The Ri plasmid A4 of Agrobacterium rhizogenes contains within its T‐DNA genetic information able to trigger root formation in infected plants. Tobacco plants regenerated from transformed roots display the hairy root (hr) syndrome. We show that DNA fragments containing the rol B locus alone are able to induce root formation both in tobacco and kalanchoe tissues. The rol A and the rol C loci by themselves are also able to induce root formation in tobacco but not in kalanchoe. This capacity to induce root formation in either host is greatly increased when the rol A and/or C loci are combined with the rol B locus. Root induction is shown to be correlated with the expression of the rol loci. Transgenic plants exhibit all the characteristics of the hairy root syndrome only when all three loci are present and expressed. Although the activity of the rol encoded functions is synergistic, each of them appears to independently influence host functions involved in the determination of root differentiation.

The combined expression of the rol A, B and C loci of Agrobacterium rhizogens Ri‐plasmids establishes, in transgenic tobacco plants, a pathological state called hairy‐root syndrome. However, when expressed separately they provoke distinct developmental abnormalities characteristic for each rol gene. Moreover, changes in their mode of expression obtained by replacing the promoters of the rol B and C genes with the cauliflower mosaic virus 35S promoter elicit new and distinct developmental patterns. These results indicate that the different rol gene products have either different targets, or have a qualitatively different effect on the same target. The target(s) must be involved in the control of plant development. Although each of the three rol genes are independently able to promote root formation in tobacco, efficient root initiation and growth is best achieved through the combined activities of more than a single rol gene. Models explaining the biological effects of A. rhizogenes‐derived TL‐DNA genes are discussed.

Transgenic tobacco and eggplants expressing the coding region of the iaaM gene from Pseudomonas syringae pv. savastanoi, under the control of the regulatory sequences of the ovule-specific DefH9 gene from Antirrhinum majus, showed parthenocarpic fruit development. Expression of the DefH9-iaaM chimeric transgene occurs during flower development in both tobacco and eggplant. Seedless fruits were produced by emasculated flowers. When pollinated, the parthenocarpic plants produced fruits containing seeds. In eggplant, the genetic manipulation allowed fruit set and growth under environmental conditions prohibitive for fruit setting in the untransformed line, which did not set fruit at all. Under normal environmental conditions, production of marketable fruits took place from pollinated and unpollinated transgenic flowers, while flowers of untransformed control plants did produce fruits of marketable size only from fertilized flowers.

BackgroundRhizobia symbionts elicit root nodule formation in leguminous plants. Nodule development requires local accumulation of auxin. Both plants and rhizobia synthesise auxin. We have addressed the effects of bacterial auxin (IAA) on nodulation by using Sinorhizobium meliloti and Rhizobium leguminosarum bacteria genetically engineered for increased auxin synthesis.ResultsIAA-overproducing S. meliloti increased nodulation in Medicago species, whilst the increased auxin synthesis of R. leguminosarum had no effect on nodulation in Phaseolus vulgaris, a legume bearing determinate nodules. Indeterminate legumes (Medicago species) bearing IAA-overproducing nodules showed an enhanced lateral root development, a process known to be regulated by both IAA and nitric oxide (NO). Higher NO levels were detected in indeterminate nodules of Medicago plants formed by the IAA-overproducing rhizobia. The specific NO scavenger cPTIO markedly reduced nodulation induced by wild type and IAA-overproducing strains.ConclusionThe data hereby presented demonstrate that auxin synthesised by rhizobia and nitric oxide positively affect indeterminate nodule formation and, together with the observation of increased expression of an auxin efflux carrier in roots bearing nodules with higher IAA and NO content, support a model of nodule formation that involves auxin transport regulation and NO synthesis.

The rolC gene of Agrobacterium rhizogenes, which drastically affects growth and development of transgenic plants, codes for a cytokinin‐beta‐glucosidase. Indeed, rolC protein expressed in Escherichia coli as a fusion protein hydrolyses cytokinin glucosides, thus liberating free cytokinins. Furthermore, beta‐glucosidase activity present in E. coli extracts expressing the rolC protein was inhibited by affinity‐purified antibodies specific for the rolC protein. Finally, rolC proteins expressed in transgenic plants were shown to be responsible for cytokinin‐beta‐glucosidase activity. Morphological and phytohormonal analysis, performed on transgenic plants that are somatic mosaics for the expression of the rolC gene, extend and confirm our interpretation that the developmental, physiological and morphological alterations caused by rolC expression in transgenic plants are primarily due to a modification of the cytokinin balance. These observations shed new light on the control of growth and differentiation in plants by growth factors.