Drought Stress-Mediated Transcriptome Profile Reveals NCED as a Key Player Modulating Drought Tolerance in Populus davidiana

Populus trichocarpa has been studied as a model poplar species through biomolecular approaches and was the first tree species to be genome sequenced. In this study, we employed a high throughput RNA-sequencing (RNA-seq) mediated leaf transcriptome analysis to investigate the response of four different Populus davidiana cultivars to drought stress. Following the RNA-seq, we compared the transcriptome profiles and identified two differentially expressed genes (DEGs) with contrasting expression patterns in the drought-sensitive and tolerant groups, i.e., upregulated in the drought-tolerant P. davidiana groups but downregulated in the sensitive group. Both these genes encode a 9-cis-epoxycarotenoid dioxygenase (NCED), a key enzyme required for abscisic acid (ABA) biosynthesis. The high-performance liquid chromatography (HPLC) measurements showed a significantly higher ABA accumulation in the cultivars of the drought-tolerant group following dehydration. The Arabidopsis nced3 loss-of-function mutants showed a significantly higher sensitivity to drought stress, ~90% of these plants died after 9 days of drought stress treatment. The real-time PCR analysis of several key genes indicated a strict regulation of drought stress at the transcriptional level in the P. davidiana drought-tolerant cultivars. The transgenic P. davidiana NCED3 overexpressing (OE) plants were significantly more tolerant to drought stress as compared with the NCED knock-down RNA interference (RNAi) lines. Further, the NCED OE plants accumulated a significantly higher quantity of ABA and exhibited strict regulation of drought stress at the transcriptional level. Furthermore, we identified several key differences in the amino acid sequence, predicted structure, and co-factor/ligand binding activity of NCED3 between drought-tolerant and susceptible P. davidiana cultivars. Here, we presented the first evidence of the significant role of NCED genes in regulating ABA-dependent drought stress responses in the forest tree P. davidiana and uncovered the molecular basis of NCED3 evolution associated with increased drought tolerance.

Transcriptome Analysis Reveals the Important Role of WRKY28 in Fusarium oxysporum Resistance

Root rot of Populus davidiana × P. alba var. pyramidalis Louche (Pdpap) is caused by Fusarium oxysporum. We used RNA sequencing to study the molecular mechanisms and response pattern of Pdpap infected by F. oxysporum CFCC86068. We cloned the PdpapWRKY28 transcription factor gene and transformed the recombinant vector pBI121-PdpapWRKY28 into Pdpap. The resistance function of PdpapWRKY28 was verified using physiological and biochemical methods. By means of RNA sequencing, we detected 1,403 differentially expressed genes (DEGs) that are common in the different treatments by F. oxysporum. Furthermore, we found that overexpression of the PdpapWRKY28 gene may significantly improve the resistance of Pdpap plants to F. oxysporum. Our research reveals a key role for PdpapWRKY28 in the resistance response of Pdpap to F. oxysporum. Additionally, our results provide a theoretical basis for in-depth research on resistance breeding to combat root rot.

Overexpression of transcription factor SlWRKY28 improved the tolerance of Populus davidiana × P. bolleana to alkaline salt stress

Background WRKY transcription factors (TFs) have been suggested to play crucial roles in the response to biotic and abiotic stresses. This study is the first to report the alkaline salt regulation of the WRKY gene. Results In this study, we cloned a WRKY gene (SlWRKY28) from the Salix linearistipularis and then transferred to the Populus davidiana × P. bolleana for expression. Sequence analysis on the transcriptome of Salix linearistipular showed the significant up-regulation of WRKY gene expression in response to salt-alkali stress in seedlings. Our data showed that SlWRKY28 localized to the nucleus. Furthermore, the expression of the SlWRKY28 from female plants increased with saline-alkali stress according to the northern blot analysis results. The results of 3,3′-Diaminobenzidine (DAB) staining showed that hydrogen peroxide (H2O2) concentration was lower under stress, but ascorbate peroxidase (APX) enzyme activity was significantly higher in the overexpressed plants than that in non-transgenic (NT) plants. Conclusions We found out the SlWRKY28 induced regulation of the enzyme gene in the reactive oxygen species (ROS) scavenging pathway is a potential mechanism for transgenic lines to improve their resistance to alkaline salt. This study shows theoretical and practical significance in determining SlWRKY28 transcription factors involved in the regulation of alkaline salt tolerance.