Tight control of sulfur assimilation: an adaptive mechanism for a plant from a severely phosphorus-impoverished habitat.

Hakea prostrata (Proteaceae) has evolved in extremely phosphorus (P)-impoverished habitats. Unlike species that evolved in P-richer environments, it tightly controls its nitrogen (N) acquisition, matching its low protein concentration, and thus limiting its P requirement for ribosomal RNA (rRNA). Protein is a major sink for sulfur (S), but the link between low protein concentrations and S metabolism in H. prostrata is unknown, although this is pivotal for understanding this species' supreme adaptation to P-impoverished soils. Plants were grown at different sulfate supplies for 5 wk and used for nutrient and metabolite analyses. Total S content in H. prostrata was unchanged with increasing S supply, in sharp contrast with species that typically evolved in environments where P is not a major limiting nutrient. Unlike H. prostrata, other plants typically store excess available sulfate in vacuoles. Like other species, S-starved H. prostrata accumulated arginine, lysine and O-acetylserine, indicating S deficiency. Hakea prostrata tightly controls its S acquisition to match its low protein concentration and low demand for rRNA, and thus P, the largest organic P pool in leaves. We conclude that the tight control of S acquisition, like that of N, helps H. prostrata to survive in P-impoverished environments.