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.

Environmental challenges to plants typically entail retardation of vegetative growth and delay or cessation of flowering. Here we report a link between the flowering time regulator, GIGANTEA (GI), and adaptation to salt stress that is mechanistically based on GI degradation under saline conditions, thus retarding flowering. GI, a switch in photoperiodicity and circadian clock control, and the SNF1-related protein kinase SOS2 functionally interact. In the absence of stress, the GI:SOS2 complex prevents SOS2-based activation of SOS1, the major plant Na þ /H þ -antiporter mediating adaptation to salinity. GI overexpressing, rapidly flowering, plants show enhanced salt sensitivity, whereas gi mutants exhibit enhanced salt tolerance and delayed flowering. Salt-induced degradation of GI confers salt tolerance by the release of the SOS2 kinase. The GI-SOS2 interaction introduces a higher order regulatory circuit that can explain in molecular terms, the long observed connection between floral transition and adaptive environmental stress tolerance in Arabidopsis.

The Ca 21-binding protein calmodulin mediates cellular Ca 21 signals in response to a wide array of stimuli in higher eukaryotes. Plants express numerous CaM isoforms. Transcription of one soybean (Glycine max) CaM isoform, SCaM-4, is dramatically induced within 30 min of pathogen or NaCl stresses. To characterize the cis-acting element(s) of this gene, we isolated an approximately 2-kb promoter sequence of the gene. Deletion analysis of the promoter revealed that a 130-bp region located between nucleotide positions 2858 and 2728 is required for the stressors to induce expression of SCaM-4. A hexameric DNA sequence within this region, GAAAAA (GT-1 cis-element), was identified as a core cis-acting element for the induction of the SCaM-4 gene. The GT-1 cis-element interacts with an Arabidopsis GT-1-like transcription factor, AtGT-3b, in vitro and in a yeast selection system. Transcription of AtGT-3b is also rapidly induced within 30 min after pathogen and NaCl treatment. These results suggest that an interaction between a GT-1 cis-element and a GT-1-like transcription factor plays a role in pathogen-and salt-induced SCaM-4 gene expression in both soybean and Arabidopsis.Plant cells, like animal cells, elevate their cytosolic free-calcium levels ([Ca 21 ] cyt ) with varying amplitude, frequency, and duration in response to a variety of external stimuli (Thomas et al., 1996; Berridge, 1997;McAinsh and Hetherington, 1998 -bound CaM transduces the signals into many cellular processes through modulation of a variety of CaM-binding proteins, including enzymes such as kinases, phosphatases, and nitric-oxide synthase, as well as receptors, ion channels, G-proteins, and transcription factors (Liao et al., 1996;Snedden and Fromm, 1998;Lee et al., 1999a;Zuhlke et al., 1999).In plant cells, in contrast to mammalian cells, multiple CaM genes code for a number of CaM isoforms. This has been shown in wheat (Triticum aestivum; Yang et al., 1996), potato (Solanum tuberosum; Takezawa et al., 1995;Poovaiah et al., 1996), and soybean (Glycine max; Lee et al., 1995a), among others. Over 30 genes encoding CaM isoforms are found in the Arabidopsis genome (The Arabidopsis Genome Initiative, 2000). We have recently cloned five CaM isoforms from soybean (SCaM-1-5). Although SCaM-1-3 are more than 90% identical to mammalian CaM, SCaM-4 and SCaM-5 exhibit only a 78% homology with SCaM-1 and are therefore the most divergent isoforms reported thus far in the plant and animal kingdoms. SCaM-4 is considered to be a bona fide CaM isoform based on the following characteristics. In its primary protein structure, SCaM-4 has four conserved putative EF-hands and a central linker region, hallmark structural features of CaM (Lee et al., 1995a). In addition, most of the nonconsensus amino acids occur outside the EF-hands, and the total number of Article, publication date, and citation information can be found at www.plantphysiol.org/cgi

Molybdenum (Mo) is an essential micronutrient for plants, serving as a cofactor for enzymes involved in nitrate assimilation, sulfite detoxification, abscisic acid biosynthesis, and purine degradation. Here we show that natural variation in shoot Mo content across 92 Arabidopsis thaliana accessions is controlled by variation in a mitochondrially localized transporter (Molybdenum Transporter 1 - MOT1) that belongs to the sulfate transporter superfamily. A deletion in the MOT1 promoter is strongly associated with low shoot Mo, occurring in seven of the accessions with the lowest shoot content of Mo. Consistent with the low Mo phenotype, MOT1 expression in low Mo accessions is reduced. Reciprocal grafting experiments demonstrate that the roots of Ler-0 are responsible for the low Mo accumulation in shoot, and GUS localization demonstrates that MOT1 is expressed strongly in the roots. MOT1 contains an N-terminal mitochondrial targeting sequence and expression of MOT1 tagged with GFP in protoplasts and transgenic plants, establishing the mitochondrial localization of this protein. Furthermore, expression of MOT1 specifically enhances Mo accumulation in yeast by 5-fold, consistent with MOT1 functioning as a molybdate transporter. This work provides the first molecular insight into the processes that regulate Mo accumulation in plants and shows that novel loci can be detected by association mapping.