Somatic embryogenesis as key technology for shaping the rubber tree of the future

Worldwide, Hevea producers face the need to replant large surfaces in the coming years. The rubber yield per ha, produced by trees grafted on heterogeneous illegitimate seedling rootstocks, has reached its maximum. For long-standing Hevea clones, as for a lot of other tree species, one of the consequences of physiological aging is reduced in vitro growth and the lack of a proper geotropic (tap) root system. Somatic embryogenesis on young inner seed integument or stamen filaments provides a mean to regain ontogenetic juvenility. The process is limited by irregular germination of the somatic embryos. Nevertheless, with the obtained in vitro plants, juvenile lines have been established of the most important profitable rubber tree clones. Currently they are micropropagated on a commercial scale. Moreover, the produced plants can serve as mother plants for propagation by means of macro-cutting. Somatic embryogenesis enables the production of transgenic Hevea brasiliensis as well. Genes conferring plant disease resistance, abiotic stress tolerance and production of foreign proteins in the lactiferous vessels will further shape the rubber tree of the future

Strigolactones as an auxiliary hormonal defence mechanism against leafy gall syndrome in Arabidopsis thaliana

Leafy gall syndrome is the consequence of modified plant development in response to a mixture of cytokinins secreted by the biotrophic actinomycete Rhodococcus fascians. The similarity of the induced symptoms with the phenotype of plant mutants defective in strigolactone biosynthesis and signalling prompted an evaluation of the involvement of strigolactones in this pathology. All tested strigolactone-related Arabidopsis thallana mutants were hypersensitive to R. fascians. Moreover, treatment with the synthetic strigolactone mixture GR24 and with the carotenoid cleavage dioxygenase inhibitor D2 illustrated that strigolactones acted as antagonistic compounds that restricted the morphogenic activity of R. fascians. Transcript profiling of the MORE ILLARY GROWTH1 (M 1), M, M 3, M, 4, and BRANCHED1 (BRC1) genes in the wild-type Columbia-0 accession and in different mutant backgrounds revealed that upregulation of strigolactone biosynthesis genes was triggered indirectly by the bacterial cytokinins via host-derived auxin and led to the activation of BRC1 expression, inhibiting the outgrowth of the newly developing shoots, a typical hallmark of leafy gall syndrome. Taken together, these data support the emerging insight that balances are critical for optimal leafy gall development: the long-lasting biotrophic interaction is possible only because the host activates a set of countermeasures including the strigolactone response in reaction to bacterial cytokinins to constrain the activity of R. fascians

A ploidy chimera reveals the effects of tetraploidy in date palm

In the oasis of Tozeur (Tunisia) a peculiar date palm ploidy chimera was discovered, showing a diploid (2n = 2x) and a tetraploid (2n =4x) sector and a uniform tetraploid offshoot. This chimera allowed to describe the effects of chromosome doubling on some selected features. Because both ploidies occur within one plant, there was no environmental bias. Tetraploid leaves are longer, bear larger leaflets and spines and have a thicker rachis than normal diploid leaves. During three seasons, tetraploid inflorescences appeared ten days later than their diploid counterparts. The tetraploid sector formed bunches with stalks bearing larger fruits, with a higher percentage of inedible parthenocarpic fruits, than bunches that arose from the diploid sector. Tetraploid fruits are significantly longer, wider and heavier than diploid fruits, with a larger pulp diameter, irrespective of whether they are seeded or parthenocarpic. Diploid parthenocarpic fruits are longer than seeded fruits, while tetraploid parthenocarpic and seeded fruits both show the same size. The occurrence of tetraploidy opens new perspectives in date palm breeding. Pollination of tetraploid pistils with pollen from diploid males gave rise to a relatively low number of triploid plantlets, suggesting a triploid block