HalALMT1 mediates malate efflux in the cortex of mature clusterrootlets of Hakea laurina, occurring naturally in severelyphosphorus-impoverished soil

Hakea laurina, a woody Proteaceae, naturally occurs in severely phosphorus (P)-impoverished habitats in southwest Australia. It develops distinctive cluster roots that exhibit a high capacity for carboxylate exudation and acid phosphatase activity, contributing to its P acquisition. However, the molecular mechanisms underlying these physiological functions remain poorly understood. We explored the cluster-root transcriptome using de novo RNA-Seq and identified Hakea laurina Aluminum-activated Malate Transporter 1 (HalALMT1), encoding an aluminum (Al)-activated malate transporter induced in mature cluster roots. We characterized HalALMT1 through electrophysiological assays and overexpression in Arabidopsis thaliana, and localized HalALMT1 expression, acid phosphatase activity, and suberized boundaries in cluster roots. Differentially expressed genes highlighted multiple increased carboxylate-related processes at cluster-root maturity. HalALMT1 released malate, an activity further enhanced by exposure to Al3+. Notably, HalALMT1 was specifically expressed in mature cortex cells of cluster rootlets, which lack a suberized exodermis. Acid phosphatase activity was pronounced throughout the cluster rootlets, unlike in noncluster roots where it was limited to the epidermis and stele. Substantial malate release and acid phosphatase activity in the cortex cells in cluster rootlets, which lack a suberized exodermis, allowed massive exudation. This study sheds light on an exquisite P-acquisition strategy of Proteaceae, enabling survival under extremely low P availability.