Regeneration of Plants From Callus Cultures of Roots Induced by Agrobacterium rhizogenes on Tobacco

From A. rhizogenes RolD to Plant P5CS: Exploiting Proline to Control Plant Development

The capability of the soil bacterium Agrobacterium rhizogenes to reprogram plant development and induce adventitious hairy roots relies on the expression of a few root-inducing genes (rol A, B, C and D), which can be transferred from large virulence plasmids into the genome of susceptible plant cells. Contrary to rolA, B and C, which are present in all the virulent strains of A. rhizogenes and control hairy root formation by affecting auxin and cytokinin signalling, rolD appeared non-essential and not associated with plant hormones. Its role remained elusive until it was discovered that it codes for a proline synthesis enzyme. The finding that, in addition to its role in protein synthesis and stress adaptation, proline is also involved in hairy roots induction, disclosed a novel role for this amino acid in plant development. Indeed, from this initial finding, proline was shown to be critically involved in a number of developmental processes, such as floral transition, embryo development, pollen fertility and root elongation. In this review, we present a historical survey on the rol genes focusing on the role of rolD and proline in plant development.

rolA locus of the Ri plasmid directs developmental abnormalities in transgenic tobacco plants

Plants containing the left T-DNA (TL) of Agrobacterium rhizogenes show a variety of developmental abnormalities that include severely wrinkled leaves, loss of apical dominance, reduced geotropism of roots, reduced internode distances, and floral hyperstyly. The TL-DNA also affects the morphology of tumor tissue at the site of inoculation on Kalanchoe diagremontiana leaves. Single mutations at four loci of the TL-DNA (rolA, rolB, rolC, and rolD) are known to affect tumor morphology on K. diagremontiana leaves. We regenerated plants from tissues transformed with TL-DNA containing mutations in each of the rol loci in order to determine which of the rol loci, if any, control the abnormal plant phenotype. Only plants regenerated after infection with bacteria containing a mutation in rolA locus showed loss of the wrinkled leaf phenotype. The rolA locus was cloned into the plant transformation vector pGA472 and introduced alone into plants. Transgenic plants containing rolA displayed the abnormal phenotype. These results indicate that rolA is the primary determinant of the severely wrinkled phenotype of Ri plasmid transgenic plants. Other rol loci may influence the degree of developmental abnormalities.

Establishment of new axenic hairy root lines by inoculation with Agrobacterium rhizogenes

Cultured hairy root lines resulting from infection by Agrobacterium rhizogenes are known for approximately thirty plant species. We extend this range by establishing forty original dicotyledonous hairy root lines with A. rhizogenes strain A4. Hairy roots have been cultured for at least 2-6 years on Murashige & Skoog medium. Some hairy root cultures such as Anagallis arvensis and Antirrhinum majus spontaneously regenerated whole plants.

Agrobacterium rhizogenes T-DNA genes capable of inducing hairy root phenotype

Segments of the TL-DNA of the agropine type Ri plasmid pRi 1855 encompassing single and groups of open-reading frames were cloned in the Ti plasmid-derived binary vector system Bin 19. Leaf disc infections on Nicotiana tabacum led to transformed plants, some of which showed typical hairy root phenotypes, such as the wrinkled leaf morphology, excessive and partially non geotropic root systems and the ability of leaf explants to differentiate roots in a hormone-free culture medium. Particularly interestingly, most of these traits were shown by plants transformed with a TL-DNA segment encompassing the single ORF 11, corresponding to the rolB locus. Hairy root can be induced by this latter T-DNA segment on wounded stems of tobacco plants; hairy root induction on carrot discs requires, on the contrary, a more complex complement of TL-DNA genes.

Hairy root transformation in alfalfa (Medicago sativa L.)

The widely cultivated forage legume alfalfa (Medicago sativa L.) was transformed with the agropine type Agrobacterium rhizogenes NCPPB 1855. Sterile root and callus cultures were derived from tumorous hairy roots which were easily obtained independent of the plant variety or genotype. Plant regeneration, via somatic embryogenesis, was achieved only when a selected alfalfa line, characterized by high regenerative capability, was utilized. Genetic transformation was confirmed by the presence of agropine and T-DNA. Phenotypic alterations, mainly affecting the root system, were observed in transformed plants. The possibility that T-DNA-induced variations could be useful in the improvement of M. sativa is discussed.

A disarmed binary vector from Agrobacterium tumefaciens functions in Agrobacterium rhizogenes : Frequent co-transformation of two distinct T-DNAs

Binary Ti plasmid vector systems consist of two plasmids in Agrobacterium, where one plasmid contains the DNA that can be transferred to plant cells and the other contains the virulence (vir) genes which are necessary for the DNA transfer but are not themselves stably transferred. We have constructed two nononcogenic vectors (pARC4 and pARC8) based on the binary Ti plasmid system of Agrobacterium tumefaciens for plant transformation. Each vector contains the left and right termini sequences from pTiT37. These sequences, which determine the extent of DNA transferred to plant cells, flank unique restriction enzyme sites and a marker gene that functions in the plant (nopaline synthase in pARC4 or neomycin phosphotransferase in pARC8). After construction in vitro, the vectors can be conjugatively transferred from E. coli to any of several Agrobacterium strains containing vir genes. Using A. rhizogenes strain A4 containing the resident Ri plasmid plus a vector with the nopaline synthase marker, we found that up to 50% of the hairy roots resulting from the infection of alfalfa or tomato synthesized nopaline. Thus, vector DNA encoding an unselected marker was frequently co-transferred with Ri plasmid DNA to an alfalfa or a tomato cell. In contrast, the frequency of co-transfer to soybean cells was difficult to estimate because we encountered a high background of non-transformed roots using this species. Up to five copies of the vector DNA between the termini sequences were faithfully transferred and maintained in most cases suggesting that the termini sequences and the vir genes from the Ri and Ti plasmids are functionally equivalent.

Identification of the genetic locus responsible for non-polar root induction by Agrobacterium rhizogenes 1855

Root proliferation can be induced by Agrobacterium rhizogenes on carrot discs both on the apical and basal surface (facing the root apex and base, respectively) or on the apical surface only, depending on the bacterial strain. This differential response on the two surfaces is denominated polarity. We correlate the polarity of some strains with the absence of an Ri plasmid genetic locus, present in non polar strains such as A. rhizogenes 1855, which bears sequence homology with the auxin genes of Ti plasmid T-DNA. We demonstrate that this locus is responsible for root induction on the basal surface since insertion of a transposon in this region of pRi1855 induces polarity in this strain.

Infection of turnip and radish storage roots with Agrobacterium rhizogenes

Within about 10 days after inoculation with Agrobacterium rhizogenes, the vascular bundles of storage root disks of turnip or radish developed small outgrowths with numerous root hairs. Thereafter, adventitious roots (hairy roots) emerged extensively from these outgrowths. The hairy roots which emerged fully supported the growth of host plants, though they lacked geotropism. An excised hairy root could be subcultured as an axenic root culture in the absence of phytohormones. Hairy root cultures with extensive lateral branches grew much more rapidly than those with few lateral branches or ordinary roots. Calli were induced from hairy root cultures in the presence of 2,4-D, and root proliferation from these calli occurred in the absence of 2,4-D. Both the primary hairy roots and the roots which grew from them synthesized agropine and mannopine.

Transformation of several species of higher plants by Agrobacterium rhizogenes: sexual transmission of the transformed genotype and phenotype

The T-DNA of the Ri plasmid from Agrobacterium rhizogenes is compatible with the regeneration of whole plants from genetically transformed roots and is transmitted through meiosis to the progeny of genetically transformed plants in carrot, tobacco, and morning glory (Convolvulus arvensis). The presence of Ri T-DNA is correlated with a phenotype that in some respects is invariable from species to species and in other respects varies as a function of species, organ clone within species, or individual. The transformed phenotype concerns a variety of morphological and physiological traits, is dominantly inherited in tobacco, but does not in general appear to be deleterious. The Ri T-DNA may provide a molecular starting point for studying a number of basic phenomena in plant morphology and physiology.