As the human population swells – and in the face of a changing and unpredictable climate – the demand for natural resources increases. This leads to distressing rates of deforestation to prepare land for agriculture, medicinal and forestry products. Related to this is an alarming reduction in species worldwide.
This can only be ameliorated through urgent, intensive and sustainable agroforestry and conservation initiatives. This involves the conservation of natural forests as well as renewable plantation efforts. But to date only a scattering of such projects are in place worldwide.
Conservation and renewable plantation efforts are trailing behind the rate of resource exploitation and species disappearance. The problem is worsened by the vast number of endangered plant species. Once disturbed from their natural habitat, they can’t easily be reintroduced. This is because many of them do not readily produce seeds, or their seeds cannot be stored to ensure longevity of the species. The result is a decreasing gene pool.
This poses further risks, as vulnerable species become marginalised. They are only suitable to shrinking ranges and more susceptible to disease. To intensify conservation while enhancing agroforestry, smarter plant breeding practices are required.
Traditional breeding has allowed for the identification, selection and propagation of plants with a superior genetic makeup, or genotype, from a given plant population. But traditional methods often fail to isolate the required superior characteristics of a species. They can also take more than five or six breeding cycles before a valuable trait is established and maintained in a plant population. The process can take decades for perennial plants, like trees.
Plant biotechnology is increasingly being used to complement traditional screening and breeding practices. Plants can be grown in test tubes under controlled laboratory conditions. Advances in biochemistry and genetics have also ushered in an understanding of the factors that influence plant growth.
Together these developments have created the opportunity to precisely identify and mass propagate superior plant varieties within a fraction of the time of traditional methods. On top of this, if required, the precise altering of the genetic makeup of plants is now also possible. This enables plant genomes to be radically enhanced so that superior genotypes can be created, maintained and propagated.
Preserving valuable genes
Maintaining superior genetics for valuable traits is fundamental in agroforestry. But to maintain superior genetics, seed production is rarely an option. In producing a seed, the sexual cross between genetically different male and female parent plants results in the dilution of valuable genes. This often leads to offspring with unpredictable genetics.
For the agroforestry industry to succeed, genotypes with predictably fast growth rates, high yields, and disease and drought resistance are needed. This will ensure land-use efficiency is maximised, which in turn will decrease ecological disturbance and protect indigenous plant species and sensitive natural forests.
One method that holds promise for preserving valuable genes is somatic embryogenesis. This is the ability to produce viable embryos from virtually any plant organ, while avoiding sexual crossing. Such embryos, when encased in alginate gel, constitute a synthetic seed. They retain all the valuable properties of the cloned parent plant.
Creating synthetic seeds
Somatic embryos may form naturally in certain plants, but can potentially be induced in any plant species and from any plant organ outside its normal biological context. This is done by altering the balance of plant hormones – the language signal in plants that controls all developmental processes.
Our research investigated the potential of inducing somatic embryos from leaves of the commercially important Eucalyptus tree. These are an important source of global timber products. Intensive efforts are under way to screen and select preferred genotypes to support environmental sustainability.
Somatic embryos mimic seeds without the lengthy breeding cycle. The germinated products are essentially clones of the parent plant from which the embryos were induced. So somatic embryogenesis allows for superior genotypes to be preserved. It also allows for the propagation of plant species that were previously excluded from standard propagation practices like traditional plant breeding or plant tissue culture.
There are other benefits too. The easily transported embryos constitute known genetics and growth properties. They could also potentially be cryopreserved, that is frozen to ultra-low temperatures indefinitely in liquid nitrogen. Importantly, because of the conditions under which they are induced, they are disease-free.
Despite its many uses somatic embryogenesis is only being used in selected agroforestry industries like sugarcane, certain conifer and forestry plantations and in a few ornamental plants. But its potential as a medium for genetic enhancement cannot be ignored, especially given recent advances in gene editing.
With the drive to sequence whole genomes of commercially important, rare or valuable plant species, scientists are presented with an opportunity to identify, understand the functions of and edit specific gene sequences to enhance plant properties.
To date, one hurdle to the success of the process has been the choice of organ when genetically editing plants.
This is why somatic embryos could be very useful for gene editing. As embryos they contain both root and shoot meristems – the precursors to a complete plant. If genes are edited at this embryonic stage, then as the embryo divides to form the complete plant all cells of the entire plant will carry the edited genome.
The advent of highly accurate gene editing methods has provided scientists with the opportunity to improve forestry, agricultural and threatened plant species. This can be done in a precise, targeted and reproducible manner. One such example is the Crispr/Cas9 system. This is a highly specific gene editing method that can be used to precisely replace whole gene sequences.
The potential exists to genetically insert tolerance to pests, disease, drought, floods and other pressures of a changing climate. Such precise gene editing will greatly benefit from readily available, disease-free embryos. In the near future, gene editing of synthetic seeds will allow extensive improvement of agricultural and forestry crops.
Planning for the future
The only historical limitation of somatic embryogenesis lay in the possibility of unplanned mutations arising from the embryo induction process. But advanced molecular screening techniques have mitigated this.
In time, we should expect to see greater use of enhanced, tolerant plant genotypes through specific gene editing of somatic embryos and synthetic seeds. What remains to be done is fervent research into the underlying mechanisms of somatic embryogenesis, their efficient conversion into synthetic seeds and successful cryopreservation. This should be done using a greater number of plant species for more efficient, productive, tolerant and sustainable agroforestry plantations, and in conservation programmes.