Cytokinins affect plant immunity to various pathogens; however, the mechanisms coupling plant-derived cytokinins to pathogen responses have been elusive. Here, we found that plant-derived cytokinins promote resistance of Arabidopsis to Pseudomonas syringae pv. tomato DC3000 (Pst). Modulated cytokinin levels or signaling activity in CKX- or IPT-overexpressing plants or in ahk2 ahk3 mutants correlated with altered resistance. In fact, the cytokinin-activated transcription factor ARR2 contributes specifically to Pst resistance. The salicylic acid (SA) response factor TGA3 binds ARR2, and mutation of TGA-binding cis-elements in the Pr1 promoter abolished cytokinin- and ARR2-dependent Pr1 activation. Cytokinin treatment did not increase pathogen resistance in tga3 plants, as the cytokinin-dependent induction of Pr1 was eliminated. Moreover, SA signaling enhanced binding of ARR2/TGA3 to the Pr1 promoter. Taken together, these results show that cytokinins modulate the SA signaling to augment resistance against Pst, a process in which the interaction between TGA3 and ARR2 is important.

SummaryA hot pepper (Capsicum annuum) cDNA clone encoding pathogenesis-related protein 10 (CaPR-10) was isolated by differential screening of a cDNA library prepared from pepper leaves inoculated with tobacco mosaic virus pathotype (TMV-P 0 ). CaPR-10 transcripts were induced in the incompatible interaction with TMV-P 0 or Xanthomonas campestris pv. vesicatoria (Xcv) but not induced in the compatible interaction. Characterization of enzymatic properties of CaPR-10 indicated that the recombinant protein exhibits a ribonucleolytic activity against TMV RNA, as well as against pepper total RNA, and shows its putative antiviral activity in several conditions. The CaPR-10 protein existed at very low level in leaf tissue but was dramatically induced as soon as plants were inoculated with TMV-P 0 , and this was correlated with the increase of its ribonucleolytic activity. Immunoblot analysis and pull-down assays using proteins extracted from pepper leaves showed that TMV-P 0 inoculation led to the phosphorylation of CaPR-10, a modi®cation that should affect its capacity for RNase function. We present data that the induction and subsequent phosphorylation of CaPR-10 increased its ribonucleolytic activity to cleave invading viral RNAs, and this activity should be important to its antiviral pathway during viral attack in vivo.

other hand, catabolism, such as nucleic acid breakdown and proteolysis, becomes active through induction of a number of hydrolytic enzymes (Matile, 1992; Noodé n, 1988; Summary Smart, 1994;Thiman, 1980;Thomas and Stoddart, 1980). Leaf senescence, although a deteriorative cellular process, Four mutants that show the delayed leaf senescence phenotype were isolated from Arabidopsis thaliana.is assumed to be an evolutionarily acquired, active genetic trait that makes an important contribution to fitness of Genetic analyses revealed that they are all monogenic recessive mutations and fall into three complementation plants, for example by remobilizing nutrients from vegetative tissues to reproductive organs (Matile, 1992; Noodé n, groups, identifying three genetic loci controlling leaf senescence in Arabidopsis. Mutations in these loci cause delay in Thiman, 1980;Thomas and Stoddart, 1980). Elucidating the genetic mechanism of leaf senescence is essential all senescence parameters examined, including chlorophyll content, photochemical efficiency of photosystem II, to understanding the senescence phenomenon itself and also for practical purposes such as improvement of plant relative amount of the large subunit of Rubisco, and RNase and peroxidase activity. Delay of the senescence productivity, pre-or post-harvest storage, and stress tolerance. However, despite the biological and practical import-symptoms was observed during both age-dependent in planta senescence and dark-induced artificial senescence ance of leaf senescence, the genetic mechanism controlling the leaf senescence process remains poorly understood. in all of the mutant plants. The results indicate that the three genes defined by the mutations are key geneticWe therefore undertook a systematic genetic screening to identify the genes that control leaf senescence, using elements controlling functional leaf senescence and provide decisive genetic evidence that leaf senescence is aArabidopsis thaliana as a model system. genetically programmed phenomenon controlled by several monogenic loci in Arabidopsis. The results further Results suggest that the three genes function at a common step of age-dependent and dark-induced senescence processes.Isolation of Arabidopsis mutants with delayed leaf

Proteasomes constitute the major machinery to degrade or process proteins by ATP/ubiquitin-mediated proteolysis. Recent findings suggest a pivotal role of the ubiquitin/proteasome pathway in the regulation of apoptosis in animal cells. Here we show that virus-induced gene silencing of two different subunits of the 26 S proteasome, the ␣6 subunit of the 20 S proteasome and RPN9 subunit of 19 S regulatory complex, both activated the programmed cell death (PCD) program, accompanied by reduced proteasome activity and accumulation of polyubiquitinated proteins. These results demonstrate that disruption of proteasome function leads to PCD in plant cells. The affected cells showed morphological markers of PCD, including nuclear condensation and DNA fragmentation, accompanied by the 10-fold higher production of reactive oxygen species and increased ion leakage for 3-fold. Similar to apoptosis in animal system, mitochondrial membrane potential was decreased, cytochrome c released from mitochondria to cytosol, and caspase 9-and caspase 3-like proteolytic activities detected in the cells. Interestingly, this proteasome-mediated PCD stimulated the expression of only a subset of transcripts that are highly induced during pathogen-mediated hypersensitive response cell death, indicating that the two PCD pathways are differentially regulated. Taken together, these results provide the first direct evidence that proteasomes play a role in the regulatory program of PCD in plants.

A sweetpotato (Ipomoea batatas cv. ‘Jinhongmi’) MADS-box protein cDNA (SRD1) has been isolated from an early stage storage root cDNA library. The role of the SRD1 gene in the formation of the storage root in sweetpotato was investigated by an expression pattern analysis and characterization of SRD1-overexpressing (ox) transgenic sweetpotato plants. Transcripts of SRD1 were detected only in root tissues, with the fibrous root having low levels of the transcript and the young storage root showing relatively higher transcript levels. SRD1 mRNA was mainly found in the actively dividing cells, including the vascular and cambium cells of the young storage root. The transcript level of SRD1 in the fibrous roots increased in response to 1000 μM indole-3-acetic acid (IAA) applied exogenously. During the early stage of storage root development, the endogenous IAA content and SRD1 transcript level increased concomitantly, suggesting an involvement of SRD1 during the early stage of the auxin-dependent development of the storage root. SRD1-ox sweetpotato plants cultured in vitro produced thicker and shorter fibrous roots than wild-type plants. The metaxylem and cambium cells of the fibrous roots of SRD1-ox plants showed markedly enhanced proliferation, resulting in the fibrous roots of these plants showing an earlier thickening growth than those of wild-type plants. Taken together, these results demonstrate that SRD1 plays a role in the formation of storage roots by activating the proliferation of cambium and metaxylem cells to induce the initial thickening growth of storage roots in an auxin-dependent manner.