evidence suggests that, despite its great metabolic versatility and possible reproductive autonomy, the chloroplast of the higher plant is not metabolically autonomous or nutritionally independent of the remainder of the plant cell. Therefore, if there is any validity to the oft‐repeated speculation that chloroplasts have evolved from ancient free‐living procaryotes, it appears that the evolution of the chloroplast has led to a considerable loss of nutritional autonomy concomitant with the development or preservation of photosynthetic competence.