Sparing and Salvaging Metals in Chloroplasts

Of the essential inorganic minerals, the micronutrients (manganese, iron, copper, and zinc) have held special interest in plant physiology because of their chemistry, duality as both nutrient and toxin, availability, and biogeochemical history. The green tissues of plants readily exhibit visual symptoms of metal deficiency, and even before the role of metals in photosynthesis was understood, the importance of metals in plant nutrition was already evident. Chlorosis is a common symptom of poor metal nutrition, which manifests as a yellowing of green tissues owing to decreased abundance of chlorophyll. Because of this readily scored phenotype, the earliest work on trace metals focused on descriptive observations of metal nutrition. It was not until the early to mid-1800s with the popularization of the mineral theory of plant nutrition, that a serious look at the requirement of metal salts for plant growth and health began (Figure 1). In the 1840s, Eusébe Gris reported that the application of iron salts to either the roots or directly to the leaves of some chlorotic plants resulted in reversing the symptom. As a result, iron deficiency became synonymous with chlorosis. Julius von Sachs is accredited with establishing the essentiality of iron in plant growth, and 40-years later Benjamin Moore reported that iron is indeed in the chloroplast and proposed that it plays a direct role in photosynthesis. In the next 30 years, the essentiality of manganese, copper, and zinc in plant growth and more specifically in photosynthesis was