BioProject

‘Pulsechip’, a boutique cDNA microarray, generated from a set of chickpea (Cicer arietinum L.) unigenes, grasspea (Lathyrus sativus L.) ESTs and lentil (Lens culinaris Med.) resistance gene analogs, was employed to generate an expression profile of chickpea genotype ICC 3996 to drought, cold and high-salinity stresses. The experiments were performed in three biological replications. The experiments were conducted in reference design where respective tissues from unstressed plants served as control. The leaf/shoot, flower/pod and/or root tissues were collected and used for hybridization to measure changes in RNA abundance of treatment vs. control. The tissues from five experimental replicate plants per biological replication were pooled together (leaf/flower/root tissues pooled separately) before RNA extraction. This RNA was used to prepare cDNA targets for expression analysis using microarray. The microarray had six technical replicate spots per EST. The transcript level for each EST/cDNA was firstly calculated as the average intensity of the six technical replicates and then the average intensity of three biological replicates. Data analysis included LOWESS normalization (LOcally WEighted polynomial regreSSion) to adjust for differences in quantity of initial RNA, labeling and detection efficiencies. A dye swap in one biological replicate adjusted dye bias, if any. The Differentially Expressed (DE) ESTs were identified as those with a 95% confidence interval for mean fold change (FC) that extended beyond the two-fold cut-off and also passed the Students t test (P<0.05) and FDR correction. These cut-offs translate into induced ESTs having a log2 ratio > 1 and repressed ESTs a ratio of < -1. Overall, 46, 54 and 266 ESTs were identified as DE under drought, cold, and high-salinity stresses, respectively. The important ones DE in response to drought stress include induction of transcripts associated with dehydrin-cognate, lipid-transfer protein precursor, and glutamate-hydrolysing asparagine synthetase, whilst repression of transcripts associated with senescence, photosynthesis/energy metabolism, auxin-repressed protein, starch metabolism, AP2/EREBP1 DNA binding domain, putative ARF1 GTPase activating protein, and histidine-containing phosphotransfer protein ATHP3. The interesting transcripts DE in response to cold-stress included induction of phosphate-induced protein, cationic peroxidase, UDP-glucose 4-epimerase, Avr9/Cf9 rapidly elicited protein, DNA-J like protein involved in intracellular protein transport, and protein kinase, whist repression of transcripts associated with a hypothetical transmembrane protein, membrane related protein CP5, lipid-transfer protein precursor, WD repeat protein and several transcripts associated with cellular metabolism, cell cycle and DNA processing, protein synthesis and photosynthesis/energy metabolism. Under high-salinity stress, several transcripts were DE only in roots at 24 hpt but DE in shoots or both, shoots and roots at 48 hpt, relating to the theory of upward movement of salt to shoots at later stages when roots fail to restrict it (Munns et al., 2002). The important transcripts DE in response to high-salinity stress included induction of aluminum-induced protein, auxin-repressed proteins, metallothionein-like protein, glutamate dehydrogenase, sucrose synthase, UDP-glucose 4-epimerase, xylose isomerase, class 10 pathogenesis related protein, disease resistance protein, SNAKIN 2 antimicrobial peptide, and DNA-J like protein involved in intra-cellular protein transport. Whilst high-salinity stress caused the repression of transcripts associated with cell rescue/death, cell cycle/DNA processing, cellular metabolism, photosynthesis/energy metabolism, and transport facilitation. Several of the above transcripts have been previously implicated to be associated with abiotic stress response in other crops. Hence, the study of more genotypes and transcriptional changes at several time points may provide a better picture of involvement of the genes being interrogated here in drought tolerance/susceptibility. Subsequently, the functionality of candidate tolerance genes detected through this approach could be validated by overexpressing the genes through transgenics or silencing them using knockout-mutants/antisense/RNAi. Keywords: abiotic stress response, drought, cold, high-salinity, chickpea Overall design: Total RNA was extracted from separately pooled leaf/flower/root tissues at each time-point (including control samples) using the RNeasy® Plant Mini Kit (Qiagen, Valencia, CA). The quantity and quality of the total RNA samples were assessed by OD260/OD280 ratios and gel electrophoresis respectively. Fluorescent-labeled targets were prepared and hybridized to array slides as described [Coram, TE. and Pang, ECK. 2006. Expression profiling of Chickpea genes differentially regulated during a resistance response to Ascochyta rabiei. Plant Biotechnology Journal. 4(6), 647–666]. All hybridizations were performed with six technical replicates and three biological replicates, incorporating dye-swapping (i.e. reciprocal labelling of Cy3 and Cy5) to eliminate any dye bias. Slides were scanned at 532 nm (Cy3 green laser) and 660 nm (Cy5 red laser) at 10 µm resolution using an Affymetrix® 428™ array scanner (Santa Clara, CA), and captured with the Affymetrix® Jaguar™ software (v. 2.0, Santa Clara, CA). Image analysis was performed using Imagene™ 5 (BioDiscovery, Marina Del Rey, CA) software. Quantified spot data was then compiled and transformed using GeneSight™ 3 (BioDiscovery, Marina Del Rey, CA). Data transformations consisted of a local background correction (mean intensity of background was subtracted from mean signal intensity for each spot), omitting flagged spots, LOWESS normalisation of the entire population, creating a Cy5/Cy3 mean signal ratio, taking a shifted log (base 2), and combination of duplicated spot data. To identify differentially expressed (DE) genes, expression ratio results were filtered to eliminate genes whose 95% confidence interval for mean fold change (FC) did not extend to 2-fold up or down, followed by Students t test with False Discovery Rate (FDR) multiple testing correction to retain only genes in which expression changes versus untreated control were significant at P < 0.05.