Vegetative Propagation in Relation to Juvenility, Maturity, and Rejuvenation

Secondary Embryogenesis of Linaloe in Temporary Immersion Bioreactor-Type RITA®

The linaloe [Bursera linanoe (La Llave) Rzed, Calderon and Medina] is an endemic species of Mexico, representative of the low deciduous forest of the states of Guerrero, Puebla, Morelos, and Oaxaca, and has been of great economic importance for the people, mainly for the artisanal use of its aromatic wood that is used to make boxes, trunks, and furniture that are manufactured in Olinala, Guerrero, Mexico; and industrial, thanks to the fine aroma of its essential oil (linalool), which is used in the manufacture of perfumes and pharmaceuticals. Overexploitation has endangered the species in recent years, and propagation by seed and/or cuttings has produced very poor results compared to those obtained with other recalcitrant Bursera species. The protection of endangered species makes urgent the need to propose new alternatives for its propagation. Somatic embryogenesis is a reliable and feasible technique, including induction, maintenance, multiplication, and maturation of embryos, often in semisolid culture media; however, the recent use of liquid media has allowed semi-automation in temporary immersion bioreactors, for example, the RITA® system, which favors both the multiplication rate and the final conversion to seedlings.

Use of bioreactor systems in the propagation of forest trees

Plant biotechnology can be used to conserve the germplasm of natural forests, and to increase the productivity and sustainability of plantations. Both goals imply working with mature trees, which are often recalcitrant to micropropagation. Conventional in vitro culture uses closed containers and gelled medium with sugar supplementation. Bioreactor culture uses liquid medium and usually incorporates aeration. The increased absorption of nutrients via the liquid medium together with the renewal of the air inside the bioreactors may improve the physiological state of the explants. In this review, we will explore the feasibility of using bioreactors to overcome the recalcitrance of many trees to micropropagation and/or to decrease the cost of large-scale propagation. We will focus on the recent use of bioreactors during the multiplication, rooting (plant conversion in the case of somatic embryos), and acclimation stages of the micropropagation of axillary shoots and somatic embryos of forest trees (including some shrubs of commercial interest), in both temporary and continuous immersion systems. We will discuss the advantages and the main obstacles limiting the widespread implementation of bioreactor systems in woody plant culture, considering published scientific reports and contributions from the business sector.

Small RNAs of Sequoia sempervirens during rejuvenation and phase change

In this work, the population of small RNAs (sRNAs) was studied in the gymnosperm Sequoia sempervirens during phase changes, specifically in the juvenile, adult and rejuvenated plants obtained in vitro. The potential target genes of Sequoia sRNAs were predicted through bioinformatics. Rejuvenation is a pivotal process in woody plants that enables them to regain their growth potential, which results in the recovery of physiologic and molecular characteristics that were lost when the juveniles mature into adult plants. The results from the five repeated graftings of juvenile, adult and rejuvenated plants in vitro showed that sRNAs could be classified into structural RNAs (Group I), small interfering RNAs (Group II), annotated microRNAs (Group III, and unannotated sRNAs (Group IV). The results indicate that only 573 among 15,485,415 sRNAs (Groups III and IV) had significantly different expression patterns associated with rejuvenation and phase change. A total of 215 sRNAs exhibited up-regulated expression patterns in adult shoots, and 358 sRNAs were down-regulated. Expression profiling and prediction of possible target genes of these unique small RNAs indicate possible functions in the control of photosynthetic efficiency and rooting competence abundance during plant rejuvenation. Moreover, the increase in SsmiR156 and decrease in SsmiR172 during plant rejuvenation suggested that these two microRNAs extensively affect phase transition.

Variations of DNA methylation in Eucalyptus urophylla×Eucalyptus grandis shoot tips and apical meristems of different physiological ages

Global DNA methylation was assessed by high-performance liquid chromatography (HPLC) for the first time in Eucalyptus urophylla×Eucalyptus grandis shoot tips comparing three outdoor and one in vitro sources of related genotypes differing in their physiological age. The DNA methylation levels found were consistent with those reported for other Angiosperms using the same HPLC technology. Notwithstanding noticeable time-related fluctuations within each source of plant material, methylation rate was overall higher for the mature clone (13.7%) than for the rejuvenated line of the same clone (12.6%) and for the juvenile offspring seedlings (11.8%). The in vitro microshoots of the mature clone were less methylated (11.3%) than the other outdoor origins, but the difference with the juvenile seedlings was not significant. Immunofluorescence investigations on shoot apices established that the mature source could be distinguished from the rejuvenated and juvenile origins by a higher density of cells with methylated nuclei in leaf primordia. Shoot apical meristems (SAMs) from the mature clone also showed a greater proportion and more methylated cells than SAMs from the rejuvenated and juvenile origins. The nuclei of these latter were characterized by fewer and more dispersed labeled spots than for the mature source. Our findings establish that physiological ageing induced quantitative and qualitative variations of DNA methylation at shoot tip, SAM and even cellular levels. Overall this DNA methylation increased with maturation and conversely decreased with rejuvenation to reach the lower scores and to show the immunolabeling patterns that characterized juvenile material nuclei.

Production of first and second generations aposporous gametophytes fromPyrrosia piloselloides (L.) Price frond strips cultured in vitro

Second generation aposporous gametophytes were obtained from sporophytes derived from first generation aposporous gametophytes, which in turn came from the mature fronds grown from spores in the laboratory. Murashige and Skoog modified medium in 1% agar supplemented with sugar alcohols (sorbitol, mannitol), auxins (NAA, 2,4-D) and cytokinin (BA) promoted a higher percentage of aposporous development from mature fronds ofPyrrosia piloselloides derived from aseptically cultured spores as compared with those obtained from plants in the field. A method using 4'6-diamidino-2-phenyl indole and fluorescence microscopy correlated the deoxyribonucleic acid contents of the aposporous gametophytes and sporophytes derived from them with their ploidy level.

Developmental changes in Sitka spruce as indices of physiological age I. Changes in needle morphology

There is a paucity of data regarding developmental changes in trees. Measurements of length, width, length/width ratio, perimeter/width and width/height ratios of tranverse sections, projected and total surface areas, dry weight, specific leaf area and the weight of epicuticular waxes per unit leaf area were made on needles sampled from the uppermost whorl of Sitka spruce trees aged 3, 5,10,15, 20 and 38 years. All characteristics, except needle length and the weight of epicuticular waxes, showed asymptotic changes with age that could adequately be described by the Gompertz growth function, and offer potential as indices of physiological age. Asymptotes were reached at different ages (width and the length/width ratio having the slowest rates of change and the perimeter/width ratio, specific leaf area, and the width/height ratio having the fastest) suggesting a predictable sequence of changes. The observed changes in needle morphology are interpreted as a transition from shade to sun leaves.