Evaluation of a SUMO E2 Conjugating Enzyme Involved in Resistance to Clavibacter michiganensis Subsp. michiganensis in Solanum peruvianum, Through a Tomato Mottle Virus VIGS Assay

SUMOylation regulates low-temperature survival and oxidative DNA damage tolerance in Botrytis cinerea

SUMOylation as one of the protein post-translational modifications plays crucial roles in multiple biological processes of eukaryotic organisms. Botrytis cinerea is a devastating fungal pathogen and capable of infecting plant hosts at low temperature. However, the molecular mechanisms of low-temperature adaptation are largely unknown in fungi. Combining with biochemical methods and biological analyses, we report that SUMOylation regulates pathogen survival at low temperature and oxidative DNA damage response during infection in B. cinerea. The heat shock protein (Hsp70) BcSsb and E3 ubiquitin ligase BcRad18 were identified as substrates of SUMOylation; moreover, their SUMOylation both requires a single unique SUMO-interacting motif (SIM). SUMOylated BcSsb regulates β-tubulin accumulation, thereby affecting the stability of microtubules and consequently mycelial growth at low temperature. On the contrary, SUMOylated BcRad18 modulates mono-ubiquitination of the sliding clamp protein proliferating cell nuclear antigen (PCNA), which is involved in response to oxidative DNA damage during infection. Our study uncovers the molecular mechanisms of SUMOylation-mediated low-temperature survival and oxidative DNA damage tolerance during infection in a devastating fungal pathogen, which provides novel insights into low-temperature adaptation and pathogenesis for postharvest pathogens as well as new targets for inhibitor invention in disease control.

Understanding SUMO-mediated adaptive responses in plants to improve crop productivity

The response to abiotic and biotic stresses in plants and crops is considered a multifaceted process. Due to their sessile nature, plants have evolved unique mechanisms to ensure that developmental plasticity remains during their life cycle. Among these mechanisms, post-translational modifications (PTMs) are crucial components of adaptive responses in plants and transduce environmental stimuli into cellular signalling through the modulation of proteins. SUMOylation is an emerging PTM that has received recent attention due to its dynamic role in protein modification and has quickly been considered a significant component of adaptive mechanisms in plants during stress with great potential for agricultural improvement programs. In the present review, we outline the concept that small ubiquitin-like modifier (SUMO)-mediated response in plants and crops to abiotic and biotic stresses is a multifaceted process with each component of the SUMO cycle facilitating tolerance to several different environmental stresses. We also highlight the clear increase in SUMO genes in crops when compared with Arabidopsis thaliana. The SUMO system is understudied in crops, given the importance of SUMO for stress responses, and for some SUMO genes, the apparent expansion provides new avenues to discover SUMO-conjugated targets that could regulate beneficial agronomical traits.

Advances in the Characterization of the Mechanism Underlying Bacterial Canker Development and Tomato Plant Resistance

Bacterial canker caused by the Gram-positive actinobacterium Clavibacter michiganensis is one of the most serious bacterial diseases of tomatoes, responsible for 10–100% yield losses worldwide. The pathogen can systemically colonize tomato vascular bundles, leading to wilting, cankers, bird’s eye lesions, and plant death. Bactericidal agents are insufficient for managing this disease, because the pathogen can rapidly migrate through the vascular system of plants and induce systemic symptoms. Therefore, the use of resistant cultivars is necessary for controlling this disease. We herein summarize the pathogenicity of C. michiganensis in tomato plants and the molecular basis of bacterial canker pathogenesis. Moreover, advances in the characterization of resistance to this pathogen in tomatoes are introduced, and the status of genetics-based research is described. Finally, we propose potential future research on tomato canker resistance. More specifically, there is a need for a thorough analysis of the host–pathogen interaction, the accelerated identification and annotation of resistance genes and molecular mechanisms, the diversification of resistance resources or exhibiting broad-spectrum disease resistance, and the production of novel and effective agents for control or prevention. This review provides researchers with the relevant information for breeding tomato cultivars resistant to bacterial cankers.

SUMO conjugating enzyme: a vital player of SUMO pathway in plants

Plants face numerous challenges such as biotic and abiotic stresses during their whole lifecycle. As they are sessile in nature, they ought to develop multiple ways to act during stressed conditions to maintain cellular homeostasis. Among various defense mechanisms, the small ubiquitin-like modifiers (SUMO) pathway is considered as the most important because several nuclear proteins regulated by this pathway are involved in several cellular functions such as response to stress, transcription, translation, metabolism of RNA, energy metabolism, repairing damaged DNA, ensuring genome stability and nuclear trafficking. In general, the SUMO pathway has its own particular set of enzymes E1, E2, and E3. The SUMO conjugating enzyme [SCE (E2)] is a very crucial member of the pathway which can transfer SUMO to its target protein even without the involvement of E3. More than just a middle player, it has shown its involvement in effective triggered immunity in crops like tomato and various abiotic stresses like drought and salinity in maize, rice, and Arabidopsis. This review tries to explore the importance of the SUMOylation process, focusing on the E2 enzyme and its regulatory role in the abiotic stress response, plant immunity, and DNA damage repair.

SUMOylation in Phytopathogen Interactions: Balancing Invasion and Resistance

Plants are constantly confronted by a multitude of biotic stresses involving a myriad of pathogens. In crops, pathogen infections result in significant agronomical losses worldwide posing a threat to food security. In order to enter plant tissues and establish a successful infection, phytopathogens have to surpass several physical, and chemical defense barriers. In recent years, post-translational modification (PTM) mechanisms have emerged as key players in plant defense against pathogens. PTMs allow a highly dynamic and rapid response in front of external challenges, increasing the complexity and precision of cellular responses. In this review, we focus on the role of SUMO conjugation (SUMOylation) in plant immunity against fungi, bacteria, and viruses. In plants, SUMO regulates multiple biological processes, ranging from development to responses arising from environmental challenges. During pathogen attack, SUMO not only modulates the activity of plant defense components, but also serves as a target of pathogen effectors, highlighting its broad role in plant immunity. Here, we summarize known pathogenic strategies targeting plant SUMOylation and, the plant SUMO conjugates involved in host-pathogen interactions. We also provide a catalog of candidate SUMO conjugates according to their role in defense responses. Finally, we discuss the complex role of SUMO in plant defense, focusing on key biological and experimental aspects that contribute to some controversial conclusions, and the opportunities for improving agricultural productivity by engineering SUMOylation in crop species.

The maize SUMO conjugating enzyme ZmSCE1b protects plants from paraquat toxicity

Paraquat (PQ) herbicide causes damage to green plant tissues by inducing the production of toxic reactive oxygen species (ROS). SUMOylation is an important post-translational modification that enables plants to defend against multiple stresses. However, it is still unknown whether the SUMOylation is involved in PQ resistance response in crops. Herein, we showed that a maize SUMO conjugating enzyme gene (ZmSCE1b) functioned in PQ resistance. The quantitative real-time PCR (qRT-PCR) analysis revealed that this gene was significantly up-regulated upon PQ exposure. The overexpression of ZmSCE1b increased the levels of SUMO conjugates and improved PQ resistance in transgenic Arabidopsis. The ZmSCE1b-transgenic plants showed lower levels of ROS and lipid peroxidation, as well as higher antioxidant enzyme activities, upon PQ exposure. Furthermore, Western blotting showed that levels of SUMOylation in these transgenic plants were significantly elevated. In addition, the abundance of transcripts of several defense-related genes was apparently up-regulated in the over-expressing lines using qRT-PCR. Collectively, our results manifested the effect of overexpression of ZmSCE1b in improving resistance to PQ, possibly by regulating the levels of SUMO conjugates, antioxidant machinery, and expression of defense genes. Findings of this study can facilitate the understanding of the regulatory mechanisms underlying the involvement of SCE-mediated SUMOylation in PQ resistance response in crop plants. Meanwhile, ZmSCE1b could be utilized for engineering PQ-resistant crops in phytoremediation.

The Maize Class-I SUMO Conjugating Enzyme ZmSCE1d Is Involved in Drought Stress Response

Post-translational modification of cellular proteins by sumoylation plays a vital role in stress responses of plants. However, the mechanisms underlying the sumoylation’s involvement in stress responses in crop species remain largely unknown. Herein, a maize class-I SUMO conjugating enzyme gene (ZmSCE1d) was identified, whose expression was upregulated upon drought stress. Over-expression of ZmSCE1d in transgenic Arabidopsis plants increased SUMO conjugates and improved drought tolerance. The ZmSCE1d-transgenic plants showed higher antioxidant enzyme activities, but lower reactive oxygen species and lipid peroxidation upon drought stress. Furthermore, transcripts of several drought-responsive genes were significantly elevated, as revealed by qPCR in the transgenic lines. Taken together, these data have demonstrated that ZmSCE1d overexpression improved drought tolerance likely by regulating sumoylation levels, antioxidant capability, and drought-responsive gene expression in transgenic plants. This study may facilitate our understanding of the mechanisms underlying SCE-mediated sumoylation under drought stress and accelerate genetic improvement of crop plants tolerant to drought stress by manipulating the SUMO system.

Exploring the regulatory levels of SUMOylation to increase crop productivity

SUMOylation is an essential post-translational modification that affects several cellular processes, from gene replication to stress response. Studies using the SUMO (de)conjugation machinery have provided evidence regarding its potential to improve crop performance and productivity under normal and adverse conditions. However, the pleiotropic effect of SUMOylation can be a disadvantage in both situations, especially when considering unpredictable environmental conditions caused by climate changes. Here, we discuss the pleiotropic effects caused by disrupting the SUMOylation machinery, and new strategies that may help to overcome pleiotropy. We propose exploring the several regulatory levels of SUMOylation recently revealed, including transcriptional, post-transcriptional regulation by alternative splicing, and post-translational modifications. These new findings may provide valuable tools to increase crop productivity.

Phloem-Triggered Virus-Induced Gene Silencing Using a Recombinant Polerovirus

The phloem-limited poleroviruses infect Arabidopsis thaliana without causing noticeable disease symptoms. In order to facilitate visual infection identification, we developed virus-induced gene silencing (VIGS) vectors derived from Turnip yellows virus (TuYV). Short sequences from the host gene AtCHLI1 required for chlorophyll biosynthesis [42 nucleotides in sense or antisense orientation or as an inverted-repeat (IR), or an 81 nucleotide sense fragment] were inserted into the 3' non-coding region of the TuYV genome to screen for the most efficient and robust silencing vector. All recombinant viruses produced a clear vein chlorosis phenotype on infected Arabidopsis plants due to the expression inhibition of the AtCHLI1 gene. The introduction of a sense-oriented sequence into TuYV genome resulted in a virus exhibiting a more sustainable chlorosis than the virus containing an IR of the same length. This observation was correlated with a higher stability of the sense sequence insertion in the viral genome. In order to evaluate the impact of the TuYV silencing suppressor P0 in the VIGS mechanism a P0 knock-out mutation was introduced into the recombinant TuYV viruses. They induced a similar but milder vein clearing phenotype due to lower viral accumulation. This indicates that P0 does not hinder the performances of the TuYV silencing effect and confirms that in the viral infection context, P0 has no major impact on the production, propagation and action of the short distance silencing signal in phloem cells. Finally, we showed that TuYV can be used to strongly silence the phloem specific AtRTM1 gene. The TuYV-derived VIGS vectors therefore represent powerful tools to easily detect and monitor TuYV in infected plants and conduct functional analysis of phloem-restricted genes. Moreover this example indicates the potential of poleroviruses for use in functional genomic studies of agronomic plants.

Fifty shades of SUMO: its role in immunity and at the fulcrum of the growth-defence balance

The sessile nature of plants requires them to cope with an ever-changing environment. Effective adaptive responses require sophisticated cellular mechanisms at the post-transcriptional and post-translational levels. Post-translational modification by small ubiquitin-like modifier (SUMO) proteins is emerging as a key player in these adaptive responses. SUMO conjugation can rapidly change the overall fate of target proteins by altering their stability or interaction with partner proteins or DNA. SUMOylation entails an enzyme cascade that leads to the activation, conjugation and ligation of SUMO to lysine residues of target proteins. In addition to their SUMO processing activities, SUMO proteases also possess de-conjugative activity capable of cleaving SUMO from target proteins, providing reversibility and buffering to the pathway. These proteases play critical roles in the maintenance of the SUMO machinery in equilibrium. We hypothesize that SUMO proteases provide the all-important substrate specificity within the SUMO system. Furthermore, we provide an overview of the role of SUMO in plant innate immunity. SUMOylation also overlaps with multiple growth-promoting and defence-related hormone signalling pathways, and hence is pivotal for the maintenance of the growth-defence balance. This review aims to highlight the intricate molecular mechanisms utilized by SUMO to regulate plant defence and to stabilize the growth-defence equilibrium.

Proteomic Study of SUMOylation During Solanum tuberosum-Phytophthora infestans Interactions

Invasive plant pathogens have developed the ability to modify the metabolism of their host, promoting metabolic processes that facilitate the growth of the pathogen at the general expense of the host. The particular enzymatic process SUMOylation, which performs posttranslational modification of target proteins, leading to changes in many aspects of protein activity and, hence, metabolism, has been demonstrated to be active in many eukaryotic organisms, both animals and plants. Here, we provide experimental evidence that indicates that, in leaves of Solanum tuberosum that have been infected by Phytophthora infestans, the SUMO (small ubiquitin-like modifier) pathway enzymes of the host are partially under transcriptional control exerted by the oomycete. Using a recently developed approach that employs three-dimensional gels, we show that, during the infection process, the abundances of most of the known SUMO conjugates of S. tuberosum change significantly, some decreasing, but many increasing in abundance. The new proteomic approach has the potential to greatly facilitate investigation of the molecular events that take place during the invasion by a pathogen of its host plant.

Phenotyping of VIGS-mediated gene silencing in rice using a vector derived from a DNA virus

Target genes in rice can be optimally silenced if inserted in antisense or hairpin orientation in the RTBV-derived VIGS vector and plants grown at 28 °C and 80% humidity after inoculation. Virus induced gene silencing (VIGS) is a method used to transiently silence genes in dicot as well as monocot plants. For the important monocot species rice, the Rice tungro bacilliform virus (RTBV)-derived VIGS system (RTBV-VIGS), which uses agroinoculation to initiate silencing, has not been standardized for optimal use. Here, using RTBV-VIGS, three sets of conditions were tested to achieve optimal silencing of the rice marker gene phytoene desaturase (pds). The effect of orientation of the insert in the RTBV-VIGS plasmid (sense, antisense and hairpin) on the silencing of the target gene was then evaluated using rice magnesium chelatase subunit H (chlH). Finally, the rice Xa21 gene, conferring resistance against bacterial leaf blight disease (BLB) was silenced using RTBV-VIGS system. In each case, real-time PCR-based assessment indicated approximately 40-80% fall in the accumulation levels of the transcripts of pds, chlH and Xa21. In the case of pds, the appearance of white streaks in the emerging leaves, and for chlH, chlorophyll levels and F _v/F _m ratio were assessed as phenotypes for silencing. For Xa21, the resistance levels to BLB were assessed by measuring the lesion length and the percent diseased areas of leaves, following challenge inoculation with Xanthomonas oryzae. In each case, the RTBV-MVIGS system gave rise to a discernible phenotype indicating the silencing of the respective target gene using condition III (temperature 28 °C, humidity 80% and 1 mM MES and 20 µM acetosyringone in secondary agrobacterium culture), which revealed the robustness of this gene silencing system for rice.