Cell type-specific protoberberine alkaloid accumulation in Thalictrum flavum

Berberine is a common benzylisoquinoline alkaloid with potent antimicrobial properties, which suggest it functions to protect some plants from pathogen challenge. Berberine was identified as the major alkaloid in meadow rue (Thalictrum flavum ssp. glaucum), a medicinal member of the Ranunculaceae, and was detected in seeds and all organs of the plant. The high level of berberine in roots, rhizomes, and older petioles is mainly responsible for the intense yellow color of these organs. In rhizomes, protoberberine alkaloids were detected throughout the pith and, to a lesser extent, the cortex, but were absent from the vascular tissues. Similarly, protoberberine alkaloids were detected in the rib parenchyma of older petioles. In roots, alkaloid accumulation was detected only in the endodermis at the onset of secondary growth. Rather than being sloughed off, the endodermis was found to undergo extensive anticlinal division leading to an expanding cellular cylinder that ultimately displaced all external tissues. Endodermal-specific protoberberine alkaloid accumulation continued throughout root development, but was extended to include 3 to 4 layers of smaller pericycle cells in the oldest roots near the base of the stem. The cell type-specific accumulation of antimicrobial alkaloids and the unusual development of the endodermis and pericycle in T flavum roots support the putative role of berberine in plant defense.