Uncovering the role of apocarotenoids in the arbuscular mycorrhiza symbiosis via mutation and knockdown of biosynthetic genes

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The arbuscular mycorrhiza(AM)-induced accumulation of certain apocarotenoids (C13 cyclohexenoneand C14 mycorradicin derivatives) is a ubiquitous phenomenon, whose importance and function in the AM symbiosis is being elucidated. Previous knockdown of two biosynthetic genes (DXS2 and CCD1) resulted in strong decreases of apocarotenoid accumulation correlated with an increase degenerating and dead arbuscules at the expense of mature ones. A current model on C13/C14 apocarotenoid mode of action proposing a role in a plantcontrolled degradation of nonfunctional arbuscules to optimize symbiotic performance and mutualism will be investigated further. A new target gene for pathway manipulation (CCD7) has emerged, which appears to constitute a crosspoint in C13/C14 and strigolactone hormone apocarotenoid biosynthesis. Mutants and knockdown transgenics for CCD7 and other pathway genes are available and additional ones will be generated and analyzed from Medicago truncatula. An AM-specific phosphate-transport mutant (mtpt4-1) producing nonfunctional arbuscules will be analyzed for a potential participation of apocarotenoids in its premature-degra-dation-of-arbuscules phenotype. Furthermore, additional downstream steps of apocarotenoid biosynthesis and metabolism will be identified and targeted for manipulation.