Caulogenic induction in cotyledons of stone pine (Pinus pinea): relationship between organogenic response and benzyladenine trends in selected families

In Vitro Plant Regeneration in Conifers: The Role of WOX and KNOX Gene Families

Conifers are a group of woody plants with an enormous economic and ecological importance. Breeding programs are necessary to select superior varieties for planting, but they have many limitations due to the biological characteristics of conifers. Somatic embryogenesis (SE) and de novo organogenesis (DNO) from in vitro cultured tissues are two ways of plant mass propagation that help to overcome this problem. Although both processes are difficult to achieve in conifers, they offer advantages like a great efficiency, the possibilities to cryopreserve the embryogenic lines, and the ability of multiplying adult trees (the main bottleneck in conifer cloning) through DNO. Moreover, SE and DNO represent appropriate experimental systems to study the molecular bases of developmental processes in conifers such as embryogenesis and shoot apical meristem (SAM) establishment. Some of the key genes regulating these processes belong to the WOX and KNOX homeobox gene families, whose function has been widely described in Arabidopsis thaliana. The sequences and roles of these genes in conifers are similar to those found in angiosperms, but some particularities exist, like the presence of WOXX, a gene that putatively participates in the establishment of SAM in somatic embryos and plantlets of Pinus pinaster.

Hormonal and gene dynamics in de novo shoot meristem formation during adventitious caulogenesis in cotyledons of Pinus pinea

KEY MESSAGE

Several members of WOX and KNOX gene families and several plant growth regulators, basically cytokinins and auxins, play a key role during adventitious caulogenesis in the conifer Pinus pinea. Similar to Arabidopsis thaliana, Pinus pinea shoot organogenesis is a multistep process. However, there are key differences between both species, which may alter the underlying physiological and genetic programs. It is unknown if the genic expression models during angiosperm development may be applicable to conifers. In this work, an analysis of the endogenous content of different plant growth regulators and the expression of genes putatively involved in adventitious caulogenesis in P. pinea cotyledons was conducted. A multivariate analysis of both datasets was also realized through partial least squares regression and principal component analysis to obtain an integral vision of the mechanisms involved in caulogenesis in P. pinea. Analyses show that cotyledons cultured in the presence of benzyladenine during long times (2-6 days) cluster separately from the rest of the samples, suggesting that the benzyladenine increase observed during the first hours of culture is sufficient to trigger the caulogenic response through the activation of specific developmental programs. In particular, the most relevant factors involved in this process are the cytokinins trans-zeatin, dihydrozeatin, trans-zeatin riboside and isopentenyl adenosine; the auxin indoleacetic acid; and the genes PpWUS, PpWOX5, PpKN2, PpKN3 and PipiRR1. WUS is functional in pines and has an important role in caulogenesis. Interestingly, WOX5 also seems to participate in the process, although its specific role has not been determined.

Endogenous cytokinin profiles and their relationships to between-family differences during adventitious caulogenesis in Pinus pinea cotyledons

Caulogenesis in mature stone pine (Pinus pinea L.) cotyledons is promoted, to varying degrees depending on genotype, by exogenous application of the cytokinin (CK) benzyladenine (BA). In the present study, endogenous CK profiles of cotyledons from open-pollinated plants and two families of stone pine with widely differing organogenic capacities were monitored during caulogenesis and linked to previously characterized BA uptake and induction phases. Changes in levels of free bases, ribosides, ribotides and glucosides of both isoprenoid and aromatic CKs were followed. Before BA application, the pool of endogenous CKs in all sets of cotyledons was dominated by isoprenoid ribotides, but 1h after BA exposure, aromatic CKs (mainly active free bases and ribosides of topolins) accounted for more than 90% of the pool. BA N-glucosides were also observed, levels of which (and topolins) rose from 2d until the end of the (six-day) culture period. The CK profiles of the two selected pine families also differed, although the general trends were similar. During the first 6h, levels of BA and meta-topolin were highest in cotyledons from the family with the strongest caulogenic responses, while levels of ribotides and aromatic glucosides were highest in cotyledons from the other family.

Cloning and characterization of a type-A response regulator differentially expressed during adventitious shoot formation in Pinus pinea L

Type-A response regulators play an important role in cytokinin-induced adventitious shoot formation, acting as negative regulators of cytokinin signal transduction. In this work, we obtained the full-length cDNA clone of a type-A response regulator from the conifer Pinus pinea, designated PipiRR1. The derived peptide sequence showed all the characteristic motifs found in angiosperms. Gene expression analysis showed that the gene was differentially expressed during adventitious shoot formation in P. pinea cotyledons, suggesting that PipiRR1 may play a role in caulogenesis in conifers. This is the first type-A response regulator identified in gymnosperms.

6-benzyladenine metabolism during reinvigoration of mature Pinus radiata buds in vitro

Maturation or phase change is a serious challenge in the deployment of superior trees of Pinus radiata D. Don because of the difficulties associated with propagation of cuttings from mature trees. We used an in vitro system to study 6-benzyladenine (BA)-induced reinvigoration of the fascicle meristems of mature buds during in vitro culture. Anatomical examinations revealed that BA inhibited the development of secondary needle primordia and 'rejuvenated' the fascicle meristems of the mature bud to produce primary needles, which are characteristic of the juvenile phase in P. radiata. Without BA supplement in the culture media, fascicle primordia continued developing secondary needles and quiescent fascicle meristems. BA metabolite analysis showed that the novel cytokinin pathway reported previously in P. radiata (H. Zhang, K.J. Horgan, P.H.S. Reynolds, G.E. Norris and P.E. Jameson. 2001. Novel cytokinins: The predominant forms in mature buds of Pinus radiata. Physiol. Plant. 112: 127-134) was mirrored in vitro, with BA converted into a variety of metabolites including 6-benzylamino-9-glucopyranosylribosyl-purine and its novel phosphorylated form, 6-benzylamino-9-glucopyranosylribosyl-purine. The culture of mature buds in the presence of BA caused a reduction in the level of endogenous cytokinins, suggesting a direct action of BA itself. Similar correlations are noted between levels of certain metabolites and the maturation status of buds from field-grown trees and buds in culture, indicating that this in vitro system may be a good model for studying the processes of maturation and reinvigoration.

Benzyladenine metabolism and temporal competence of Pinus pinea cotyledons to form buds in vitro

Germination negatively affects adventitious shoot formation induced by cytokinins in pine cotyledons. To investigate the causes of this decrease in the organogenic response, uptake and metabolism of benzyladenine (BA) were studied in stone pine cotyledons (Pinus pinea) isolated from in vitro germinating embryos and cultured in bud induction medium. As embryos grew, cotyledons showed a progressive decrease in the amount of BA taken up from the medium. BA was barely metabolized; however, a BA metabolite previously undescribed in conifers was found. It was identified as a glucoside of the BA riboside, a type of metabolite recently described in other gymnosperms. Data revealed that differences in the organogenic capacity of P. pinea cotyledons associated with embryo germination are related primarily to their ability to absorb BA from the bud induction medium.