Genes selected for their role in modifying post-harvest life

Once a plant is harvested it starts a process of decay that will result in the inevitable death of the organism and the deterioration of its organic matter. This process is even more accentuated if the harvested tissue is a particular organ (fruit, leaf, root, etc.) instead of the whole plant. In an ideal world this fact should not be a problem, we would grow our vegetables and fruits in our backyard and consume them fresh every day. In real life most of the food we consume has been grown hundreds or even thousands of kilometres away from the shop where we bought them. Therefore we are daily confronted with the fact that food crops, after they are grown, need to be transported to intermediate destinations, stored, transported again and distributed before finally reaching the consumer. Aside from the natural/physiological reasons stated above, sociological aspects need to be considered; the backyard self-sufficient growing strategy will not work nowadays because many of us do not have a backyard. The world is evolving from a predominantly rural population to a new demographic distribution based on high human concentration on urban areas. Areas of food production are therefore far away from areas of food consumption. We are consequently fighting a continuous battle to bring the right food to the right place with a minimum of losses. Sadly, although important advances have been made in post-harvest technology, we are still losing the battle.

Post-harvest problems can ac count for substantial losses, the magnitude of which depends on the crop, the country and the year. It is important to stress that post-harvest losses are one of the most significant factors limiting agricultural production in third-world and developing countries. Whereas technically advanced countries such as the USA, Japan, Australia and European countries can apply relatively sophisticated technologies to minimise losses; developing countries cannot afford them. In addition, many of the developing regions in the world are situated in the tropics, where high temperature and humidity exacerbate the problem.

Senescence, the final stage in the life of a plant (or a particular organ) was once thought to have an essentially chaotic nature in which cell components break down without a particular order. Nevertheless, recent advances in our understanding of the process have revealed that senescence is a very well programmed developmental stage, involving highly coordinated cellular events that require the sequential action of many genes. Our fundamental knowledge of senescence has greatly improved in the last few years but we are still far from having a good understanding of the underlying biochemical and molecular mechanisms. There are two kinds of senescence processes, a natural one that comes after the end of the useful life of an organ is reached, and a stress- or environmentally-induced senescence. Fruits will develop and ripen to entice predators and ensure seed dispersal. When the seeds contained in the fruit are not viable any more, a natural senescence process will eventually end up with the spoilage of the fruit. Old leaves shrivel and fall while new ones develop to keep the photosynthetic capacity of the plant. These events are just natural processes pre-programmed in the genetic code. Harvesting plant tissue for commercialisation causes a series of stresses in the detached tissue that will inexorably trigger senescence. Natural and induced senescence do not always share the same mechanisms.

The main objective of post-harvest technology is to increase the useful life of a particular foodstuff but increasing the useful life is a very ambiguous term. For some produce it means keeping the tissue turgor for longer (we all want our lettuces, broccoli and apples to have that fresh crispy feel), for other foods such as mangoes and bananas it means keeping the right degree of softness for longer without over-ripening (we like our mangoes soft but not mushy). Horticultural produce is extremely heterogeneous with a large variety of plant tissues being commercialised such as fruit, leaves, flowers, roots and tubers. Even though senescence has some common features, each tissue has specific characteristics and therefore needs to be studied separately. This is the reason why there is not a single universal post-harvest treatment useful for all horticultural crops. Leafy vegetables must be treated in a different way from fruits and even fruits have a wide variety of post-harvest problems depending on the fruit and even the commercial variety.

From a biotechnological point of view it is therefore important to establish the nature of the crop and the particular problem before establishing the approach to be attempted. It is also important to emphasise that the same problem can be tackled with very different approaches. Metabolic engineering can target internal processes but is not restricted to endogenous genes. Modern genetic engineering techniques allow us to cross species barriers (and even kingdom barriers) and therefore genes that would not normally be accessible by conventional breeding can now be incorporated into the plant species being targeted.

Post-harvest technologies such as atmosphere control and (CO2 humidity), refrigeration, irradiation, etc., have proven useful in controlling post-harvest losses, but the implementation of many of the existing and new technologies is quite difficult in many developing countries due to the lack of infrastructure and specialised personnel in rural areas. Many crops are also grown in small farms meaning that a single grower is not able to afford the economic cost of setting up treatment plants and has no access to specialised packaging requirements. Regional centres, either governmental or private, can alleviate the situation but require a level of organisation commonly non-existent.