ALUMINUM-ACTIVATED MALATE TRANSPORTER 1 (ALMT1) partially acidifies the rhizosphere in Pi deficiency-induced inhibition of primary root growth

The inhibition of primary root (PR) growth is a major developmental response of Arabidopsis (<i>Arabidopsis thaliana</i>) to phosphate (Pi) deficiency. Previously, our laboratory demonstrated that under Pi deficiency, a blue light-triggered malate-mediated photo-Fenton reaction and a canonical Fenton reaction in root apoplasts together form an Fe redox cycle, which results in Pi deficiency-induced inhibition of PR growth by continuously producing hydroxyl radicals (·OH). In this model, blue light, malate, Fe²⁺, Fe³⁺, H₂O₂, low pH, and low Pi are critical components, and the LPR1/LPR2 and STOP1-ALMT1 modules are key regulators that affect the occurrence and extent of these chemical reactions. However, whether the function of ALMT1 in the Pi deficiency-induced inhibition of PR growth relies on low pH in the rhizosphere and, conversely, whether ALMT1 is involved in regulating rhizosphere acidification remain elusive. Here, we show that low pH in the rhizosphere is required for malate-mediated inhibition of PR growth under Pi deficiency. Moreover, although not the principal factor, ALMT1 facilitates rhizosphere acidification under Pi deficiency. Our results shed new light on the function of ALMT1 and rhizosphere acidification under Pi deficiency.