Developmental Physiology of Cluster-Root Carboxylate Synthesis and Exudation in Harsh Hakea. Expression of Phosphoenolpyruvate Carboxylase and the Alternative Oxidase

Harsh hakea (Hakea prostrata R.Br.) is a member of the Proteaceae family, which is highly represented on the extremely nutrientimpoverishedsoils in southwest Australia. When phosphorus is limiting, harsh hakea develops proteoid or cluster roots thatrelease carboxylates that mobilize sparingly soluble phosphate in the rhizosphere. To investigate the physiology underlying thesynthesis and exudation of carboxylates from cluster roots in Proteaceae, we measured O2 consumption, CO2 release, internalcarboxylate concentrations and carboxylate exudation, and the abundance of the enzymes phosphoenolpyruvate carboxylaseand alternative oxidase (AOX) over a 3-week time course of cluster-root development. Peak rates of citrate and malateexudation were observed from 12- to 13-d-old cluster roots, preceded by a reduction in cluster-root total protein levels anda reduced rate of O2 consumption. In harsh hakea, phosphoenolpyruvate carboxylase expression was relatively constant incluster roots, regardless of developmental stage. During cluster-root maturation, however, the expression of AOX proteinincreased prior to the time when citrate and malate exudation peaked. This increase in AOX protein levels is presumablyneeded to allow a greater flow of electrons through the mitochondrial electron transport chain in the absence of rapid ATPturnover. Citrate and isocitrate synthesis and accumulation contributed in a major way to the subsequent burst of citrate andmalate exudation. Phosphorus accumulated by harsh hakea cluster roots was remobilized during senescence as part of theirefficient P cycling strategy for growth on nutrient impoverished soils.