Bundle sheath cells are responsible for direct root regeneration from leafexplants of Helianthus occidentalis L.

Direct root organogenesis from isolated leaf explants of Helianthus occiden talis L. was induced using MS culture media supplemented with alpha-naphtha lene acetic acid (NAA) and benzyladenine (BA). The highest numbers of roots were formed de novo from bundle sheath cells located around leaf veins whe n explants were cultured on induction medium supplemented with 2.7 mu mol/L NAA and 0.44 mu mol/L BA. The structural events accompanying reactivation and division of competent bundle sheath cells and cells derived from them w ere investigated employing light, transmission and scanning electron micros copy in time course experiments. Vein parenchyma and bundle sheath cells lo cated around leaf veins started to divide first after 2 days of culture. Ve in parenchyma cells, however, ceased their divisions by day 5, while reacti vated bundle sheath cells produced cambium-like cells. These later cells ga ve origin to circular meristematic tissue, and subsequently to root primord ia. Root primordia appeared after 7 days and regenerated roots emerged from leaf explants after 15 days of culture. Ultrastructure of reactivated rege neration competent cells is described and is compared with other examples o f regeneration in vitro.