A transgenic dTph1 insertional mutagenesis system for forward genetics in mycorrhizal phosphate transport of Petunia

The active endogenous dTph1 system of the Petunia hybrida mutator line W138 has been used in several forward-genetic mutant screens that were based on visible phenotypes such as flower morphology and color. In contrast, defective symbiotic phosphate (P(i)) transport in mycorrhizal roots of Petunia is a hidden molecular phenotype as the symbiosis between plant roots and fungi takes place below ground, and, while fungal colonization can be visualized histochemically, P(i) transport and the activity of P(i) transporter proteins cannot be assessed visually. Here, we report on a molecular approach in which expression of a mycorrhiza-inducible bi-functional reporter transgene and insertional mutagenesis in Petunia are combined. Bi-directionalization of a mycorrhizal P(i) transporter promoter controlling the expression of two reporter genes encoding firefly luciferase and GUS allows visualization of mycorrhiza-specific P(i) transporter expression. A population of selectable transposon insertion mutants was established by crossing the transgenic reporter line with the mutator W138, from which the P(i)transporter downregulated (ptd1) mutant was identified, which exhibits strongly reduced expression of mycorrhiza-inducible P(i) transporters in mycorrhizal roots.