How Agrobacterium rhizogenes triggers de novo root formation in a recalcitrant woody plant: an integrated histological, ultrastructural and molecular analysis

Abstract

Adventitious rooting might be induced in recalcitrant woody genotypes by infection with Agrobacterium rhizogenes, and, in some cases, might also require exogenous auxin. The objective of the present study was to determine how agrobacteria trigger root formation in the stem of a recalcitrant woody microcutting, which cytological events result from the combined presence of infection and exogenous auxin, and which types of roots are induced by infection. Microcuttings of a recalcitrant walnut (Fuglans regia), infected or not with A. rhizogenes strain 1855, were cultured with either indolebutyric acid (IBA), IAA, or without exogenous hormones, to induce rhizogenesis. They were cytohistologically and ultrastructurally investigated at various times in culture. Southern blot and PCR analyses were performed to verify the frequency of transgenic, chimeric and bacterium-containing roots. The infection was sufficient per se to stimulate rhizogenesis. Rooting on the infected cuttings was enhanced by exogenous IBA, which accelerated and increased root meristemoid formation, in comparison with without hormone treatment. Meristemoids were organized both directly by the cambial cells and indirectly by the callus, and showed a pluricellular origin. Inter and intracellular bacteria were observed in the stem throughout the culture period (30 d). They were preferentially present in the vessels, and mainly in those showing polyphenol deposition. In the infected IAA-treated cultures, a high level of secondary xylem formation occurred instead of rhizogenesis. Nontransformed roots were preferentially produced by the infected cuttings treated with the auxins. Bacterium-containing and chimeric roots were produced by infected cuttings independently of the treatment. Thus, in a recalcitrant walnut, nontransformed root meristemoids are stimulated by combining infection and exogenous indolebutyric acid. Furthermore, the persistence of bacteria in the stem during the culture and the pluricellular origin of the meristemoids explain the presence of the bacterium-containing and chimeric roots

Root induction in radiata pine using Agrobacterium rhizogenes

“…In these cases, root induction was probably due to an improvement of rooting environment by the bacterium (McAfee et al 1993;Simpson et al 1986). Recently, similar results were obtained by Falasca et al 2000 who conducted an experiment to investigate how A. rhizogenes triggers de novo root formation in a recalcitrant walnut plant. They found that rooting on the infected cuttings was enhanced by IBA, which accelerated and increased root meristemoid formation, in comparison with the treatment without hormone.…”

Biological Activity of Essential Oils from Leaves and Fruits of Pepper Tree (Schinus molle L.) to Control Rice Weevil (Sitophilus oryzae L.)

“…Furthermore, the transformed roots were able to regenerate transgenic plants or clones that are viable, genetically stable (Narasu and Giri, 2000), and phenotypically normal (Sánchez-Olate et al, 1997). This could be indicating the interaction between endogenous auxins and the A. rhizogenes effect (Falasca et al, 2000).…”

“…The living cells would transmit diffuse signals to other healthy cells that are capable of initiating the rhizogenesis process (Falasca et al, 2000). According to Vahdati et al (2002), the rol genes derived from the T-DNA of A. rhizogenes are involved in changing the following characteristics in the transformed plants: rol A: wrinkled leaves, condensed inflorescences, increment in the size of the stigma and large flowers; rol B: increments the rooting potential as a result of increasing sensitivity to the tissue auxins, alters the morphology, and increases flower size; rol C: reduces internode length, produces flowering abnormalities, and increments ramification; rol D: causes dwarfism and early blooming.…”

“…Furthermore, the transformed roots were able to regenerate transgenic plants or clones that are viable, genetically stable (Narasu and Giri, 2000), and phenotypically normal (Sánchez-Olate et al, 1997). This could be indicating the interaction between endogenous auxins and the A. rhizogenes effect (Falasca et al, 2000).In spite of the observed differences in the rhizogenic rate obtained via IBA or A. rhizogenes, the use of the bacterial vector can be a powerful tool to reproduce selected cultivars that are advanced in age, or recuperate high-value cultivars in a state of deterioration, given that the results obtained with this inductor were comparable to those obtained with microshoots of embryonic origin (Ripetti et al, 1994;Caboni et al, 1996).Finally, considering that the commercial importance of J. regia generates a high demand for grafted plants with germoplasma quality, and that the results of the grafting programs are significantly less than the rooting percentages achieved in this study, the use of rhizogenic induction agents can mean plant conversion at higher rates than those achieved via traditional grafting. …”

1,3-di(benzo[d]oxazol-5-yl)urea acts as either adventitious rooting adjuvant or xylogenesis enhancer in carob and pine microcuttings depending on the presence/absence of exogenous indole-3-butyric acid

“…In in vitro systems, endogenous auxin accumulation occurs independently of the fact that xylogenesis needs a preceding cell proliferation phase or results from a trans-differentiation event (Sundberg et al 2000;McCann et al 2000). Moreover, in Arabidopsis TCLs, xylogenesis occurs as xylary nodules and strands in addition to, and in competition with, AR formation (Falasca et al 2004;Della Rovere et al 2015), but the same occurs in other culture systems, e.g., the IBA-cultured tobacco, Prunus avium and walnut microcuttings (Biondi et al 1994;Falasca et al 2000;Faivre-Rampant et al 2003).…”