The quest for sterile seed in black wattle

Introduction to our ‘Guest Blog’ below (by Marnie Light)

Shelagh and I thought that an article by Dr Sascha Beck-Pay would be an interesting addition to the blog. Sascha, one of my colleagues, worked on a project for several years with the aim of developing a sterile black wattle tree in order to reduce the seed load of this commercially important, yet invasive species, in South Africa. So, in one sense this research does fly in the face of seed scientists (we love to see seeds germinate!), but it does highlight some interesting research…

Guest Blog by Sascha Beck-Pay (Senior Research Scientist at the Institute for Commercial Forestry Research, South Africa)

In South Africa, black wattle (Acacia mearnsii) is a leading commercially grown forestry tree species. However, it is also considered to be an exotic invader of indigenous vegetation. This ‘alien invader’ status adds pressure on the forestry industry, making future afforestation to black wattle difficult, particularly for small-scale growers. Thus, the production of a sterile or seedless variety of black wattle was considered a favourable solution to help reduce the contribution of seed to the existing seedbank, thereby potentially removing the invasive status of black wattle whilst allowing the forestry industry to continue as a thriving contributor to the South African economy.

A project was conducted within the Acacia Breeding Programme at the Institute for Commercial Forestry Research (ICFR) in South Africa to try and produce a sterile or seedless tree using the following two approaches:

  • The production of a triploid variety; and
  • The use of gamma irradiation techniques to produce sterile trees.

With the first approach, tetraploids were successfully induced and controlled-crosses were made with diploids. Due to the fact that no triploids were produced from the controlled-crosses, reproductive biology studies were conducted to investigate possible barriers that could be preventing this. Fluorescent microscopy revealed that, in the cross between diploids and tetraploids, the pollen tubes were reaching the ovary and ovules indicating that fertilisation was possible and that the barrier preventing seed maturation was located within the ovary. Embryo rescue, using tissue culture techniques, was then attempted at varying stages of pod development post-fertilisation. Due to the small size of the pods, however, it was impossible to identify and isolate a clean embryo, devoid of maternal tissue, until 11 months post-fertilisation, at which stage embryo abortion would have occurred.

The second approach tested the use of gamma irradiation to induce sterility in seeds. Samples of seed were irradiated, the seedlings raised in a nursery, and then established in a field trial. It was apparent that reductions in flowering and seed production occurred at doses of 350, 400, 450 and 520 Gy. Although seed production was not completely eliminated, reductions of up to 50 % were recorded. Where seed was produced, the seed was determined to be viable, leading to the conclusion that, although seed production can be reduced, the viability of the seed is not affected and thus gamma irradiation does not affect the female reproductive structures of the plant. This suggested that reduced flowering and resultant lower seed production could be as a result of male sterility, although this still needs further investigation. At the doses tested, there was no detrimental effect on the growth of the trees, which makes this approach to reducing seed production quite promising.

The project was conducted over a period of 12 years from 2000 to 2012 and, although a number of advancements were achieved in trying to produce a sterile or seedless variety of black wattle, the project was closed due to funding constraints. For more information contact Sascha Beck-Pay, or for click here for further reading.

Black wattle inflorescences Diploid and tetraloid polyadsStigma with multiple polyads attached

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