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,...
Sorghum is a cereal crop with key agronomic traits of drought and heat stress tolerance, making it an ideal food and industrial commodity for hotter and more arid climates. These stress tolerances also present a useful scientific resource for studying the molecular basis for environmental resilience. Here we provide an extensive review of current transcriptome and proteome works conducted with laboratory, greenhouse, or field-grown sorghum plants exposed to drought, osmotic stress, or treated with the drought stress-regulatory phytohormone, abscisic acid. Large datasets from these studies reveal changes in gene/protein expression across diverse signaling and metabolic pathways. Together, the emerging patterns from these datasets reveal that the overall functional classes of stress-responsive genes/proteins within sorghum are similar to those observed in equivalent studies of other drought-sensitive model species. This highlights a monumental challenge of distinguishing key regulatory genes/proteins, with a primary role in sorghum adaptation to drought, from genes/proteins that change in expression because of stress. Finally, we discuss possible options for taking the research forward. Successful exploitation of sorghum research for implementation in other crops may be critical in establishing climate-resilient agriculture for future food security.
Plants reprogram gene expression as an adaptive response to survive high temperatures. While the identity and functions of intracellular heat stress-responsive proteins have been extensively studied, the heat response of proteins secreted to the extracellular matrix is unknown. Here, we used Sorghum bicolor, a species adapted for growth in hot climates, to investigate the extracellular heat-induced responses. When exposed to 40 C for 72 h, heat-sensitive Arabidopsis cell suspension cultures died, while ICSB338 sorghum cell cultures survived by activation of a transcriptional response characterized by the induction of HSP70 and HSP90 genes. Quantitative proteomic analysis of proteins recovered from cell culture medium revealed specific heat stress-induced protein accumulation within the sorghum secretome. Of the 265 secreted proteins identified, 31 responded to heat (2-fold change), with 84% possessing a predicted signal peptide for targeting to the classical secretory pathway. The differentially accumulated proteins have putative functions in metabolism, detoxification, and protein modifications. A germin (SORBI_3003G427700) was highly heat-inducible at both protein and gene level. Overall, our study reveals new insights into sorghum responses to heat and provides a useful resource of extracellular proteins that could serve as targets for developing thermotolerant crops. Data are available via ProteomeXchange with identifier PXD021536.
Sorghum, a naturally drought tolerant crop, is genetically diverse and provides a wide gene pool for exploitation in crop breeding. In this study, we experimentally assessed friable callus induction rates of seven sorghum varieties using shoot explant for the generation of cell suspension cultures. The cell suspensions were characterized in terms of cell growth and viability profiles as well as gene expression following 400 mM sorbitol-induced osmotic stress for 72 h. Only ICSB 338, a drought susceptible variety, was readily amenable to friable callus formation. Cell culture growth plots of both ICSB 338 and White sorghum (used as a reference line) depicted typical sigmoidal curves. Interestingly, Evans blue assay showed that ICSB 338 cell cultures are more susceptible to osmotic stress than the White sorghum cells. The osmotic stress treatment also triggered differential expression of eight target genes between the two cell culture lines. Overall, these results suggest that the genetic diversity of sorghum germplasm influences friable callus induction rates and molecular responses to osmotic stress, and could be further exploited in plant stress biology studies. Therefore, we have developed a valuable resource for use in molecular studies of sorghum in response to a range of biotic and abiotic stresses.
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