Understanding “Stress” and How To Measure It

Understanding what stress in seeds is, and developing analytical methods in the search for potential markers of seed viability.

High-Performance Liquid Chromatography (HPLC) is one of the analytical methods used to measure chemical compounds involved in the seed stress response.

Conventional germination tests are convenient to assess seed viability in species of which certain seed traits are known. However, germination tests have been developed for crops, but not for the majority of the species stored in the Millennium Seed Bank, where we are challenged with a biodiversity of thousands of wild species.

Seed stress arises following exposure to unfavourable conditions both during development on the mother plant and following dispersal. Environmental factors such as temperature and humidity can affect seed development and also have consequences for seed longevity. In addition, exposure to toxic pollutants e.g. heavy metals can result in stress leading to reduction in seed viability.

We endeavour to use methods of biochemistry and molecular biology to find markers of viability and germination to diagnose and quantify ‘stress’, and to understand the regulatory switchboards that induce stress responses leading to acclimation and adaption. This project is one of a series in the research theme ‘Stress and Survival”.

Prior to stress marker development it must be understood what “stress” in seeds is, and if and how we can “put a number on it” as argued by Lord Kelvin in his famous quote:

“When you can measure what you are speaking about and express it in numbers you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind” (Kelvin, 1883).

We have recently developed a new stress model for seeds (see Annex 1), based on the General Adaptation Syndrome (Selye, 1936), which is the perhaps most accepted biomedical stress model since 1936. In the ‘alarm phase’ of this triphasic seed stress model, a stress factor is perceived by a signalling network of plant hormones and reactive oxygen species (ROS), and signals are transduced to activate the stress response systems. Under continuing stress, the ‘resistance phase’ is reached when the stress response is sufficient to protect cells from damage and to repair damage suffered, maintaining plant viability. The ‘exhaustion phase’ is reached when the protection and repair mechanisms fail, leading to the breakdown of essential molecules, including DNA, proteins and lipids, ultimately resulting in seed death.

The seed stress response is being investigated using a range of analytical approaches including omics-based methodology to gain a detailed picture of the processes initiated following exposure to stressful conditions. The analysis of certain compounds that respond to stressful environmental conditions is often a major challenge in itself. The chemical composition of plant seeds is dominated by their reserves, carbohydrates, lipids and proteins. When co-extracted with, for example nucleic acids, these compounds can interfere with conventional assays. In addition, many seeds are rich in (poly)phenols, which can be oxidized to quinones during extraction and then covalently bind to the analyte, for example, antioxidants, leading to erroneous results. Modifying existing methods, finding adequate combinations of existing methods to deal with several interferents simultaneously, or developing new assays for reproducible and reliable measurement of a certain group of chemical compounds is a challenging and often very time-consuming task for the analytical chemist, and we put significant effort into developing appropriate assays for our research.