Bundle Sheath Cells are Responsible for Direct Root Regeneration from Leaf Explants of Helianthus occidentalis L.

Summary Direct root organogenesis from isolated leaf explants of Helianthus occidentalis L. was induced using MS culture media supplemented with (x-naphthalene acetic acid (NAA) and benzyladenine (BA). The highest numbers of roots were formed de novo from bundle sheath cells located around leaf veins when explants were cultured on induction medium supplemented with 2.7 pmol/L NAA and 0.44 pmol/L BA. The structural events accompanying reactivation and division of competent bundle sheath cells and cells derived from them were investigated employing light, transmission and scanning electron microscopy in time course experiments. Vein parenchyma and bundle sheath cells located around leaf veins started to divide first after 2 days of culture. Vein parenchyma cells, however, ceased their divisions by day 5, while reactivated bundle sheath cells produced cambium-like cells. These later cells gave origin to circular meristematic tissue, and subsequently to root primordia. Root primordia appeared after 7 days and regenerated roots emerged from leaf explants after 15 days of culture. Ultrastructure of reactivated regeneration competent cells is described and is compared with other examples of regeneration in vitro.