SUMO proteases ULP1c and ULP1d are required for development and osmotic stress responses in Arabidopsis thaliana

Abstract

Sumoylation is an essential post-translational regulator of plant development and the response to environmental stimuli. SUMO conjugation occurs via an E1-E2-E3 cascade, and can be removed by SUMO proteases (ULPs). ULPs are numerous and likely to function as sources of specificity within the pathway, yet most ULPs remain functionally unresolved. In this report we used loss-of-function reverse genetics and transcriptomics to functionally characterize Arabidopsis thaliana ULP1c and ULP1d SUMO proteases. GUS reporter assays implicated ULP1c/d in various developmental stages, and subsequent defects in growth and germination were uncovered using loss-of-function mutants. Microarray analysis evidenced not only a deregulation of genes involved in development, but also in genes controlled by various drought-associated transcriptional regulators. We demonstrated that ulp1c ulp1d displayed diminished in vitro root growth under low water potential and higher stomatal aperture, yet leaf transpirational water loss and whole drought tolerance were not significantly altered. Generation of a triple siz1 ulp1c ulp1d mutant suggests that ULP1c/d and the SUMO E3 ligase SIZ1 may display separate functions in development yet operate epistatically in response to water deficit. We provide experimental evidence that Arabidopsis ULP1c and ULP1d proteases act redundantly as positive regulators of growth, and operate mainly as isopeptidases downstream of SIZ1 in the control of water deficit responses.

Exploiting protein modification systems to boost crop productivity: SUMO proteases in focus

“…As protein and/or transcript levels of almost all characterized ULP SUMO proteases (the northern blot performed in esd4 mutants is inconclusive) are known to be elevated in developing flowers (Murtas et al , 2003; Hermkes et al , 2011; Castro et al , 2016; Kong et al , 2017), we suspect the ULP mutants may be harbouring undiscovered floral phenotypes. For example, esd4 mutants exhibit deformed and irregularly placed siliques (Reeves, 2002) reminiscent of sum1-1 amiR-SUM2 SUM1/2 knockdown plants (van den Burg et al , 2010), which may be caused by defective flower or seed formation earlier in development.…”

“…Both the siz1 and the ots1ots2 double mutant exhibit a late germination phenotype (Castro et al , 2016; Kim et al , 2016) and abscisic acid hypersensitivity (Miura and Hasegawa, 2010), while SUMO overexpression leads to abscisic acid hyposensitivity (Lois et al , 2003). …”

“…Meanwhile, esd4 , ots1/2 , and spf1 mutants exhibit higher levels of SUMO conjugation in non-stressed conditions (Conti et al , 2008; Hermkes et al , 2011; Liu et al , 2017), while the ability to recover from stress-induced SUMO conjugate accumulation was shown to be compromised in spf1 mutants (Kong et al , 2017). Phenotypically, ots1/2 mutants are more sensitive to salt, osmotic stress, and abscisic acid (Conti et al , 2008; Castro et al , 2016; Srivastava et al , 2017). The salinity tolerance of the esd4 mutant has not yet been studied but, as it is known to be hypersensitive to abscisic acid (Miura and Hasegawa, 2010), it may also exhibit a salinity phenotype.…”

Arabidopsis thaliana SPF1 and SPF2 are nuclear-located ULP2-like SUMO proteases that act downstream of SIZ1 in plant development

“…S3A). Given that SPF1 and SPF2 are phylogenetically and topologically close (Figs 1, 2) and that functional redundancy has been displayed by other gene family members (Castro et al , 2016), we also generated a double-mutant spf1-1 spf2-2 (hereafter referred to as spf1/2 ). We confirmed abolished gene expression in the mutant backgrounds using semi-quantitative RT-PCR (Supplementary Fig.…”

“…For instance, ESD4 loss-of-function results in a pleiotropic phenotype (severe dwarfism), while the closely related ELS1 does not have such a severe phenotype (Murtas et al , 2003; Hermkes et al , 2011). OTS1 and OTS2 act redundantly in flowering transition, plant growth, and photomorphogenesis, as well as in pathogen defence, and salt and osmotic stress responses (Conti et al , 2008, 2014; Sadanandom et al , 2015; Bailey et al , 2016; Castro et al , 2016). The function of SPF1 (also designated ASP1) and SPF2 has been recently associated with the control of flowering time, and gamete and embryo development (Kong et al , 2017; Liu et al , 2017 a ).…”