The half-size ABC transporters STR1 and STR2 are indispensable for mycorrhizal arbuscule formation in rice

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Abstract

The central structure of the symbiotic association between plants and arbuscular mycorrhizal (AM) fungi is the fungal arbuscule that delivers minerals to the plant. Our earlier transcriptome analyses identified two half-size ABCG transporters that displayed enhanced mRNA levels in mycorrhizal roots. We now show specific transcript accumulation in arbusculated cells of both genes during symbiosis. Presently, arbuscule-relevant factors from monocotyledons have not been reported. Mutation of either of theOryza sativa(rice) ABCG transporters blocked arbuscule growth of different AM fungi at a small and stunted stage, recapitulating the phenotype ofMedicago truncatula stunted arbuscule 1and2(str1andstr2) mutants that are deficient in homologous ABCG genes. This phenotypic resemblance and phylogenetic analysis suggest functional conservation of STR1 and STR2 across the angiosperms. Malnutrition of the fungus underlying limited arbuscular growth was excluded by the absence of complementation of thestr1phenotype by wild-type nurse plants. Furthermore, plant AM signaling was found to be intact, as arbuscule-induced marker transcript accumulation was not affected instr1mutants. Strigolactones have previously been hypothesized to operate as intracellular hyphal branching signals and possible substrates of STR1 and STR2. However, full arbuscule development in the strigolactone biosynthesis mutantsd10andd17suggested strigolactones to be unlikely substrates of STR1/STR2. Interestingly, rice STR1 is associated with acis-natural antisense transcript (antiSTR1). Analogous toSTR1andSTR2, at the root cortex level, theantiSTR1transcript is specifically detected in arbusculated cells, suggesting unexpected modes of STR1 regulation in rice.

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