The converging path of protein SUMOylation in phytohormone signalling: highlights and new frontiers

Post-translational modifications control the signal at the crossroads of plant-pathogen interactions

The co-evolution of plants and pathogens has enabled them to 'outsmart' each other by promoting their own defence responses and suppressing those of the other. While plants are reliant on their sophisticated immune signalling pathways, pathogens make use of effector proteins to achieve the objective. This entails rapid regulation of underlying molecular mechanisms for prompt induction of associated signalling events in both plants as well as pathogens. The past decade has witnessed the emergence of post-translational modification (PTM) of proteins as a key a factor in modulating cellular responses. The ability of PTMs to expand the functional diversity of the proteome and induce rapid changes at the appropriate time enables them to play crucial roles in the regulation of plant-pathogen interactions. Therefore, this review will delve into the intricate interplay of five major PTMs involved in plant defence and pathogen countermeasures. We discuss how plants employ PTMs to fortify their immune networks, and how pathogen effectors utilize/target host modification systems to gain entry into plants and cause disease. We also emphasize the need for identification of novel PTMs and propose the use of PTM pathways as potential targets for genome editing approaches.

Insights into the role of SUMO in regulating drought stress responses in pigeonpea (Cajanus cajan)

KEY MESSAGE

We have identified and analyzed 28 SUMO-pathway proteins from pigeonpea. Enhanced transcripts of pathway genes and increased SUMO conjugation under drought signifies the role of SUMO in regulating stress. Being a protein-rich and nutrient-dense legume crop, pigeonpea (Cajanus cajan) holds a vital position in a vegetarian meal. It is a resilient crop capable of striving in harsh climates and provides a means of subsistence to small-holding farmers. Nevertheless, extremes of water scarcity and drought conditions, especially during seedling and reproductive stages, remains a major issue severely impacting the growth and overall productivity of pigeonpea. Small ubiquitin-like modifier (SUMO), a post-translational modification system, plays a pivotal role in fortifying plants against stressful conditions by rapid reprogramming of molecular events. In this study, we have scanned the entire pigeonpea genome and identified 28 candidates corresponding to SUMO machinery components of pigeonpea. qRT-PCR analysis of different SUMO machinery genes validated their presence under natural conditions. The analysis of the promoters of identified SUMO machinery genes revealed the presence of abiotic stress-related cis-regulatory elements highlighting the potential involvement of the genes in abiotic stress responses. The transcript level analysis of selected SUMO machinery genes and global SUMO status of pigeonpea proteins in response to drought stress suggests an integral role of SUMO in regulating drought stress conditions in pigeonpea. Collectively, the work puts forward a detailed in silico analysis of pigeonpea SUMO machinery candidates and highlights the essential role of SUMOylation in drought stress responses. Being the first report on a pulse crop, the study will serve as a resource for devising strategies for counteracting drought stress in pigeonpea that can be further extended to other pulse crops.

Arabidopsis SUMO E3 ligase SIZ1 enhances cadmium tolerance via the glutathione-dependent phytochelatin synthesis pathway

Sumoylation is a posttranslational modification (PTM) in which SUMO (small ubiquitin-like modifier) is covalently conjugated to protein substrates via a range of enzymes. SUMO E3 ligase SIZ1 is involved in mediating several essential or nonessential element-responsive SUMO conjugations in Arabidopsis. However, whether SIZ1 is involved in the cadmium (Cd) response remains to be identified. In this study, we found that SIZ1 positively regulates plant Cd tolerance. The loss-of-function siz1-2 mutant exhibited impaired resistance to Cd exposure and accumulated more reactive oxygen species (ROS). Moreover, the transcription of GSH1, GSH2, PCS1, and PCS2 was suppressed while the accumulation of Cd was enhanced in the siz1-2 mutant under Cd exposure. Further analysis revealed that the higher Cd sensitivity of the siz1-2 mutant was partially rescued by the overexpression of GSH1. Consistently, Cd stress stimulated the accumulation of SUMO1 conjugates in wild-type plants but not in the siz1-2 mutant. Together, our results demonstrate that Cd-induced SIZ1 activates GSH- and PC synthesis-related gene expression to increase the synthesis of GSH- and PCs, thereby leading to higher Cd tolerance in plants.