Salmonella virulence in animals depends on effectors injected by Type III Secretion Systems (T3SSs). In this report we demonstrate that Salmonella mutants that are unable to deliver effectors are also compromised in infection of Arabidopsis thaliana plants. Transcriptome analysis revealed that in contrast to wild type bacteria, T3SS mutants of Salmonella are compromised in suppressing highly conserved Arabidopsis genes that play a prominent role during Salmonella infection of animals. We also found that Salmonella originating from infected plants are equally virulent for human cells and mice. These results indicate a high degree of conservation in the defense and infection mechanism of animal and plant hosts during Salmonella infection.

SUMMARYMembrane-delimited events play a crucial role for ABA signaling and PYR/PYL/RCAR ABA receptors, clade A PP2Cs and SnRK2/CPK kinases modulate the activity of different plasma membrane components involved in ABA action. Therefore, the turnover of PYR/PYL/RCARs in the proximity of plasma membrane might be a step that affects receptor function and downstream signaling. In this study we describe a single-subunit RING-type E3 ubiquitin ligase RSL1 that interacts with the PYL4 and PYR1 ABA receptors at the plasma membrane. Overexpression of RSL1 reduces ABA sensitivity and rsl1 RNAi lines that impair expression of several members of the RSL1/RFA gene family show enhanced sensitivity to ABA. RSL1 bears a C-terminal transmembrane domain that targets the E3 ligase to plasma membrane. Accordingly, bimolecular fluorescent complementation (BiFC) studies showed the RSL1-PYL4 and RSL1-PYR1 interaction is localized to plasma membrane. RSL1 promoted PYL4 and PYR1 degradation in vivo and mediated in vitro ubiquitylation of the receptors. Taken together, these results suggest ubiquitylation of ABA receptors at plasma membrane is a process that might affect their function via effect on their half-life, protein interactions or trafficking.

Seed longevity is crucial for agriculture and plant genetic diversity, but it is limited by cellular damage during storage. Seeds are protected against aging by cellular defenses and by structures such as the seed coat. We have screened an activation-tagging mutant collection of Arabidopsis (Arabidopsis thaliana) and selected four dominant mutants with improved seed longevity (isl1-1D to isl4-1D) under both natural and accelerated aging conditions. In the isl1-1D mutant, characterized in this work, overexpression of the transcription factor ARABIDOPSIS THALIANA HOMEOBOX25 (ATHB25; At5g65410) increases the expression of GIBBERELLIC ACID3-OXIDASE2, encoding a gibberellin (GA) biosynthetic enzyme, and the levels of GA 1 and GA 4 are higher (3.2-and 1.4-fold, respectively) in the mutant than in the wild type. The morphological and seed longevity phenotypes of the athb25-1D mutant were recapitulated in transgenic plants with moderate (4-to 6-fold) overexpression of ATHB25. Simultaneous knockdown of ATHB25, ATHB22, and ATHB31 expression decreases seed longevity, as does loss of ATHB25 and ATHB22 function in a double mutant line. Seeds from wild-type plants treated with GA and from a quintuple DELLA mutant (with constitutive GA signaling) are more tolerant to aging, providing additional evidence for a role of GA in seed longevity. A correlation was observed in several genotypes between seed longevity and mucilage formation at the seed surface, suggesting that GA may act by reinforcing the seed coat. This mechanism was supported by the observation of a maternal effect in reciprocal crosses between the wild type and the athb25-1D mutant.

BackgroundMitogen-activated protein kinases (MAPKs) are key regulators of immune responses in animals and plants. In Arabidopsis, perception of microbe-associated molecular patterns (MAMPs) activates the MAPKs MPK3, MPK4 and MPK6. Increasing information depicts the molecular events activated by MAMPs in plants, but the specific and cooperative contributions of the MAPKs in these signalling events are largely unclear.ResultsIn this work, we analyse the behaviour of MPK3, MPK4 and MPK6 mutants in early and late immune responses triggered by the MAMP flg22 from bacterial flagellin. A genome-wide transcriptome analysis reveals that 36% of the flg22-upregulated genes and 68% of the flg22-downregulated genes are affected in at least one MAPK mutant. So far MPK4 was considered as a negative regulator of immunity, whereas MPK3 and MPK6 were believed to play partially redundant positive functions in defence. Our work reveals that MPK4 is required for the regulation of approximately 50% of flg22-induced genes and we identify a negative role for MPK3 in regulating defence gene expression, flg22-induced salicylic acid accumulation and disease resistance to Pseudomonas syringae. Among the MAPK-dependent genes, 27% of flg22-upregulated genes and 76% of flg22-downregulated genes require two or three MAPKs for their regulation. The flg22-induced MAPK activities are differentially regulated in MPK3 and MPK6 mutants, both in amplitude and duration, revealing a highly interdependent network.ConclusionsThese data reveal a new set of distinct functions for MPK3, MPK4 and MPK6 and indicate that the plant immune signalling network is choreographed through the interplay of these three interwoven MAPK pathways.

HghlightWe provide the first evidence for a mechanism of growth inhibition by ABA during germination and seedling establishment based on inhibition of PM H+-ATPase and altered pH, K+, and anion homeostasis.

SUMMARYIntracellular pH must be kept close to neutrality to be compatible with cellular functions, but the mechanisms of pH homeostasis and the responses to intracellular acidification are mostly unknown. In the plant Arabidopsis thaliana, we found that intracellular acid stress generated by weak organic acids at normal external pH induces expression of several chaperone genes, including ROF2, which encodes a peptidyl-prolyl cis-trans isomerase of the FK506-binding protein class. Loss of function of ROF2, and especially double mutation of ROF2 and the closely related gene ROF1, results in acid sensitivity. Over-expression of ROF2 confers tolerance to intracellular acidification by increasing proton extrusion from cells. The activation of the plasma membrane proton pump (H + -ATPase) is indirect: over-expression of ROF2 activates K + uptake, causing depolarization of the plasma membrane, which activates the electrogenic H + pump. The depolarization of ROF2 over-expressing plants explains their tolerance to toxic cations such as lithium, norspermidine and hygromycin B, whose uptake is driven by the membrane potential. As ROF2 induction and intracellular acidification are common consequences of many stresses, this mechanism of pH homeostasis may be of general importance for stress tolerance.

SummaryIn order to investigate the effects of a permanent increase in cellular H 2 O 2 on cation homeostasis we have studied a T-DNA insertion mutant of the Arabidopsis CATALASE 2 gene. This mutant (cat2-1) exhibits 20% of wild-type leaf catalase activity and accumulates more H 2 O 2 than the wild type under normal growth conditions. In addition to reduced size, a pale green color and great reduction in secondary roots, the cat2-1 mutant exhibited increased sensitivity to H 2 O 2 , NaCl, norspermidine, high light and cold stress. On the other hand, the germination of the cat2-1 mutant is more tolerant to lithium than the wild type. This novel phenotype cannot be explained by changes in lithium transport. Actually, the uptake of lithium (and of other toxic cations such as sodium and norspermidine) is increased in the cat2-1 mutant while K + levels were decreased. The lithium tolerance of this mutant seems to result both from insensitivity to the inhibitory ethylene induced by this cation and a reduced capability for ethylene production. Accordingly, induction by ethylene of responsive genes such as PR4 and EBP/ERF72 is decreased in cat2-1. Mutants insensitive to ethylene such as etr1-1 and ein3-3 are lithium tolerant, and inhibition of ethylene biosynthesis with 2-aminoisobutyrate protects against lithium toxicity. Microarray analysis of gene expression indicates that the expression of genes related to cation transport and ethylene synthesis and perception was not altered in the cat2-1 mutant, suggesting that H 2 O 2 modulates these processes at the protein level. These results uncover a cross-talk between oxidative stress, cation homeostasis and ethylene.

SUMMARYLight is a major regulator of plant growth and development by antagonizing gibberellins (GA), and we provide evidence for a role of light perception and GA in seed coat formation and seed tolerance to deterioration. We have identified two activation-tagging mutants of Arabidopsis thaliana, cog1-2D and cdf4-1D, with improved seed tolerance to deterioration linked to increased expression of COG1/DOF1.5 and CDF4/DOF2.3, respectively. These encode two homologous DOF transcription factors, with COG1 most highly expressed in seeds. Improved tolerance to seed deterioration was reproduced in transgenic plants overexpressing these genes, and loss of function from RNA interference resulted in opposite phenotypes. Overexpressions of COG1 and CDF4 have been described to attenuate various light responses mediated by phytochromes. Accordingly, we found that phyA and phyB mutants exhibit increased seed tolerance to deterioration. The phenotype of tolerance to deterioration conferred by gain of function of COG1 and by loss of function of phytochromes is of maternal origin, is also observed under natural aging conditions and correlates with a seed coat with increased suberin and reduced permeability. In developing siliques of the cog1-2D mutant the expression of the GA biosynthetic gene GA3OX3 and levels of GA 1 are higher than in the wild type. These results explain the antagonism between phytochromes and COG1 in terms of the inhibition and the activation, respectively, of GA action.