Plant growth response to Agrobacterium rhizogenes : A thesis submitted in partial fulfilment of the requirements for the degree of Master of Applied Science in the University of Canterbury

Abstract

Agrobacterium rhizogenes, a soil inhabiting bacteria, has a natural genetic engineering ability to induce the development of functional roots (hairy roots) at the site of inoculation on a range of dicotyledonous plant species. Inoculation of some important horticultural crop plants with eleven strains of A. rhizogenes revealed that strains A4T and TR105 were the most effective i producing hairy roots at the inoculation site. The initiation and development of hairy roots depended on the bacterial strain, plant species, cultivar, environment at the inoculation site, type of tissue inoculated and the physiological state of plant growth. The most effective method for testing the Agrobacterium rhizogenes strain:host plant growth response involved inoculating juvenile plants immediately after the removal of the epidermis and outer cortex tissue, then encompassing the inoculation site by a moist and warm (ca. 25°C) environment. Under hydroponic growing conditions the presence of hairy roots incited by strain A4T showed no detrimental effects on growth over a full growing season of tomato (Lycopersicon esculemum cv. Moneymaker) even when live Agrobacterium rhizogenes were still present. This suggests that at least under these conditions Agrobacterium rhizogenes should not be considered pathogenic. Since enhancement of certain growth parameters (root dry weight, root length, leaf dry weight) resulted from an increase in hairy roots and the rapid establishment of tomato seedlings solely on a hairy root system was observed, this indicates these genetically transformed roots have similar functions to normal roots. The intentional inoculation of forest, nursery, fruit tree, or vegetable transplants with A. rhizogenes has potential to promote rapid establishment after transplanting and increase the efficiency of plant propagation. It may also provide a potential method to genetically modify the root system to alter top growth and promote plant survival under extreme soil environments.