Compatible solute, transporter protein, transcription factor, and hormone-related gene expression provides an indicator of drought stress in Paulownia fortunei

Abstract

Drought is one of the most devastating effects of global climate change. Leaves contribute significantly to the management of water deficit and plant adaptation to drought stress. In this study, we compared the transcriptomes of leaves of two genotypes of Paulownia fortunei with different drought tolerances. Solexa sequencing and qRT-PCR were used for gene expression analysis and validation. Variations in leaf growth were found between drought-treated and well-watered samples in both genotypes. Drought-treated samples from diploid and autotetraploid P. fortunei cultivars showed inward leaf rolling and smaller blade size compared with the well-watered ones. High throughput transcriptome sequencing generated 266,700,100 high-quality reads representing 110,586 unigenes from the leaves. The drought-treated samples responded to water deficiency by inducing various genes and pathways, such as photosynthesis, carbon fixation in photosynthetic organisms, stress response, plant hormone signal transduction, and flavonoid pathways. Regulatory genes, such as WRKY, and transcription factors, such as NAC, known for leaf development under drought stress, were highly expressed in the drought-treated samples, and so were the genes related to compatible solutes (sugars, sugar alcohols, amino sugars, amino acids, or betaine), hormones, and various transporters. This study illustrates changes in the expression pattern of genes induced in response to drought stress and provides a comprehensive and specific set of drought-responsive genes in P. fortunei.

Fig. 1

Tissues used for the transcriptome analysis. a PF2W75 well-watered diploid, b PF2W25-12D 12 days drought-treated diploid, c PF4W75 well-watered tetraploid, d PF4W25-12D 12 days drought-treated tetraploid

Fig. 2

Effects of drought stress on P. fortunei physiology. PF2 diploid P. fortunei, PF4 autotetraploid P. fortunei. a Effect of drought stress on leaf relative water content. b Effect of drought stress on leaf chlorophyll content. c Effect of drought stress on leaf SOD activities. d Effect of drought stress on leaf soluble protein content. e Effect of drought stress on leaf soluble sugar content. f Effect of drought stress on leaf proline content

Fig. 3

Characteristics of homology search of Illumina sequences against the nr database. a The e-value distribution of BLAST hits for each unique sequence with a cutoff e-value of 1.0E − 5. b Similarity distribution of the top BLAST hits for each sequence. c Species distribution is shown as a percentage of the total homologous sequences with an e-value of at least 1.0E − 5

Fig. 4

Classification of the clusters of orthologous groups (COG) (http://www.ncbi.nlm.nih.gov/COG/) for the transcriptome of P. fortunei all-unigenes; 29,510 unigenes (26.69 % of the total) were annotated and divided into 25 specific categories

Fig. 5

Statistics of differentially expressed genes in each pairwise comparison. Red bars represent the up-regulated genes, while blue bars represent the down-regulated ones. PF4W25-12D 12 days drought-treated tetraploid, PF4W75 well-watered tetraploid, PF2W25-12D 12 days drought-treated diploid, PF2W75 well-watered diploid

Fig. 6

Quantitative RT-PCR analysis of candidate drought response genes. CL4426 a peptide/nitrate transporter, CL10709 a dehydrin, CL5735 a flavonoid glycosyltransferase, CL8793 a glycine-rich protein precursor, Unigene28894 an alcohol dehydrogenase, Unigene18828 a chlorophyll A/B binding protein, CL2903 a disease resistance response protein, CL6055 an auxin-induced protein X10A. 18S rRNA was used as the internal reference gene. For each group, the highest expression level was considered as 100 %, and other samples were normalized accordingly. Standard error of the mean for three technical replicates is represented by the error bars. a PF2W25-12D 12 days drought-treated diploid, PF2W25-9D 9 days drought-treated diploid, PF2W25-6D 6 days drought-treated diploid, PF2W25-3D 3 days drought-treated diploid, PF2W75, well-watered diploid. b PF4W25-12D 12 days drought-treated tetraploid, PF4W25-9D 9 days drought-treated tetraploid, PF4W25-6D 6 days drought-treated tetraploid, PF4W25-3D 3 days drought-treated tetraploid, PF4W75 well-watered tetraploid

Fig. 7

Zeatin biosynthesis pathway in P. fortunei. Up-regulated expressed genes in both PF2W25-12D and PF4W25-12D are in red boxes. Double solid line plasma membranes of the leaf of P. fortunei. Dashed line nuclear membranes of the leaf cell