SummaryReversible modifications of target proteins by small ubiquitin-like modifier (SUMO) proteins are involved in many cellular processes in yeast and animals. Yet little is known about the function of sumoylation in plants.Here, we show that the SIZ1 gene, which encodes an Arabidopsis SUMO E3 ligase, regulates innate immunity. Mutant siz1 plants exhibit constitutive systemic-acquired resistance (SAR) characterized by elevated accumulation of salicylic acid (SA), increased expression of pathogenesis-related (PR) genes, and increased resistance to the bacterial pathogen Pseudomonas syringae pv. tomato (Pst) DC3000. Transfer of the NahG gene to siz1 plants results in reversal of these phenotypes back to wild-type. Analyses of the double mutants, npr1 siz1, pad4 siz1 and ndr1 siz1 revealed that SIZ1 controls SA signalling. SIZ1 interacts epistatically with PAD4 to regulate PR expression and disease resistance. Consistent with these observations, siz1 plants exhibited enhanced resistance to Pst DC3000 expressing avrRps4, a bacterial avirulence determinant that responds to the EDS1/PAD4-dependent TIR-NBS-type R gene. In contrast, siz1 plants were not resistant to Pst DC3000 expressing avrRpm1, a bacterial avirulence determinant that responds to the NDR1-dependent CC-NBS-type R gene. Jasmonic acid (JA)-induced PDF1.2 expression and susceptibility to Botrytis cinerea were unaltered in siz1 plants. Taken together, these results demonstrate that SIZ1 is required for SA and PAD4-mediated R gene signalling, which in turn confers innate immunity in Arabidopsis.

Mitogen-activated protein kinase (MAPK) cascades are known to transduce plant defense signals, but the downstream components of the MAPK have as yet not been elucidated. Here, we report an MAPK from rice (Oryza sativa), BWMK1, and a transcription factor, OsEREBP1, phosphorylated by the kinase. The MAPK carries a TDY phosphorylation motif instead of the more common TEY motif in its kinase domain and has an unusually extended C-terminal domain that is essential to its kinase activity and translocation to the nucleus. The MAPK phosphorylates OsEREBP1 that binds to the GCC box element (AGCCGCC) of the several basic pathogenesis-related gene promoters, which in turn enhances DNA-binding activity of the factor to the cis element in vitro. Transient co-expression of the BWMK1 and OsEREBP1 in Arabidopsis protoplasts elevates the expression of the ␤-glucuronidase reporter gene driven by the GCC box element. Furthermore, transgenic tobacco (Nicotiana tabacum) plants overexpressing BWMK1 expressed many pathogenesis-related genes at higher levels than wild-type plants with an enhanced resistance to pathogens. These findings suggest that MAPKs contribute to plant defense signal transduction by phosphorylating one or more transcription factors.Mitogen-activated protein kinase (MAPK) cascades are known to play essential roles in the signal transduction pathways involved in numerous eukaryotic cellular processes from cell division to cell death (Davis, 2000; Ligterink and Hirt, 2001). In the last few years, it has become apparent that MAPK cascades also play vital roles in signal transduction pathways of plants, including plant defense signaling (Innes, 2001; Tena et al., 2001; Zhang and Klessig, 2001). The Arabidopsis genome sequence has revealed the presence of 23 MAPK genes in the genome, which suggests that the MAPK cascades in plants may be quite complex.Accumulating lines of evidence indicate that plants rapidly activate MAPKs when exposed to a variety of abiotic and biotic stress stimuli (Ligterink et al., 1997; Zhang et al., 1998; Seo et al., 1999; Cardinale et al., 2000; Ichimura et al., 2000). These include pathogens, pathogen-derived elicitors, and defense-related second messengers. In tobacco (Nicotiana tabacum), two MAPKs, SIPK and WIPK, are activated by both various pathogen-related signals and diverse abiotic stresses, indicating that pathogen defense signaling is part of an integrated stress-signaling network in plants. Orthologs of SIPK and WIPK in Arabidopsis (AtMPK6 and AtMAPK3, respectively) and alfalfa (Medicago sativa; SIMK and SAMK, respectively) are also activated by both biotic and abiotic stresses (Seo et al., 1995; Zhang and Klessig, 1997; Nuhse et al., 2000). Recently, the MAPKK, NtMEK2, was identified to operate in the cascade upstream of SIPK and WIPK because a constitutively active NtMEK2 activates endogenous SIPK and WIPK molecules in transiently transformed tobacco cells. Furthermore, the constitutively active NtMEK2 induces hypersensitive cell death and the expression of defense genes (Yan...

Many plants measure changes in day length to synchronize their flowering time with appropriate seasons for maximum reproductive success. In Arabidopsis, the day-length-dependent regulation of CONSTANS (CO) protein stability is crucial to induce FLOWERING LOCUS T (FT) expression for flowering in long days. The FLAVIN-BINDING, KELCH REPEAT, F-BOX1 (FKF1) protein binds to CO protein specifically in the long-day afternoon and stabilizes it, although the mechanism remains unknown. Here we demonstrated that the FKF1-interacting proteins GIGANTEA (GI) and ZEITLUPE (ZTL) are involved in CO stability regulation. First, our immunoprecipitationmass spectrometry analysis of FKF1 revealed that FKF1 forms an S-phase kinase-associated protein 1 (Skp1)/Cullin(CUL)/F-box complex through interactions with Arabidopsis Skp1-like 1 (ASK1), ASK2, and CUL1 proteins and mainly interacts with GI protein in vivo. GI interacts with CO directly and indirectly through FKF1. Unexpectedly, the gi mutation increases the CO protein levels in the morning in long days. This gi-dependent destabilization of CO protein was cancelled by the fkf1 mutation. These results suggest that there are other factors likely influenced by both gi and fkf1 mutations that also control CO stability. We found that ZTL, which interacts with GI and FKF1, may be one such factor. ZTL also interacts with CO in vivo. The CO protein profile in the ztl mutant resembles that in the gi mutant, indicating that ZTL activity also may be changed in the gi mutant. Our findings suggest the presence of balanced regulation among FKF1, GI, and ZTL on CO stability regulation for the precise control of flowering time.photoperiodic flowering | blue-light photoreceptor | E3 ubiquitin ligase | protein degradation

Cellular Na+/K+ ratio is a crucial parameter determining plant salinity stress resistance. We tested the function of plasma membrane Na+/K+ cotransporters in the High-affinity K+ Transporter (HKT) family from the halophytic Arabidopsis (Arabidopsis thaliana) relative Thellungiella salsuginea. T. salsuginea contains at least two HKT genes. TsHKT1;1 is expressed at very low levels, while the abundant TsHKT1;2 is transcriptionally strongly up-regulated by salt stress. TsHKT-based RNA interference in T. salsuginea resulted in Na+ sensitivity and K+ deficiency. The athkt1 mutant lines overexpressing TsHKT1;2 proved less sensitive to Na+ and showed less K+ deficiency than lines overexpressing AtHKT1. TsHKT1;2 ectopically expressed in yeast mutants lacking Na+ or K+ transporters revealed strong K+ transporter activity and selectivity for K+ over Na+. Altering two amino acid residues in TsHKT1;2 to mimic the AtHKT1 sequence resulted in enhanced sodium uptake and loss of the TsHKT1;2 intrinsic K+ transporter activity. We consider the maintenance of K+ uptake through TsHKT1;2 under salt stress an important component supporting the halophytic lifestyle of T. salsuginea.