The mechanisms of drought stress tolerance in the crop Sorghum bicolor

Abstract

Drought stress can have a major impact upon plant survival and crop productivity. Sorghum bicolor is an important cereal crop grown in the arid and semi-arid regions of >98 different countries. It is well adapted to the harsh drought-prone environments in which it is grown however; relatively few studies have investigated the molecular basis of these adaptations. Breeding programs have lead to the identification of ‘stay-green’ varieties, so-called due to their ability to maintain green photosynthetic leaf area for longer under drought conditions. However, despite extensive breeding efforts to select for this trait we have very little understanding of the fundamental biological processes that underlie it.

Microarray analysis was used to identify gene expression changes in sorghum following heat stress, drought stress and combined heat and drought stress. These microarrays were additionally used to compare gene expression in stay-green (drought-tolerant) and senescent (drought-sensitive) sorghum lines. Ontological analysis of the genes expressed to higher levels in the stay-green lines identified key processes hypothesised to be associated with the trait. These include genes associated with proline and betaine biosynthesis, glutathione S-transferase (GST) activity and the regulation of stomatal aperture and density. Both proline levels and GST activity were found to be higher in the stay-green lines thus validating that the changes at the gene expression level result in changes at the protein level. Stay-green lines were also shown to have reduced transpiration and reduced numbers of stomata.

Two signalling genes, DREB1A and SDIR1 were expressed to higher levels in the stay-green varieties. Transgenic lines overexpressing these genes were generated in order to test their function. Based on the gene expression data, putative mechanisms underlying two QTL for the stay-green trait (Stg1 and StgB) were generated. Further validation of these genes and processes could not only improve our understanding of drought tolerance mechanisms in sorghum, but also facilitate the improvement of future sorghum cultivars.