The Plant Journal

Chen, Changbin; Zhang, Wei; Timofejeva, Ljudmilla; Gerardin, Ylaine; Ma, Hong

doi: 10.1111/j.1365-313X.2005.02461.xpmid: 16045469

Recent studies in Saccharomyces cerevisiae have unveiled that meiotic recombination crossovers are formed by two genetically distinct pathways: a major interference‐sensitive pathway and a minor interference‐insensitive pathway. Several proteins, including the MSH4/MSH5 heterodimer and the MER3 DNA helicase, are indispensable for the interference‐sensitive pathway. MSH4 homologs have been identified in mice and Arabidopsis and shown to be required for normal levels of crossovers, suggesting that the function of MSH4 may be conserved among major eukaryotic kingdoms. However, it is not known whether an MER3‐like function is also required for meiosis in animals and plants. We have identified an Arabidopsis gene that encodes a putative MER3 homolog and is preferentially expressed in meiocytes. T‐DNA insertional mutants of this gene exhibit defects in fertility and meiosis. Detailed cytological studies indicate that the mutants are defective in homolog synapsis and crossover formation, resulting in a reduction of bivalents and in the formation of univalents at late prophase I. We have named this gene ROCK‐N‐ROLLERS (RCK) to reflect the mutant phenotype of chromosomes undergoing the meiotic ‘dance’ either in pairs or individually. Our results demonstrate that an MER3‐like function is required for meiotic crossover in plants and provide further support for the idea that Arabidopsis, like the budding yeast, possesses both interference‐sensitive and insensitive pathways for crossover formation.

Fitzgerald, Heather A.; Canlas, Patrick E.; Chern, Maw‐Sheng; Ronald, Pamela C.

doi: 10.1111/j.1365-313X.2005.02457.xpmid: 16045470

In dicotyledonous plants broad‐spectrum resistance to pathogens is established after the induction of the systemic acquired resistance (SAR) response. In Arabidopsis the NPR1 protein can regulate SAR by interacting with members of the TGA class of basic, leucine‐zipper transcription factors to alter pathogenesis‐related (PR) gene expression. Overexpression of (At)NPR1 in Arabidopsis enhances resistance to multiple pathogens. Similarly, overexpression of (At)NPR1 in rice enhances resistance to the bacterial pathogen, Xanthomonas oryzae pv. oryzae (Xoo). These results suggest that components of the (At)NPR1‐mediated SAR defense response may be conserved between monocots and dicots. To determine whether or not rice TGA factors are involved in disease resistance responses, the effect of altering the function of rice TGA2.1 was analyzed in transgenic plants. Transgenic rice overexpressing an rTGA2.1 mutant, that can no longer bind DNA, and transgenic rice that have the endogenous rTGA2.1 silenced by dsRNA‐mediated silencing were generated. Both types of transgenic rice displayed increased tolerance to Xoo, were dwarfed, and had altered accumulation of PR genes. The results presented in this study suggest that wild‐type rTGA2.1 has primarily a negative role in rice basal defense responses to bacterial pathogens.

Dubos, Christian; Willment, Janet; Huggins, David; Grant, Guy H.; Campbell, Malcolm M.

doi: 10.1111/j.1365-313X.2005.02458.xpmid: 16045471

Ionotropic glutamate receptors (iGluRs) play important roles in neurotransmission in animals. There is growing evidence that iGluRs also play important roles in plants. Using a chemical genetics approach, which combined a pH‐homeostasis mutant of Arabidopsis thaliana (de‐etiolated3), several different iGluR agonists, molecular modelling, and reporter gene expression in transgenic plants, we provide evidence that iGluR agonism can induce dramatic changes in plant development and metabolism. Systematic hypothesis testing revealed a signalling circuit that integrates amino acid and sugar signals to affect elongation growth and the deposition of carbon into starch and lignins. The data show that aminoglycoside antibiotics, such as kanamycin, and polyamines impinge upon this circuit. These findings provide a mechanism for the conversion of amino acid and sugar signals into an appropriate response at the gene expression level, and underline the similarities in iGluR agonism between animals and plants.

Zhou, Qingwen; Hare, Peter D.; Yang, Seong Wook; Zeidler, Mathias; Huang, Li‐Fang; Chua, Nam‐Hai

doi: 10.1111/j.1365-313X.2005.02453.xpmid: 16045472

Phytochrome A (phyA) plays a primary role in initiating seedling de‐etiolation and is the only plant photoreceptor known to be activated by far‐red light (FR). The signaling intermediate FHY1 appears to either participate directly in relaying the phyA signal or to positively regulate a critical signaling event(s) downstream of phyA activation. Here we identify a homolog of FHY1 named FHL (FHY1‐like) as a novel signaling factor essential for complete responsiveness to phyA. FHL possesses functional nuclear localization and nuclear export signals. Lines in which FHL function was abolished by insertional mutagenesis or attenuated by RNAi‐mediated suppression displayed a weaker hyposensitivity to continuous FR than fhy1 null mutants and most reported phyA signaling mutants. However, hypocotyl elongation assays indicated that suppression of FHL expression in fhy1‐3 caused an insensitivity of hypocotyl elongation to FR and blue light (B) indistinguishable from that seen in phyA. Real‐time PCR indicates that in FR, FHY1 transcripts are approximately 15‐fold more abundant than FHL transcripts. Although both FHY1 and FHL are capable of homo‐ and hetero‐interaction via their C‐termini, the ability of FHL overexpression to restore wild‐type (WT) morphological and molecular phenotypes to fhy1‐3 seedlings suggests that the extreme insensitivity to FR associated with suppression of FHL expression in fhy1‐3 cannot be accounted for by a critical role for FHY1‐FHL heterodimers in phyA signal transmission. Rather, we suggest that the relative abundances of FHY1 and FHL in WT plants account for the differences in the severity of fhy1 and fhl mutations. As for FHY1, FHL transcript accumulation is dependent on FHY3 and is decreased after exposure to FR, R or B light. These findings reiterate the prevalence of partial degeneracy in plant signaling networks that regulate responses crucial to survival.

Quint, Marcel; Ito, Hironori; Zhang, Wenjing; Gray, William M.

doi: 10.1111/j.1365-313X.2005.02449.xpmid: 16045473

Selective protein degradation by the ubiquitin‐proteasome pathway has emerged as a key regulatory mechanism in a wide variety of cellular processes. The selective components of this pathway are the E3 ubiquitin‐ligases which act downstream of the ubiquitin‐activating and ‐conjugating enzymes to identify specific substrates for ubiquitinylation. SCF‐type ubiquitin‐ligases are the most abundant class of E3 enzymes in Arabidopsis. In a genetic screen for enhancers of the tir1‐1 auxin response defect, we identified eta1/axr6‐3, a recessive and temperature‐sensitive mutation in the CUL1 core component of the SCFTIR1 complex. The axr6‐3 mutation interferes with Skp1 binding, thus preventing SCF complex assembly. axr6‐3 displays a pleiotropic phenotype with defects in numerous SCF‐regulated pathways including auxin signaling, jasmonate signaling, flower development, and photomorphogenesis. We used axr6‐3 as a tool for identifying pathways likely to be regulated by SCF‐mediated proteolysis and propose new roles for SCF regulation of the far‐red light/phyA and sugar signaling pathways. The recessive inheritance and the temperature‐sensitive nature of the pleiotropically acting axr6‐3 mutation opens promising possibilities for the identification and investigation of SCF‐regulated pathways in Arabidopsis.

Singh, Sunil Kumar; Eland, Cathlene; Harholt, Jesper; Scheller, Henrik Vibe; Marchant, Alan

doi: 10.1111/j.1365-313X.2005.02455.xpmid: 16045474

Despite the fact that several hundred glycosyltransferases have been identified from sequencing of plant genomes, the biological functions of only a handful have been established to date. A Poplar glycosyltransferase 64 (GT64) family member that is differentially expressed during the cell division and elongation phases of cambial growth was identified from previously generated transcript profiling of cambium tissues. The predicted Poplar GT64 protein has a closely related Arabidopsis homolog ECTOPICALLY PARTING CELLS (EPC1). Mutation of the EPC1 gene, one of three Arabidopsis GT64 family members, results in plants with a dramatically reduced growth habit, defects in vascular formation and reduced cell–cell adhesion properties in hypocotyl and cotyledon tissues. Secondary growth is enhanced in epc1 hypocotyl tissues and it is proposed that this results from the abnormal cell–cell adhesion within the cortical parenchyma cell layers. Loss of cell–cell contacts within cotyledon and leaf tissues is also proposed to account for vascular patterning defects and the fragile nature of epc1 tissues. The EPC1 protein thus plays a critical role during plant development in maintaining the integrity of organs via cell–cell adhesion, thereby providing mechanical strength and facilitating the movement of metabolites throughout the plant.

Delvallé, David; Dumez, Sylvain; Wattebled, Fabrice; Roldán, Isaac; Planchot, Véronique; Berbezy, Pierre; Colonna, Paul; Vyas, Darshna; Chatterjee, Manash; Ball, Steven; Mérida, Ángel; D'Hulst, Christophe

doi:

Wang, Yang; Ying, Jifeng; Kuzma, Monika; Chalifoux, Maryse; Sample, Angela; McArthur, Charlene; Uchacz, Tina; Sarvas, Carlene; Wan, Jiangxin; Dennis, David T.; McCourt, Peter; Huang, Yafan

doi: 10.1111/j.1365-313X.2005.02463.x

Alcázar, Rubén; García‐Martínez, José L.; Cuevas, Juan C.; Tiburcio, Antonio F.; Altabella, Teresa

doi: 10.1111/j.1365-313X.2005.02465.xpmid: 16045477

We have obtained Arabidopsis thaliana transgenic plants constitutively overexpressing ADC2, one of the two genes encoding arginine decarboxylase (ADC) in Arabidopsis. These plants contained very high levels of putrescine (Put) but no changes were observed in spermidine and spermine contents. The results obtained from quantification of free and conjugated polyamines suggest that conjugation may be a limiting step for control of Put homeostasis within a non‐toxic range for plant survival. Transgenic plants with increased levels of ADC2 transcript and elevated Put content showed dwarfism and late‐flowering, and the phenotype was rescued by gibberellin A3 (GA3) application. The contents of bioactive GA4 and GA1, and of GA9 (a precursor of GA4), as well as the levels of AtGA20ox1, AtGA3ox1 and AtGA3ox3 transcripts (quantified by real‐time PCR) were lower in the ADC2 overexpressor plants than in the wild type. No change in the expression of genes encoding earlier enzymes in the GA biosynthesis pathway was detected by microarray analysis. These results suggest that Put accumulation affects GA metabolism through the repression of biosynthetic steps catalyzed by GA 20‐oxidase and GA 3‐oxidase.

Thomann, Alexis; Brukhin, Vladimir; Dieterle, Monika; Gheyeselinck, Jacqueline; Vantard, Marylin; Grossniklaus, Ueli; Genschik, Pascal

doi: 10.1111/j.1365-313X.2005.02467.xpmid: 16045478

Cullin (CUL)‐dependent ubiquitin ligases form a class of structurally related multisubunit enzymes that control the rapid and selective degradation of important regulatory proteins involved in cell cycle progression and development, among others. The CUL3‐BTB ligases belong to this class of enzymes and despite recent findings on their molecular composition, our knowledge on their functions and substrates remains still very limited. In contrast to budding and fission yeast, CUL3 is an essential gene in metazoans. The model plant Arabidopsis thaliana encodes two related CUL3 genes, called CUL3A and CUL3B. We recently reported that cul3a loss‐of‐function mutants are viable but exhibit a mild flowering and light sensitivity phenotype. We investigated the spatial and temporal expression of the two CUL3 genes in reproductive tissues and found that their expression patterns are largely overlapping suggesting possible functional redundancy. Thus, we investigated the consequences on plant development of combined Arabidopsis cul3a cul3b loss‐of‐function mutations. Homozygous cul3b mutant plants developed normally and were fully fertile. However, the disruption of both the CUL3A and CUL3B genes reduced gametophytic transmission and caused embryo lethality. The observed embryo abortion was found to be under maternal control. Arrest of embryogenesis occurred at multiple stages of embryo development, but predominantly at the heart stage. At the cytological level, CUL3 loss‐of‐function mutations affected both embryo pattern formation and endosperm development.