The <i>cis</i>‐acting <scp>CTTC</scp>–P1<scp>BS</scp> module is indicative for gene function of <i><scp>L</scp>j<scp>VTI</scp>12</i>, a <scp>Q</scp>b‐<scp>SNARE</scp> protein gene that is required for arbuscule formation in <i><scp>L</scp>otus japonicus</i>

Abstract

Summary The majority of land plants live in symbiosis with arbuscular mycorrhizal fungi from the phylum G lomeromycota. This symbiosis improves acquisition of phosphorus ( P ) by the host plant in exchange for carbohydrates, especially under low‐ P availability. The symbiosome, constituted by root cortex cells accommodating arbuscular mycorrhizal fungal hyphae, is the site at which bi‐directional exchange of nutrients and metabolites takes place. Uptake of orthophosphate ( P i) in the symbiosome is facilitated by mycorrhiza‐specific plant P i transporters. Modifications of the potato P i transporter 3 ( StPT3 ) promoter were analysed in transgenic mycorrhizal roots, and it was found that the CTTC cis‐ regulatory element is necessary and sufficient for a transcriptional response to fungal colonization under low‐Pi conditions. Phylogenetic footprinting also revealed binary combination of the CTTC element with the Pi starvation response‐associated PHR 1‐binding site (P1 BS ) in the promoters of several mycorrhiza‐specific Pi transporter genes. Scanning of the L otus japonicus genome for gene promoters containing both cis ‐regulatory elements revealed a strong over‐representation of genes involved in transport processes. One of these, L j VTI 12 , encoding a member of the SNARE family of proteins involved in membrane transport, exhibited enhanced transcript levels in L otus roots colonized with the arbuscular mycorrhizal fungus G lomus intraradices . Down‐regulation of L j VTI 12 by RNA interference resulted in a mycorrhiza‐specific phenotype characterized by distorted arbuscule morphology. The results highlight cooperative cis ‐regulation which integrates mycorrhiza and Pi starvation signaling with vesicle trafficking in symbiosome development.