Exogenous abscisic acid treatment regulates protein secretion in sorghum cell suspension cultures

Abstract

Drought stress adversely affects plant growth, often leading to total crop failure. Upon sensing soil water deficits, plants switch on biosynthesis of abscisic acid (ABA), a stress hormone for drought adaptation. Here, we used exogenous ABA application to dark-grown sorghum cell suspension cultures as an experimental system to understand how a drought-tolerant crop responds to ABA. We evaluated intracellular and secreted proteins using isobaric tags for relative and absolute quantification. While the abundance of only ~ 7% (46 proteins) intracellular proteins changed in response to ABA, ~32% (82 proteins) of secreted proteins identified in this study were ABA responsive. This shows that the extracellular matrix is disproportionately targeted and suggests it plays a vital role in sorghum adaptation to drought. Extracellular proteins responsive to ABA were predominantly defense/detoxification and cell wall-modifying enzymes. We confirmed that sorghum plants exposed to drought stress activate genes encoding the same proteins identified in the in vitro cell culture system with ABA. Our results suggest that ABA activates defense and cell wall remodeling systems during stress response. This could underpin the success of sorghum adaptation to drought stress.

Keywords: ABA; Sorghum; cell suspension cultures; drought stress; extracellular matrix; gene expression; iTRAQ; protein secretion; secretome; total soluble protein.

Figure 1.

Predictions of signal peptides and cellular locations of ABA-responsive proteins of sorghum cell suspension cultures. (a) signal peptide predictions for ABA-responsive secreted proteins were done using the SignalP 6.0 server. (b) gene ontology terms for cellular components of ABA-responsive secreted and total soluble proteins were retrieved from the UniProt database.

Figure 2.

Biological processes of ABA-responsive secreted and total soluble proteins of sorghum cell suspension cultures. Gene Ontology terms for biological processes were retrieved from the UniProt database.

Figure 3.

Putative functional groupings of the intracellular and extracellular ABA-responsive proteins of sorghum cell suspension cultures.

Figure 4.

Gene expression analysis of sorghum root tissue following drought stress. Drought-susceptible ICSB338 and drought-tolerant SA1441 sorghum plants were exposed to drought stress by withholding water for 12 days. Root tissue samples were harvested for gene expression analysis using quantitative reverse transcription – polymerase chain reaction. Bars represent mean ± SD (n = 3). *, ** and *** represent statistical significance at p ≤.05, 0.01 and 0,001, respectively using a Student’s t-test.