Homogalacturonan-modifying enzymes: structure, expression, and roles in plants.

Understanding the changes affecting the plant cell wall is a key element in addressing its functional role in plant growth and in the response to stress. Pectins, which are the main constituents of the primary cell wall in dicot species, play a central role in the control of cellular adhesion and thereby of the rheological properties of the wall. This is likely to be a major determinant of plant growth. How the discrete changes in pectin structure are mediated is thus a key issue in our understanding of plant development and plant responses to changes in the environment. In particular, understanding the remodelling of homogalacturonan (HG), the most abundant pectic polymer, by specific enzymes is a current challenge in addressing its fundamental role. HG, a polymer that can be methylesterified or acetylated, can be modified by HGMEs (HG-modifying enzymes) which all belong to large multigenic families in all species sequenced to date. In particular, both the degrees of substitution (methylesterification and/or acetylation) and polymerization can be controlled by specific enzymes such as pectin methylesterases (PMEs), pectin acetylesterases (PAEs), polygalacturonases (PGs), or pectate lyases-like (PLLs). Major advances in the biochemical and functional characterization of these enzymes have been made over the last 10 years. This review aims to provide a comprehensive, up to date summary of the recent data concerning the structure, regulation, and function of these fascinating enzymes in plant development and in response to biotic stresses.

Pectin Remodeling and Involvement of AtPME3 in the Parasitic Plant-Plant Interaction, Phelipanche ramosa-Arabidospis thaliana.

Phelipanche ramosa is a root parasitic plant fully dependent on host plants for nutrition and development. Upon germination, the parasitic seedling develops inside the infected roots a specific organ, the haustorium, thanks to the cell wall-degrading enzymes of haustorial intrusive cells, and induces modifications in the host's cell walls. The model plant Arabidopsis thaliana is susceptible to P. ramosa; thus, mutants in cell wall metabolism, particularly those involved in pectin remodeling, like Atpme3-1, are of interest in studying the involvement of cell wall-degrading enzymes in the establishment of plant-plant interactions. Host-parasite co-cultures in mini-rhizotron systems revealed that parasite attachments are twice as numerous and tubercle growth is quicker on Atpme3-1 roots than on WT roots. Compared to WT, the increased susceptibility in AtPME3-1 is associated with reduced PME activity in the roots and a lower degree of pectin methylesterification at the host-parasite interface, as detected immunohistochemically in infected roots. In addition, both WT and Atpme3-1 roots responded to infestation by modulating the expression of PAE- and PME-encoding genes, as well as related global enzyme activities in the roots before and after parasite attachment. However, these modulations differed between WT and Atpme3-1, which may contribute to different pectin remodeling in the roots and contrasting susceptibility to P. ramosa. With this integrative study, we aim to define a model of cell wall response to this specific biotic stress and indicate, for the first time, the role of PME3 in this parasitic plant-plant interaction.

Seasonal Variation in Cell Wall Composition and Carbohydrate Metabolism in the Seagrass Posidonia oceanica Growing at Different Depths.

Posidonia oceanica is a common seagrass in the Mediterranean Sea that is able to sequester large amounts of carbon. The carbon assimilated during photosynthesis can be partitioned into non-structural sugars and cell-wall polymers. In this study, we investigated the distribution of carbon in starch, soluble carbohydrates and cell-wall polymers in leaves and rhizomes of P. oceanica. Analyses were performed during summer and winter in meadows located south of the Frioul archipelago near Marseille, France. The leaves and rhizomes were isolated from plants collected in shallow (2 m) and deep water (26 m). Our results showed that P. oceanica stores more carbon as starch, sucrose and cellulose in summer and that this is more pronounced in rhizomes from deep-water plants. In winter, the reduction in photoassimilates was correlated with a lower cellulose content, compensated with a greater lignin content, except in rhizomes from deep-water plants. The syringyl-to-guaiacyl (S/G) ratio in the lignin was higher in leaves than in rhizomes and decreased in rhizomes in winter, indicating a change in the distribution or structure of the lignin. These combined data show that deep-water plants store more carbon during summer, while in winter the shallow- and deep-water plants displayed a different cell wall composition reflecting their environment.

Identification of pectin methylesterase 3 as a basic pectin methylesterase isoform involved in adventitious rooting in Arabidopsis thaliana.

• Here, we focused on the biochemical characterization of the Arabidopsis thaliana pectin methylesterase 3 gene (AtPME3; At3g14310) and its role in plant development. • A combination of biochemical, gene expression, Fourier transform-infrared (FT-IR) microspectroscopy and reverse genetics approaches were used. • We showed that AtPME3 is ubiquitously expressed in A. thaliana, particularly in vascular tissues. In cell wall-enriched fractions, only the mature part of the protein was identified, suggesting that it is processed before targeting the cell wall. In all the organs tested, PME activity was reduced in the atpme3-1 mutant compared with the wild type. This was related to the disappearance of an activity band corresponding to a pI of 9.6 revealed by a zymogram. Analysis of the cell wall composition showed that the degree of methylesterification (DM) of galacturonic acids was affected in the atpme3-1 mutant. A change in the number of adventitious roots was found in the mutant, which correlated with the expression of the gene in adventitious root primordia. • Our results enable the characterization of AtPME3 as a major basic PME isoform in A. thaliana and highlight its role in adventitious rooting.

New insights into pectin methylesterase structure and function.

In bacteria, fungi and plants, pectin methylesterases are ubiquitous enzymes that modify the degree of methylesterification of pectins, which are major components of plant cell walls. Such changes in pectin structure are associated with changes in cellular adhesion, plasticity, pH and ionic contents of the cell wall and influence plant development and stress responses. In plants, pectin methylesterases belong to large multigene families, are regulated in a highly specific manner, and are involved in vegetative and reproductive processes, including wood and pollen formation, in addition to plant-pathogen interactions. Although, overall, protein structures are highly conserved between isoforms, recent data indicate that structural variations might be associated with the targeting and functions of specific pectin methylesterases.

New insights into diet breadth of polyphagous and oligophagous aphids on two Arabidopsis ecotypes.

We investigated whether plant ecotype might affect aphid performance and behavior. The probing behaviors of the polyphagous aphid Myzus persicae and the oligophagous aphid Brevicoryne brassicae on two ecotypes of Arabidopsis thaliana, WS and Col-0 were recorded using the direct current electrical penetration graph method (DC-EPG). Myzus persicae displayed a significant preference for the WS ecotype but was not greatly disturbed on Col-0, while B. brassicae discriminated between the two A. thaliana ecotypes, feeding less on WS than on Col-0. A Principal Component Analysis of aphid probing behavior data recorded on Col-0 and WS ecotypes showed that the one of M. persicae was positively correlated with the phloem ingestion phases while the one of B. brassicae was more related to nonfeeding phase. The survival of the aphid species was followed during early larval stages on the two ecotypes and a significantly higher mortality was observed of B. brassicae neonates compared to M. persicae, both reared on WS. Moreover, transcriptomic analysis of noninfested plant leaves from both ecotypes was monitored and underlined constitutive differences between Col-0 and WS gene expression that might explain the different aphid behaviors. Among a unigene set comprising 39 042 sequences for A. thaliana, 6% were differently expressed affecting, for example, the secondary metabolites and cell wall pathways: two third upregulated in WS and one third upregulated in Col-0. Thus, the "ecotype" variable should be taken into account when setting up a plant-insect experimental research.

The lack of a systematic validation of reference genes: a serious pitfall undervalued in reverse transcription-polymerase chain reaction (RT-PCR) analysis in plants.

Reverse transcription-polymerase chain reaction (RT-PCR) approaches have been used in a large proportion of transcriptome analyses published to date. The accuracy of the results obtained by this method strongly depends on accurate transcript normalization using stably expressed genes, known as references. Statistical algorithms have been developed recently to help validate reference genes, and most studies of gene expression in mammals, yeast and bacteria now include such validation. Surprisingly, this important approach is under-utilized in plant studies, where putative housekeeping genes tend to be used as references without any appropriate validation. Using quantitative RT-PCR, the expression stability of several genes commonly used as references was tested in various tissues of Arabidopsis thaliana and hybrid aspen (Populus tremula x Populus tremuloides). It was found that the expression of most of these genes was unstable, indicating that their use as references is inappropriate. The major impact of the use of such inappropriate references on the results obtained by RT-PCR is demonstrated in this study. Using aspen as a model, evidence is presented indicating that no gene can act as a universal reference, implying the need for a systematic validation of reference genes. For the first time, the extent to which the lack of a systematic validation of reference genes is a stumbling block to the reliability of results obtained by RT-PCR in plants is clearly shown.

Major changes in the cell wall during silique development in Arabidopsis thaliana.

Fruit development is a highly complex process, which involves major changes in plant metabolism leading to cell growth and differentiation. Changes in cell wall composition and structure play a major role in modulating cell growth. We investigated the changes in cell wall composition and the activities of associated enzymes during the dry fruit development of the model plant Arabidopsis thaliana. Silique development is characterized by several specific phases leading to fruit dehiscence and seed dispersal. We showed that early phases of silique growth were characterized by specific changes in non-cellulosic sugar content (rhamnose, arabinose, xylose, galactose and galacturonic acid). Xyloglucan oligosaccharide mass profiling further showed a strong increase in O-acetylated xyloglucans over the course of silique development, which could suggest a decreased capacity of xyloglucans to be associated with each other or to cellulose. The degree of methylesterification, mediated by the activity of pectin methylesterases (PMEs), decreased over the course of silique growth and dehiscence. The major changes in cell wall composition revealed by our analysis suggest that it could be major determinants in modulating cell wall rheology leading to growth or growth arrest.

Compatible plant-aphid interactions: how aphids manipulate plant responses.

To access phloem sap, aphids have developed a furtive strategy, their stylets progressing towards sieve tubes mainly through the apoplasmic compartment. Aphid feeding requires that they overcome a number of plant responses, ranging from sieve tube occlusion and activation of phytohormone-signalling pathways to expression of anti-insect molecules. In addition to bypassing plant defences, aphids have been shown to affect plant primary metabolism, which could be a strategy to improve phloem sap composition in nutrients required for their growth. During compatible interactions, leading to successful feeding and reproduction, aphids cause alterations in their host plant, including morphological changes, modified resource allocation and various local as well as systemic symptoms. Repeated salivary secretions injected from the first probe in the epidermal tissue up to ingestion of sieve-tube sap may play a crucial role in the compatibility between the aphid and the plant.

S-nitrosoglutathione reductase affords protection against pathogens in Arabidopsis, both locally and systemically.

Nitric oxide and S-nitrosothiols (SNOs) are widespread signaling molecules that regulate immunity in animals and plants. Levels of SNOs in vivo are controlled by nitric oxide synthesis (which in plants is achieved by different routes) and by S-nitrosoglutathione turnover, which is mainly performed by the S-nitrosoglutathione reductase (GSNOR). GSNOR is encoded by a single-copy gene in Arabidopsis (Arabidopsis thaliana; Martínez et al., 1996; Sakamoto et al., 2002). We report here that transgenic plants with decreased amounts of GSNOR (using antisense strategy) show enhanced basal resistance against Peronospora parasitica Noco2 (oomycete), which correlates with higher levels of intracellular SNOs and constitutive activation of the pathogenesis-related gene, PR-1. Moreover, systemic acquired resistance is impaired in plants overexpressing GSNOR and enhanced in the antisense plants, and this correlates with changes in the SNO content both in local and systemic leaves. We also show that GSNOR is localized in the phloem and, thus, could regulate systemic acquired resistance signal transport through the vascular system. Our data corroborate the data from other authors that GSNOR controls SNO in vivo levels, and shows that SNO content positively influences plant basal resistance and resistance-gene-mediated resistance as well. These data highlight GSNOR as an important and widely utilized component of resistance protein signaling networks conserved in animals and plants.

Fatty acid hydroperoxides and H2O2 in the execution of hypersensitive cell death in tobacco leaves.

We initially compared lipid peroxidation profiles in tobacco (Nicotiana tabacum) leaves during different cell death events. An upstream oxylipin assay was used to discriminate reactive oxygen species (ROS)-mediated lipid peroxidation from 9- and 13-lipoxygenase (LOX)-dependent lipid peroxidation. Free radical-mediated membrane peroxidation was measured during H(2)O(2)-dependent cell death in leaves of catalase-deficient plants. Taking advantage of these transgenic plants, we demonstrate that, under light conditions, H(2)O(2) plays an essential role in the execution of cell death triggered by an elicitor, cryptogein, which provokes a similar ROS-mediated lipid peroxidation. Under dark conditions, however, cell death induction by cryptogein was independent of H(2)O(2) and accompanied by products of the 9-LOX pathway. In the hypersensitive response induced by the avirulent pathogen Pseudomonas syringae pv syringae, both 9-LOX and oxidative processes operated concurrently, with ROS-mediated lipid peroxidation prevailing in the light. Our results demonstrate, therefore, the tight interplay between H(2)O(2) and lipid hydroperoxides and underscore the importance of light during the hypersensitive response.

Runaway cell death, but not basal disease resistance, in lsd1 is SA- and NIM1/NPR1-dependent.

LSD1 was defined as a negative regulator of plant cell death and basal disease resistance based on its null mutant phenotypes. We addressed the relationship between lsd1-mediated runaway cell death and signaling components required for systemic acquired resistance (SAR), namely salicylic acid (SA) accumulation and NIM1/NPR1. We present two important findings. First, SA accumulation and NIM1/NPR1 are required for lsd1-mediated runaway cell death following pathogen infection or application of chemicals that mimic SA action. This implies that lsd1-dependent cell death occurs 'downstream' of the accumulation of SA. As SA application triggers runaway cell death in lsd1 but not wild-type plants, we infer that LSD1 negatively regulates an SA-dependent signal leading to cell death. Thus SA is both a trigger and a required mediator of lsd1 runaway cell death. Second, neither SA accumulation nor NIM1/NPR1 function is required for the basal resistance operating in lsd1. Therefore LSD1 negatively regulates a basal defense pathway that can act upstream or independently of both NIM1/NPR1 function and SA accumulation following avirulent or virulent pathogen challenge. Our data, together with results from other studies, point to the existence of an SA-dependent 'signal potentiation loop' controlling HR. Continued escalation of signaling in the absence of LSD1 leads to runaway cell death. We propose that LSD1 is a key negative regulator of this signal potentiation.

LESION SIMULATING DISEASE 1 is required for acclimation to conditions that promote excess excitation energy.

The lsd1 mutant of Arabidopsis fails to limit the boundaries of hypersensitive cell death response during avirulent pathogen infection and initiates unchecked lesions in long day photoperiod giving rise to the runaway cell death (rcd) phenotype. We link here the initiation and propagation of rcd to the activity of photosystem II, stomatal conductance and ultimately to photorespiratory H(2)O(2). A cross of lsd1 with the chlorophyll a/b binding harvesting-organelle specific (designated cao) mutant, which has a reduced photosystem II antenna, led to reduced lesion formation in the lsd1/cao double mutant. This lsd1 mutant also had reduced stomatal conductance and catalase activity in short-day permissive conditions and induced H(2)O(2) accumulation followed by rcd when stomatal gas exchange was further impeded. All of these traits depended on the defense regulators EDS1 and PAD4. Furthermore, nonphotorespiratory conditions retarded propagation of lesions in lsd1. These data suggest that lsd1 failed to acclimate to light conditions that promote excess excitation energy (EEE) and that LSD1 function was required for optimal catalase activity. Through this regulation LSD1 can influence the effectiveness of photorespiration in dissipating EEE and consequently may be a key determinant of acclimatory processes. Salicylic acid, which induces stomatal closure, inhibits catalase activity and triggers the rcd phenotype in lsd1, also impaired acclimation of wild-type plants to conditions that promote EEE. We propose that the roles of LSD1 in light acclimation and in restricting pathogen-induced cell death are functionally linked.