Ubiquitin Extension Protein UEP1 Modulates Cell Death and Resistance to Various Pathogens in Tobacco

Tobacco NtUBC1 and NtUBQ2 enhance salt tolerance by reducing sodium accumulation and oxidative stress through proteasome activation in Arabidopsis

The ubiquitin/proteasome system plays a crucial role in the regulation of plant responses to environmental stress. Here, we studied the involvement of the UBC1 and UBQ2 genes encoding a ubiquitin conjugating enzyme (E2) and ubiquitin extension protein, respectively, in the response to salt stress. Our results showed that the constitutive expression of tobacco NtUBC1 and NtUBQ2 in Arabidopsis thaliana improved salt tolerance, along with the lower Na+ level and higher K+/Na+ ratio compared to control plants. Moreover, the expression levels of sodium transporters, including AtHKT1 (High-Affinity K+ Transporter1) and AtSOS1 (Salt Overly Sensitive 1), were higher in NtUBC1- and NtUBQ2-Arabidopsis. However, the transcript level of AtNHX1 (Na+/H+ Exchanger 1) was similar between control and transgenic plants. After salt exposure, the activity of the 26S proteasome markedly increased in NtUBC1- and NtUBQ2-expressing plants; however, ubiquitinated protein levels decreased compared to control plants. Furthermore, higher activity of antioxidant enzymes and lower ROS production were observed in UBC1- and UBQ2-expressing plants. We further challenged atubc1, atubc2, and atubq2 single mutants and atubc1ubc2 double mutant lines with salt stress; interestingly, the salt sensitivity and sodium levels of the studied mutants were enhanced, while the potassium levels were reduced. However, the atubc1ubc2 double mutant illustrated a more severe phenotype than the single mutants, probably due to the redundant function of UBC1 and UBC2 in Arabidopsis. Taken together, NtUBC1 and NtUBQ2 enhance salt tolerance by enhancing 26S proteasome activity and reducing Na+ accumulation, ROS, and ubiquitinated/salt-denatured proteins.

The Hypersensitive Response to Plant Viruses

Plant proteins with domains rich in leucine repeats play important roles in detecting pathogens and triggering defense reactions, both at the cellular surface for pattern-triggered immunity and in the cell to ensure effector-triggered immunity. As intracellular parasites, viruses are mostly detected intracellularly by proteins with a nucleotide binding site and leucine-rich repeats but receptor-like kinases with leucine-rich repeats, known to localize at the cell surface, have also been involved in response to viruses. In the present review we report on the progress that has been achieved in the last decade on the role of these leucine-rich proteins in antiviral immunity, with a special focus on our current understanding of the hypersensitive response.

The interaction between the lemon ribosomal protein ClRPS9-2 and citrus yellow vein clearing virus coat protein affects viral infection and gene silencing suppressor activity

Citrus yellow vein clearing virus (CYVCV) is an emerging virus that causes serious economic damage to the lemon industry worldwide. The coat protein (CP) of CYVCV is a strong RNA silencing suppressor and is associated with the severity of symptoms in citrus, yet the interaction between CP and host factors remains unknown. In this study, the 40S ribosomal subunit protein S9-2 (ClRPS9-2) was identified as a CP-binding partner using the yeast two-hybrid system from a lemon (cv. Eureka) cDNA library, and the interaction between CP and ClRPS9-2 was demonstrated by in vivo methods. The results suggest that the N-terminal 8-108 amino acid sequence of ClRPS9-2 is crucial for its interaction with CP and may be associated with the nuclear localization of ClRPS9-2. The accumulation and silencing suppressor activity of CP were reduced by transient expression of ClRPS9-2 in Nicotiana benthamiana. Reverse transcription-quantitative PCR analysis showed that the content of CYVCV in ClRPS9-2 transgenic Eureka lemon plants was approximately 50% of that in CYVCV-infected wild-type plants 1 month after inoculation, and mild yellowing and vein clearing symptoms were observed in the transgenic plants. These findings demonstrate that ClRPS9-2 plays a role in host defensive reactions, and the enhanced resistance of transgenic plants to CYVCV may be associated with the up-regulation of salicylic acid-related and R genes.

Autopolyploidization affects transcript patterns and gene targeting frequencies in Physcomitrella

In Physcomitrella, whole-genome duplications affected the expression of about 3.7% of the protein-encoding genes, some of them relevant for DNA repair, resulting in a massively reduced gene-targeting frequency. Qualitative changes in gene expression after an autopolyploidization event, a pure duplication of the whole genome (WGD), might be relevant for a different regulation of molecular mechanisms between angiosperms growing in a life cycle with a dominant diploid sporophytic stage and the haploid-dominant mosses. Whereas angiosperms repair DNA double-strand breaks (DSB) preferentially via non-homologous end joining (NHEJ), in the moss Physcomitrella homologous recombination (HR) is the main DNA-DSB repair pathway. HR facilitates the precise integration of foreign DNA into the genome via gene targeting (GT). Here, we studied the influence of ploidy on gene expression patterns and GT efficiency in Physcomitrella using haploid plants and autodiploid plants, generated via an artificial WGD. Single cells (protoplasts) were transfected with a GT construct and material from different time-points after transfection was analysed by microarrays and SuperSAGE sequencing. In the SuperSAGE data, we detected 3.7% of the Physcomitrella genes as differentially expressed in response to the WGD event. Among the differentially expressed genes involved in DNA-DSB repair was an upregulated gene encoding the X-ray repair cross-complementing protein 4 (XRCC4), a key player in NHEJ. Analysing the GT efficiency, we observed that autodiploid plants were significantly GT suppressed (p < 0.001) attaining only one third of the expected GT rates. Hence, an alteration of global transcript patterns, including genes related to DNA repair, in autodiploid Physcomitrella plants correlated with a drastic suppression of HR.

The Ubiquitin Proteasome System as a Double Agent in Plant-Virus Interactions

The ubiquitin proteasome is a rapid, adaptive mechanism for selective protein degradation, crucial for proper plant growth and development. The ubiquitin proteasome system (UPS) has also been shown to be an integral part of plant responses to stresses, including plant defence against pathogens. Recently, significant progress has been made in the understanding of the involvement of the UPS in the signalling and regulation of the interaction between plants and viruses. This review aims to discuss the current knowledge about the response of plant viral infection by the UPS and how the viruses counteract this system, or even use it for their own benefit.