Low levels of ribosomal RNA partly account for the very high photosynthetic phosphorus-use efficiency of Proteaceae species

Proteaceae species in south-western Australia occur on phosphorus- (P) impoverished soils. Their leaves contain very low P levels, but have relatively high rates of photosynthesis. We measured ribosomal RNA (rRNA) abundance, soluble protein, activities of several enzymes and glucose 6-phosphate (Glc6P) levels in expanding and mature leaves of six Proteaceae species in their natural habitat. The results were compared with those for Arabidopsis thaliana. Compared with A.thaliana, immature leaves of Proteaceae species contained very low levels of rRNA, especially plastidic rRNA. Proteaceae species showed slow development of the photosynthetic apparatus (delayed greening'), with young leaves having very low levels of chlorophyll and Calvin-Benson cycle enzymes. In mature leaves, soluble protein and Calvin-Benson cycle enzyme activities were low, but Glc6P levels were similar to those in A.thaliana. We propose that low ribosome abundance contributes to the high P efficiency of these Proteaceae species in three ways: (1) less P is invested in ribosomes; (2) the rate of growth and, hence, demand for P is low; and (3) the especially low plastidic ribosome abundance in young leaves delays formation of the photosynthetic machinery, spreading investment of P in rRNA. Although Calvin-Benson cycle enzyme activities are low, Glc6P levels are maintained, allowing their effective use. Proteaceae species in south-western Australia occur on phosphorus- impoverished soils. Their leaves contain very low phosphorus levels, but have relatively high rates of photosynthesis and activities of enzymes involved in primary metabolism, suggesting they do not compromise their metabolic machinery in order to save P. In contrast, we show that low ribosome abundance contributes to the high phosphorus-use efficiency of these Proteaceae species in three ways: less ribosomes means less P investment; the rate of growth and, hence, demand for phosphorus is decreased; young leaves show a delay in the formation of the photosynthetic machinery, exhibiting very low plastidic ribosome abundance and spreading investment of phosphorus in ribosomes over a longer time. Commentary: