Stress and quality in in vitro culture

The response of animals to unfavourable environmental conditions is to migrate. Plants, lacking this ability, rely on an array of stress defences. These responses are clearly defined for most stresses, e.g. temperature, drought, pathogen attack etc. They involve stress perception, stress signalling, activation of transcription factors, genetic reprogramming and characterised changes in the proteome and metabolome. In so far as tissue culture is an unnatural process, the plant tissues are exposed to stresses and stress combinations that they may not have encountered in nature in their long evolution. It is a remarkable reflection on the plasticity of the plant genome that it can decipher and respond to novel in vitro stresses. The first stress that plant tissues in culture are exposed to is the oxidative stress resulting from severe wounding at excision of the tissue explant. This is associated with activation of the cell cycle, cell dedifferentiation and expression of totipotency. It is also associated, in some genotypes, with genome instability (somaclonal variation). Following establishment in culture, the plant tissues may be stressed by the high salt or unbalanced mineral composition of the medium and by the effects of plant growth regulators in the medium. When microshoots form they often respond to the culture environment by developing abnormal morphology. The extreme version of this is hyperhydricity where the cells become enlarged and the tissues appear waterlogged. Associated symptoms are tip and leaf necrosis, callusing and in extreme cases, tissue death. In less extreme conditions, the morphology of the microplants can vary from an etiolated habit to a juvenile leaf morphology to a more normal in vivo leaf morphology. The habit of microshoots is economically important as it can affect labour costs; also some habits are more amenable to automated cutting than others. Optimization of the in vitro habit for ease of cutting may adversely affect the post vitrum performance of the propagules. Biotization (in vitro or post vitrum inoculation with beneficial microorganisms) has been shown to improve the biotic and abiotic stress tolerance of microplants and may contribute to the production of added value propagules. Managing stress in in vitro cultures can result in reduction of the harmful and exploitation of the beneficial aspects.