Response of starch potato (Solanum tuberosum L.) genotypes to osmotic stress in vitro and drought stress in vivo

Abstract

Potato (Solanum tuberosum L.) is one of the most important crops in the world. In addition to food and fodder, potato is also used for industrial purposes like production of adhesives, paper, and cosmetics. The vegetative growth phase of potato correlates with dry periods in spring and early summer, which are increased by climate change. Drought stress leads to morphological, physiological, and biochemical changes in the plant that have an extensive negative impact on the size and quality of the tubers. Since potato is a drought-sensitive species with its shallow root system, the interest in drought-tolerant cultivars is immense. Because ex vitro test systems are expensive and labor-intense and because additional parameters like other abiotic and biotic stressors influence the stress response, investigations in vitro are of great interest. Advantages of in vitro systems are the controlled light intensity, temperature, and supply of nutrients. Furthermore, pathogens can be excluded from the culture, and experiments require less space. Osmotic stress in vitro can be induced by adding an osmoticum, which lowers the osmotic potential in the culture medium. In the context of this work, an existing in vitro test system was optimised. For this purpose, the solid medium was replaced by liquid medium to enable that sorbitol can be added stepwise with increasing concentration. This resulted in two advantages: 1. the stress induction was gradual, and thus no osmotic shock was induced; 2. the plants were able to establish roots prior to the addition of the osmoticum, which allowed the stress that occurred by cutting the explants to be mitigated. This experiment showed that sorbitol was probably taken up by the roots and transported into the shoots, where it was detected by GC-MS. Furthermore, selected potato genotypes were investigated for their early drought stress response in open greenhouse and shelter experiments and their early osmotic stress response in vitro. For this purpose, candidate proteins for drought stress were selected after identification by LC-MS in material from rainout shelter trials. Identified proteins were further selected based on differential abundance in the genotypes ‘Eurostarch’ and ‘Tomba', which were postulated to be rather tolerant. From the identified candidate proteins, eight genes were selected, and their expression was investigated by RT-qPCR in leaves after seven days of water withdrawal in two trials in an open greenhouse, where differences between treatments but no genotypic effects were detected. Expression of peroxidase 51-like (POD), subtilase family protein (SBT1.7), and cell wall/vacuolar inhibitor of fructosidase (INH1) responded strongly to drought stress in all genotypes. Dry masses of the shoots also demonstrated stress induction ex vitro without reaching the permanent wilting point in the open greenhouse. The analysis under osmotic stress in two experiments in vitro also showed altered shoot dry mass and differential gene expression under osmotic stress. SBT1.7 was regulated in vitro in all genotypes under osmotic stress. POD showed similar regulation to the open greenhouse experiments in three of the four genotypes analysed. Furthermore, INH1 was only regulated in ‘Eurostrach’ and 'Tomba'. Additionally, 13-LOX, a gene of the family of lipoxygenases linked to osmotic adjustment, was upregulated in all genotypes under osmotic stress. Finally, differentially abundant proteins were identified in leaves of two shelter experiments under drought stress, nitrogen deficiency, and combined stress in two genotypes that differ in tolerance towards those stresses. Results showed differences in proteomic responses under combined as well as single stresses. The sensitive genotype ‘Kiebitz’ showed a higher abundance of proteases, whereas the rather tolerant genotype ‘Tomba’ showed a lower abundance of such proteins. In summary, important insights into the stress response of potato to drought stress and osmotic stress were gained. Further studies with earlier sampling could help to better understand genotypic differences and develop biomarkers for early drought stress. An alternative osmoticum for the in vitro system should be considered.