Distinct Preflowering Drought Tolerance Strategies of Sorghum bicolor Genotype RTx430 Revealed by Subcellular Protein Profiling

Abstract

Drought is the largest stress affecting agricultural crops, resulting in substantial reductions in yield. Plant adaptation to water stress is a complex trait involving changes in hormone signaling, physiology, and morphology. Sorghum (Sorghum bicolor (L.) Moench) is a C4 cereal grass; it is an agricultural staple, and it is particularly drought-tolerant. To better understand drought adaptation strategies, we compared the cytosolic- and organelle-enriched protein profiles of leaves from two Sorghum bicolor genotypes, RTx430 and BTx642, with differing preflowering drought tolerances after 8 weeks of growth under water limitation in the field. In agreement with previous findings, we observed significant drought-induced changes in the abundance of multiple heat shock proteins and dehydrins in both genotypes. Interestingly, our data suggest a larger genotype-specific drought response in protein profiles of organelles, while cytosolic responses are largely similar between genotypes. Organelle-enriched proteins whose abundance significantly changed exclusively in the preflowering drought-tolerant genotype RTx430 upon drought stress suggest multiple mechanisms of drought tolerance. These include an RTx430-specific change in proteins associated with ABA metabolism and signal transduction, Rubisco activation, reactive oxygen species scavenging, flowering time regulation, and epicuticular wax production. We discuss the current understanding of these processes in relation to drought tolerance and their potential implications.

Keywords: FLOWERING LOCUS C; abiotic stress; abscisic acid; adaptive mechanisms; drought stress; drought tolerance; postflowering; preflowering; proteomics; reactive oxygen species; rubisco activase; sorghum.

Figure 1

Evaluation of cytosolic and organelle enrichment. Ribosomal proteins abundant in the cytoplasm were more abundant in cytosolic-enriched samples (A), while photosynthesis-related chloroplastic (B) and nuclear histone proteins (C) were more abundant in organelle-enriched samples. Color and scale bars represent average Z-score transformed protein abundances of all biological replicates for each sample. Protein identifiers correspond to STRING or UniProt accessions. Wat., Watered; Drt., Drought.

Figure 2

Principal component analysis of organelle- (A) and cytosolic-enriched protein profiles (B) in response to drought. Correlation between RTx430 and BTx642 drought responses in (C) organelle- and (E) cytoplasm-enriched samples. Venn diagram summary of proteins with adj. p-value < 0.05 in each genotype for (D) organelle- and (F) cytosolic-enriched samples. Purple data points are significantly changed in either genotype, while red points correspond to RTx430-specific changes (adj. p-value < 0.05). D, Drought; W, Watered; PC, Principal Component.

Figure 3

Comparison of genotype-specific drought responses in organelle-enriched samples of RTx430 (A) and BTx642 (B). Red datapoints passed a limma adjusted p-value threshold of 0.05. Pink circles indicate proteins that changed significantly only in RTx430 (limma adj. p-value < 0.05). Protein names correspond to STRING identifiers, or when unavailable, UniProt accessions.