Cytological analysis of hybrid embryos of intergeneric crosses between Salix viminalis and Populus species

Abstract

Intergeneric hybridisation between Salix viminalis L. as the female and four Populus species (Populus trichocarpa, P. tremula, P. × canadensis and P. simonii) as male pollen donors was performed by in vitro stigma pollination. To overcome postzygotic barriers, transfer of hybrid embryos to new medium is necessary. We carried out detailed ultrastructural analyses to establish: (i) at which stage of embryo development the first signs of programmed cell death (PCD) could be detected; and (ii) at which stage the lack of serious or irreversible changes guaranteed that advanced development of hybrid plants could occur after embryo rescue. Transmission electron microscopy and confocal laser scanning microscopy revealed the presence of both developing and degenerating embryos. Developing globular, heart-shaped, and early cotyledonary embryos contained cells of correct ultrastructure. The only sign of intergeneric hybridisation was a delay in development for a few days, in comparison with control embryos. The earliest indicators of embryo degeneration were noted at 9 days after pollination (DAP). The most common indicators were excessive embryo vacuolisation, which was characterised by a large number of vesicles and formation of small vacuoles, as well as enlarged central vacuoles. Extended plastid thylakoids, folding of the cell wall, and autophagosomes were observed. Our detailed investigation of PCD in hybrid embryos enabled us to conclude that the embryo rescue technique was most effective in intergeneric willow × poplar crosses if applied between 9 and 16 DAP.

Avoiding transport bottlenecks in an expanding root system: Xylem vessel development in fibrous and pioneer roots under field conditions

“…In this case, mega‐autophagy plays a role in cell death and the recycling of cell material, suggesting that other types of autophagy could occur in developmental PCD. Cytological observations of degenerating intergeneric embryos (Bagniewska‐Zadworna et al, 2010) revealed that different types of autophagy occurred at the same time, although two different types of vacuolization of embryo cells were noticed, resulting in different modes of PCD.…”

Significance of stigma receptivity in intergeneric cross-pollination of Salix × Populus

“…There is no doubt that germination occurred despite the limited contact of Populus pollen with Salix papillae, although the frequency of fertilization was rather low [9,10]. It is apparent that the consequence of the micro-incompatibility of both structures was a slow germination and a greater pollen tube attrition exhibited by poplar pollen.…”

“…Breeding of willows involves interspecific hybridization or selection of natural hybrids [5][6][7][8]. Some research on willows has already been carried out in Poland to increase the degree of genetic variability within the species [9][10][11][12]. However, some aspects of the flowering biology and stigma receptivity in Salix are still insufficiently recognized.…”

65 years of in vitro culture in Poland

“…The objective of these attempts was to introgress the genes of poplar, the genus with large biomass increments, into the genome of the fast-growing short rotation willow, and are therefore important from the point of view of sustainable energy economy [93][94][95][96][97]. Intergeneric hybridization has also been used to improve other species grown for their rapid biomass accumulation, such as Miscanthus ×giganteus [98][99][100].…”