Enhanced expression of heat-shock proteins in thermo-tolerant lines of sunflower and their progenies selected on the basis of temperature-induction response

Abstract

The major lacuna in developing stress-tolerant lines through breeding is the lack of suitable techniques for screening the segregating population. We report here the development of an efficient technique for identifying high-temperature-tolerant lines in sunflower. The rationale behind this technique is that the stress-responsive genes are expressed during sub-lethal (induction) stress, and its products impart tolerance at subsequent lethal stresses. The genetic variability in gene expression upon induction stress is responsible for the differential survival and recovery following exposure to severe lethal stress in a heterogeneous population. Optimization of induction and subsequent lethal temperature levels is a pre-requisite for developing a standardized screening protocol. The optimum induction temperature in sunflower was identified by subjecting the germinated seedlings to various sub-lethal temperatures followed by exposure to a specific lethal temperature for a fixed duration. Gradual temperature induction was found to be optimum in bringing about a maximum response in terms of the recovery growth of seedlings after exposure to a lethal temperature. Following the optimum induction treatment, seedlings were subjected to a specific high temperature for different periods to arrive at a high-stringency lethal temperature treatment. By adopting this approach an open-pollinated population of Helianthus annuus L. (morden) was screened for high-temperature tolerance. The seedlings which survived at the high-stringency lethal temperature following the optimum induction treatment were identified as thermo-tolerant lines. At the plant level too these identified lines showed higher tolerance as reflected by a higher membrane integrity and recovery leaf-area. The progeny population of these identified lines also exhibited higher tolerance to temperature compared to the original population, indicating the persistence of the selected trait. This tolerance was associated with a higher accumulation of heat-shock proteins (HSPs). We propose that this technique can be used as a potential tool to identify and select temperature-tolerant lines from a heterogeneous/segregating population.