Cell-specific production and antimicrobial activity of naphthoquinones in roots of Lithospermum erythrorhizon

Pigmented naphthoquinone derivatives of shikonin are produced at specific t imes and in specific cells of Lithospermum erythrorhizon roots. Normal pigm ent development is limited to root hairs and root border cells in hairy roo ts grown on "noninducing" medium, whereas induction of additional pigment p roduction by abiotic (CuSO4) or biotic (fungal elicitor) factors increases the amount of total pigment, changes the ratios of derivatives produced, an d initiates production of pigment de novo in epidermal cells. When the biol ogical activity of these compounds was tested against soil-borne bacteria a nd fungi, a wide range of sensitivity was recorded. Acetyl-shikonin and bet a-hydroxyisovaleryl-shikonin, the two most abundant derivatives in both Agr obacterium rhizogenes-transformed "hairy-root" cultures and greenhouse-grow n plant roots, were the most biologically active of the seven compounds tes ted. Hyphae of the pathogenic fungi Rhizoctonia solani, Pythium aphaniderma tum, and Nectria hematococca induced localized pigment production upon cont act with the roots. Challenge by R. solani crude elicitor increased shikoni n derivative production 30-fold. We have studied the regulation of this sui te of related, differentially produced, differentially active compounds to understand their role(s) in plant defense at the cellular level in the rhiz osphere.