Joseli Schwambach

Phenotypic Plasticity of Adventitious Rooting in Arabidopsis Is Controlled by Complex Regulation of AUXIN RESPONSE FACTOR Transcripts and MicroRNA Abundance

The development of shoot-borne roots, or adventitious roots, is indispensable for mass propagation of elite genotypes. It is a complex genetic trait with a high phenotypic plasticity due to multiple endogenous and environmental regulatory factors. We demonstrate here that a subtle balance of activator and repressor AUXIN RESPONSE FACTOR (ARF) transcripts controls adventitious root initiation. Moreover, microRNA activity appears to be required for fine-tuning of this process. Thus, ARF17, a target of miR160, is a negative regulator, and ARF6 and ARF8, targets of miR167, are positive regulators of adventitious rooting. The three ARFs display overlapping expression domains, interact genetically, and regulate each others expression at both transcriptional and posttranscriptional levels by modulating miR160 and miR167 availability. This complex regulatory network includes an unexpected feedback regulation of microRNA homeostasis by direct and nondirect target transcription factors. These results provide evidence of microRNA control of phenotypic variability and are a significant step forward in understanding the molecular mechanisms regulating adventitious rooting.

Carbohydrates as regulatory factors on the rooting of Eucalyptus saligna Smith and Eucalyptus globulus Labill

Comparisons between related species with different rooting capacities can provide insights into the mechanisms controlling adventitious root development. The availability of carbohydrates is often considered exclusively as an energetic requirement to drive root development; the major regulatory role in the process is often attributed to phytohormones, particularly auxin. The roles of light quantity (irradiance) and carbohydrate supply available to young aseptic donor-plants on the adventitious rooting response of Eucalyptus globulus (rooting recalcitrant) and Eucalyptus saligna (easy-to-root) were examined. The effects of the type of carbohydrate supply (sucrose or glucose) on the rooting response of cuttings was also evaluated. Light intensity supplied to mother-plants (30 or 60xa0μmolxa0m−2xa0s−1) had limited influence on the rooting response of both species, whereas dark periods were detrimental, particularly for E. globulus. In E. globulus, rooting was promoted by the absence of sucrose in donor-plant media. Presence of sucrose in donor plant medium promoted root number but did not affect rooting percentage of E. saligna. A positive effect of glucose on cutting rhizogenesis was found if this hexose was supplied during the root induction phase, followed by sucrose in the root formation step, especially for E. globulus. The same effect was not seen with fructose. The beneficial effect of glucose in the induction phase on root number was also evident under suboptimal auxin concentrations.

Brachycerine, a novel monoterpene indole alkaloid from Psychotria brachyceras

Brachycerine (1), an unusual alkaloid from the leaves of Psychotria brachyceras, was characterized through spectroscopic data interpretation and its stereochemistry established by NOE difference techniques. Brachycerine (1) was found to be restricted to shoots in rooted cuttings of P. brachyceras (0.018 +/- 0.004% dry weight), and accumulation was unaffected by root induction treatment with auxin.

Adventitious rooting of Eucalyptus globulus × maidennii mini-cuttings derived from mini-stumps grown in sand bed and intermittent flooding trays: a comparative study

Eucalyptus globulus Labill and hybrids thereof have low lignin content, favoring cellulose extraction, but are often recalcitrant to clonal propagation. This work analyzed biochemical and morphological changes during adventitious rooting of mini-cuttings of E. globulusxa0×xa0maidenni obtained from mini-stumps cultured in drip fertigated sand bed or intermittent flooding tray commercial propagation systems. Morphological (% rooting, root number and length, mean rooting time) and biochemical parameters (peroxidase activity, total phenolic content and flavonoid content) were monitored to characterize the rooting phases. All of the rooting parameters were equivalent in both systems, indicating comparable efficiency of both methods in clonal propagation. Kinetic profiles of biochemical parameters were also similar, although the activity of peroxidases was an order of magnitude higher and the phenolic content about three times lower in cuttings derived from intermittent flooding-grown mini-stumps than in those derived from sand bed-grown mini-stumps. Taken together, results suggest that rooting phases were similar in both systems: induction before dayxa05, formation from dayxa05 to 15, and elongation from dayxa015 to 45. These data may contribute to the development of rooting phase-specific mineral nutrient solutions to maximize clonal propagation and plant survival.

Identification of new adventitious rooting mutants amongst suppressors of the Arabidopsis thaliana superroot2 mutation

Summary Auxin is a major regulator of adventitious rooting and, to better understand its role, we identified suppressor mutants of superroot2-1. This provides new resources for the discovery of genetic players involved in auxin signalling or auxin crosstalk with other hormones.

Immunoadjuvant saponin production in seedlings and micropropagated plants of Quillaja brasiliensis

The leaves of Quillaja brasiliensis, a native tree species of southern Brazil, accumulate saponins which have adjuvant activity in vaccines. An efficient micropropagation system for Q. brasiliensis was developed. Plants were obtained from aseptically germinated seeds. High rates of germination were observed on filter paper and aseptic medium substrate (sucrose and light exposure did not affect the germination response). Stem nodal segments were inoculated into Murashige and Skoog (MS) medium supplemented with 6-benzylaminopurine. Each explant produced multiple shoots, which elongated satisfactorily on the same medium. Elongated shoots were rooted in a modified MS medium with or without auxins. Best rooting responses were observed in medium containing 10xa0mg l−1 indole-3-acetic acid under continuous exposure. The rooted explants were acclimatized and successfully transferred to soil, yielding approximately 95% survival after 10xa0mo. Leaf content of immunoadjuvant saponins in micropropagated plants was not affected by auxin type used for rooting and was comparable to that of field-grown trees. The results indicate that Q. brasiliensis represents an alternative and readily renewable source of biomass for the production of bioactive saponins.