Lateral roots (LRs) increase the contact area of the root with the rhizosphere and thereby improve water and nutrient uptake from the soil. LRs are generated either via a developmentally controlled mechanism or through induction by external stimuli, such as water and nutrient availability. Auxin regulates LR organogenesis via transcriptional activation by an auxin complex receptor. Endocytic trafficking to the vacuole positively regulates LR organogenesis independently of the auxin complex receptor in Arabidopsis (Arabidopsis thaliana). Here, we demonstrate that phosphatidylinositol 4-phosphate (PI4P) biosynthesis regulated by the phosphatidylinositol 4-kinases PI4KIIIb1 and PI4KIIIb2 is essential for the LR organogenesis driven by endocytic trafficking to the vacuole. Stimulation with Sortin2, a biomodulator that promotes protein targeting to the vacuole, altered PI4P abundance at both the plasma membrane and endosomal compartments, a process dependent on PI4K activity. These findings suggest that endocytic trafficking to the vacuole regulated by the enzymatic activities of PI4KIIIb1 and PI4KIIIb2 participates in a mechanism independent of the auxin complex receptor that regulates LR organogenesis in Arabidopsis. Surprisingly, loss-of-function of PI4KIIIb1 and PI4KIIIb2 induced both LR primordium formation and endocytic trafficking toward the vacuole. This LR primordium induction was alleviated by exogenous PI4P, suggesting that PI4KIIIb1 and PI4KIIIb2 activity constitutively negatively regulates LR primordium formation. Overall, this research demonstrates a dual role of PI4KIIIb1 and PI4KIIIb2 in LR primordium formation in Arabidopsis.
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