Reduction of methylviologen-mediated oxidative stress tolerance in antisense transgenic tobacco seedlings through restricted expression of StAPX

Transcriptome Analysis Reveals the Biocontrol Mechanism of Endophytic Bacterium AM201, Rhodococcus sp., against Root Rot Disease of Atractylodes macrocephala

Atractylodes macrocephala Koidz (AMK) is a perennial herb from the plant family Asteraceae (formerly Compositae). This herb is mainly distributed in mountainous wetlands in Zhejiang, Sichuan, Yunnan, and Hunan provinces of China. Its medicinal production and quality, however, are severely impacted by root rot disease. In our previous study, endophytic bacterium designated AM201 exerted a high biocontrol effect on the root rot disease of AMK. However, the molecular mechanisms underlying this effect remain unclear. In this study, the identity of strain AM201 as Rhodococcus sp. was determined through analysis of its morphology, physiological and biochemical characteristics, as well as 16S rDNA sequencing. Subsequently, we performed transcriptome sequencing and bioinformatics analysis to compare and analyze the transcriptome profiles of root tissues from two groups: AM201 (AMK seedlings inoculated with Fusarium solani [FS] and AM201) and FS (AMK seedlings inoculated with FS alone). We also conducted morphological, physiological, biochemical, and molecular identification analyses for the AM201 strain. We obtained 1,560 differentially expressed genes, including 187 upregulated genes and 1,373 downregulated genes. We screened six key genes (GOLS2, CIPK25, ABI2, egID, PG1, and pgxB) involved in the resistance of AM201 against AMK root rot disease. These genes play a critical role in reactive oxygen species (ROS) clearance, Ca2+ signal transduction, abscisic acid signal inhibition, plant root growth, and plant cell wall defense. The strain AM201 was identified as Rhodococcus sp. based on its morphological characteristics, physiological and biochemical properties, and 16S rDNA sequencing results. The findings of this study could enable to prevent and control root rot disease in AMK and could offer theoretical guidance for the agricultural production of other medicinal herbs.

Biofilms Positively Contribute to Bacillus amyloliquefaciens 54-induced Drought Tolerance in Tomato Plants

Drought stress is a major obstacle to agriculture. Although many studies have reported on plant drought tolerance achieved via genetic modification, application of plant growth-promoting rhizobacteria (PGPR) to achieve tolerance has rarely been studied. In this study, the ability of three isolates, including Bacillus amyloliquefaciens 54, from 30 potential PGPR to induce drought tolerance in tomato plants was examined via greenhouse screening. The results indicated that B. amyloliquefaciens 54 significantly enhanced drought tolerance by increasing survival rate, relative water content and root vigor. Coordinated changes were also observed in cellular defense responses, including decreased concentration of malondialdehyde and elevated concentration of antioxidant enzyme activities. Moreover, expression levels of stress-responsive genes, such as lea, tdi65, and ltpg2, increased in B. amyloliquefaciens 54-treated plants. In addition, B. amyloliquefaciens 54 induced stomatal closure through an abscisic acid-regulated pathway. Furthermore, we constructed biofilm formation mutants and determined the role of biofilm formation in B. amyloliquefaciens 54-induced drought tolerance. The results showed that biofilm-forming ability was positively correlated with plant root colonization. Moreover, plants inoculated with hyper-robust biofilm (ΔabrB and ΔywcC) mutants were better able to resist drought stress, while defective biofilm (ΔepsA-O and ΔtasA) mutants were more vulnerable to drought stress. Taken altogether, these results suggest that biofilm formation is crucial to B. amyloliquefaciens 54 root colonization and drought tolerance in tomato plants.

Overexpression of a harpin-encoding gene popW from Ralstonia solanacearum primed antioxidant defenses with enhanced drought tolerance in tobacco plants

KEY MESSAGE

The tobacco plants transformed with popW gene showed enhanced drought tolerance, and the mechanism was found with primed antioxidant defenses and reduced drought stress damages in the transgenic lines. Harpin proteins are elicitors produced by several gram-negative plant pathogenic bacteria, triggering multiple beneficial responses in plants, such as induction of defense response against diverse pathogens and insects, growth promotion, and drought tolerance. In this study, the harpin-encoding gene popW derived from Ralstonia solanacearum ZJ3721 was transferred to tobacco. We examined the tolerance of transgenic tobacco plants toward drought stress under greenhouse conditions and analyzed the molecular mechanisms underlying the enhanced drought tolerance. The results revealed that the transgenic lines primed antioxidant defenses and reduced drought stress damages. In addition, they displayed lower malondialdehyde and relative electrical conductivity, while higher relative water content and recovery intension than the tobacco plants transformed with empty vector pBI121 and the wild-type (WT) plants under drought stress. Furthermore, the transgenic lines displayed a significant increase in peroxidase, superoxide dismutase, catalase activities, and ascorbic acid content compared with control plants under drought stress, and these levels were up to 1.95, 1.68, 1.34, and 1.43 times higher than those of WT plants, respectively. Overexpression of popW in tobacco also significantly enhanced the relative transcript levels of oxidative stress-responsive genes NtAPX, NtCAT1, NtGST, and NtCu/Zn-SOD under drought stress. The relative transcript levels of these genes in the transgenic line PW12 were up to 1.94, 2.36, 5.24, and 3.62 times higher than those of WT plants, respectively. These results confirmed that the popW gene, which was transformed into tobacco primed antioxidant responses, increased tolerance to drought stress in tobacco plants.

MAP kinase phosphatase 1 harbors a novel PTS1 and is targeted to peroxisomes following stress treatments

In Arabidopsis thaliana, twenty mitogen-activated protein kinases (MAPKs/MPKs) are regulated by five MAP kinase phosphatases (MKPs). Arabidopsis MKP1 has an important role in biotic, abiotic and genotoxic stresses and has been shown to interact with and negatively regulate specifically MPK3 and MPK6. MKP1 has been reported to have a role in negative regulation of reactive oxygen species (ROS) and salicylic acid (SA) production. As essential organelles involved in production of ROS and SA, peroxisomes could possibly be an important compartment for MKP1 activity, however MKP1 was previously reported to be cytosolic. By screening Arabidopsis protein phosphatases for peroxisomal targeting signal 1 (PTS1), we identified MKP1 as a putative peroxisomal protein. Arabidopsis MKP1 was found to harbor a non-canonical PTS1-like tripeptide (Ser-Ala-Leu>) that is conserved in MKP1 orthologs. We show experimentally that the C-terminal Ser-Ala-Leu> can function as a novel PTS1, and alanine in position -2, adds more relaxation to the plant PTS1 motif. The full-length MKP1 remained in the cytosol when transiently expressed in Arabidopsis mesophyll protoplasts under standard conditions. When different biotic and abiotic stresses were applied to mesophyll protoplasts, the full length protein changed its targeting to unidentified organelle-like structures that subsequently fused with peroxisomes. Our results identify MKP1 as a protein dually targeted to cytosol and peroxisomes. The finding that MKP1 targets peroxisomes by a non-canonical PTS1 under stressful conditions highlights the complexity of peroxisomal targeting mechanism.