SIZ1 (for yeast SAP and MIZ1) encodes the sole ortholog of mammalian PIAS (for protein inhibitor of activated STAT) and yeast SIZ SUMO (for small ubiquitin-related modifier) E3 ligases in Arabidopsis (Arabidopsis thaliana). Four conserved motifs in SIZ1 include SAP (for scaffold attachment factor A/B/acinus/PIAS domain), PINIT (for proline-isoleucine-asparagine-isoleucinethreonine), SP-RING (for SIZ/PIAS-RING), and SXS (for serine-X-serine, where X is any amino acid) motifs. SIZ1 contains, in addition, a PHD (for plant homeodomain) typical of plant PIAS proteins. We determined phenotypes of siz1-2 knockout mutants transformed with SIZ1 alleles carrying point mutations in the predicted domains. Domain SP-RING is required for SUMO conjugation activity and nuclear localization of SIZ1. Salicylic acid (SA) accumulation and SA-dependent phenotypes of siz1-2, such as diminished plant size, heightened innate immunity, and abscisic acid inhibition of cotyledon greening, as well as SA-independent basal thermotolerance were not complemented by the altered SP-RING allele of SIZ1. The SXS domain also controlled SA accumulation and was involved in greening and expansion of cotyledons of seedlings germinated in the presence of abscisic acid. Mutations of the PHD zinc finger domain and the PINIT motif affected in vivo SUMOylation. Expression of the PHD and/or PINIT domain mutant alleles of SIZ1 in siz1-2 promoted hypocotyl elongation in response to sugar and light. The various domains of SIZ1 make unique contributions to the plant's ability to cope with its environment.Posttranslational protein modifications regulate the function of proteins, affecting activity, stability, interaction type, and interaction strength with partners as well as subcellular localization. The known protein modifications that elicit such changes are phosphorylation, prenylation, myristoylation, methylation, ubiquitination, and SUMO (for small ubiquitin-related modifier) conjugation. Although ubiquitination and SUMOylation of target proteins occur by similar mechanisms, both have vastly different functions. Ubiquitination is used to label proteins destined for degradation, whereas SUMOylation is a transient and reversible process that elicits long-term metabolic and developmental effects that are largely antagonistic to ubiquitination (Hay, 2005). In animals and yeast, SUMOylation has been extensively studied. SUMO modifications control cell cycle progression, DNA repair, and subcellular localization and are involved in transcriptional regulation (Matunis et al