DEVELOPMENTAL ANATOMY OF ROOTS INDUCED BY AGROBACTERIUM-RHIZOGENES INMALUS-PUMILA M26 SHOOTS GROWN-INVITRO

Malus pumila 'M.26' apple shoots grown in vitro were induced to root b y treating with naphthaleneacetic acid (NAA) or by infecting with Agro bacterium rhizogenes strain A4. A histological study was conducted to follow the development of the adventitious roots. In response to NAA a dventitious roots formed endogenously, initiating within the vascular tissues of the stem. A clear connection between roots and shoot was ap parent. Roots formed along the base of the stem, up to 2 mm from the c ut surface. Adventitious roots formed in response to infection with A. rhizogenes showed both endogenous and exogenous development. In endog enous root formation, callus formed within the cortex with subsequent formation of tracheid nests, causing a bulge in the stem. Exogenous ca llus, formed at the base of the shoot, also produced tracheid nests, a nd in some specimens was quite extensive. Roots developed from both of these callus structures. We hypothesized that the response to A. rhiz ogenes involves auxins through either increased concentration or incre ased sensitivity of the infected cells since the morphological respons e closely resembled that obtained when tissue was treated with auxins.