When exposed to drought stress many plants reprogram their gene expression to activate adaptive biochemical and physiological responses for survival. However, most of the well-studied adaptive responses are common between drought-sensitive and drought-tolerant species, making it difficult to identify the key mechanisms underpinning successful drought tolerance in crops. We developed a sorghum experimental system that compares between drought-sensitive (ICSB338) and enhanced drought-tolerant (SA1441) varieties. We show that sorghum activates a swift and robust stomatal shutdown to preserve leaf water content when water stress has been sensed. Water uptake is enhanced via increasing root cell water potential through the rapid biosynthesis of predominantly glycine betaine and an increased root-to-shoot ratio to explore more soil volume for water. in addition to stomatal responses, there is a prompt accumulation of proline in leaves and effective protection of chlorophyll during periods of water limitation. Root and stomatal functions rapidly recover from water limitation (within 24 h of re-watering) in the drought-tolerant variety, but recovery is impaired in the drought-sensitive sorghum variety. Analysis of the root proteome revealed complex protein networks that possibly underpin sorghum responses to water limitation. common and unique protein changes between the two sorghum varieties provide new targets for future use in investigating sorghum drought tolerance. Drought stress is a major barrier to agricultural productivity 1 , primarily causing damaging water deficits in plant cells. Drought-induced osmotic stress triggers loss of cell turgor, which affects the rate of cell expansion and overall cell size, ultimately restricting plant growth and development 2. In sub-Saharan Africa, drought is often associated with very high temperatures, the combination of which severely limits crop productivity 1 , particularly in rain-fed agricultural systems. Several climatic models predict increases in surface temperatures and drought episodes by the year 2050 3 , further threatening global food security 4. Therefore, it is important to understand plant adaptive responses to drought in order to improve breeding for drought tolerant crops 5. Plants respond to the effects of drought stress through a complex network of mechanisms aimed at sustaining cellular metabolism under the prevailing conditions. The mechanisms of drought avoidance, escape and tolerance in a variety of plant species have been extensively reviewed 2,6-8. While drought escape is important in desert ephemerals that rapidly complete their life cycles before the onset of water deficits 2 , drought avoidance and tolerance are significant response mechanisms in many other plant species. For instance, plants can avoid drought by conserving water through various anatomical features. These include an extensive root system, a thick waxy cuticle, and reduced leaf surface area attained through leaf rolling, folding or shedding. In addition, changes in stomatal number,...

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Drought stress triggers remarkable physiological changes and growth impediments, which significantly diminish plant biomass and crop yield. However, certain plant species show notable resilience, maintaining nearly normal yields under severe water deficits. For example, sorghum is a naturally drought-tolerant crop, which is ideal for studying plant adaptive responses to drought. Here we used sorbitol treatments to simulate drought-induced osmotic stress in sorghum cell suspension cultures and analysed fractions enriched for extracellular matrix proteins using isobaric tags for relative and absolute quantification technology. Sorbitol induced an overall increase in protein secretion, with putative redox proteins, proteases, and glycosyl hydrolases featuring prominently among the responsive proteins. Gene expression analysis of selected candidates revealed regulation at the transcriptional level. There was a notable differential gene expression between drought-tolerant and drought-sensitive sorghum varieties for some of the candidates. This study shows that protein secretion is a major component of the sorghum response to osmotic stress. Additionally, our data provide candidate genes, which may have putative functions in sorghum drought tolerance, and offer a pool of genes that could be developed as potential biomarkers for rapid identification of drought tolerant lines in plant breeding programs.

Sorghum is one of the most important cereal crops grown in the world for human consumption, animal feed and bio-energy. The objective of the study was to estimate the genetic variability, heritability and genetic gain of some agro-physiological traits of 98 sorghum accessions in South Africa. The studies were conducted at Makhathini in KwaZulu-Natal and Burgershall in Mpumalanga provinces. The experiments were laid out in an alpha lattice design replicated twice. The plant height, panicle length, panicle width and panicle exsertion, rachis number, panicle weight, thousand seed weight, and grain yield per panicle were recorded. The data were subjected to analysis of variance using generalised linear model and means were separated using Duncan Multiple Range Test. Coefficients of variation were also determined. There were significant differences among the traits measured. Plant height was significant and positively associated with panicle length (r =0.139), panicle width (r = 0.127) and panicle weight (r = 0.1457). Panicle exsertion was highly significant and negatively correlated with all other traits except with plant height. The Principal Component analysis revealed three most important PCs contributing a total variation of 92.36%. The PC1, PC2, and PC3 contributed 42.53, 34.65 and 15.18%, respectively. Low, medium and high estimates of broad sense heritability were found in different plant characters under study. Highest heritability estimates were found in plant height, thousand seed weight, panicle length and rachis number. Values of genetic advance ranged between 0.39 to 17655.04 and the genetic gain (of the mean percent) ranged from 3.27 to 372.52. Greater magnitude of broad sense heritability coupled with higher genetic advance in traits studied provided the evidence that these were under the control of additive genetic effects indicating that selection in the germplasm should lead to a fast improvement of the traits.