Plant parasite control and soil fauna diversity

The use of pesticides to control plant parasites and diseases has generated serious problems of public health and environmental quality, leading to the promotion of alternative Integrated Pest Management strategies that tend to rely more on natural processes and the active participation of farmers as observers and experimenters in their own fields. We present three case studies that point at different options provided by locally available populations of soil organisms, the maintenance of diverse populations of pests or increased resistance of plants to pest attacks by their interactions with earthworms and other useful soil organisms. These examples demonstrate the diversity of options offered by the non-planned agro-ecosystem diversity in pest control and the need to identify management options that maintain this biodiversity.     

Mutations in QARS,Encoding Glutaminyl-tRNA Synthetase,Cause Progressive Microcephaly,Cerebral-Cerebellar Atrophy,and Intractable Seizures

Progressive microcephaly is a heterogeneous condition with causes including mutations in genes encoding regulators of neuronal survival. Here, we report the identification of mutations in QARS (encoding glutaminyl-tRNA synthetase [QARS]) as the causative variants in two unrelated families affected by progressive microcephaly, severe seizures in infancy, atrophy of the cerebral cortex and cerebellar vermis, and mild atrophy of the cerebellar hemispheres. Whole-exome sequencing of individuals from each family independently identified compound-heterozygous mutations in QARS as the only candidate causative variants. QARS was highly expressed in the developing fetal human cerebral cortex in many cell types. The four QARS mutations altered highly conserved amino acids, and the aminoacylation activity of QARS was significantly impaired in mutant cell lines. Variants p.Gly45Val and p.Tyr57His were located in the N-terminal domain required for QARS interaction with proteins in the multisynthetase complex and potentially with glutamine tRNA, and recombinant QARS proteins bearing either substitution showed an over 10-fold reduction in aminoacylation activity. Conversely, variants p.Arg403Trp and p.Arg515Trp, each occurring in a different family, were located in the catalytic core and completely disrupted QARS aminoacylation activity in vitro. Furthermore, p.Arg403Trp and p.Arg515Trp rendered QARS less soluble, and p.Arg403Trp disrupted QARS-RARS (arginyl-tRNA synthetase 1) interaction. In zebrafish, homozygous qars loss of function caused decreased brain and eye size and extensive cell death in the brain. Our results highlight the importance of QARS during brain development and that epilepsy due to impairment of QARS activity is unusually severe in comparison to other aminoacyl-tRNA synthetase disorders.     

Omega-3 fatty acid desaturase (FAD3, FAD7, FAD8) gene expression and linolenic acid content in cowpea leaves submitted to drought and after rehydration

Membrane polyunsaturated fatty acids (PUFA) and particularly linolenic acid (18:3, LA) are known to be implicated in plant tolerance to low temperature. Their role in resistance to drought is much less investigated. In this work, three full-length cDNAs corresponding to omega-3 fatty acid desaturases: fad3 (endoplasmic reticulum), fad7 and fad8 (chloroplastic) were isolated from Vigna unguiculata leaves. Two cowpea cultivars, one drought-tolerant, EPACE-1, and one drought-susceptible, 1183, were compared in terms of fad isoform gene expression and leaf LA contents in plants submitted to water stress followed by rehydration. In EPACE-1, LA content in the main leaf polar lipids increased in response to mild water deficit. Severe water deficits induced a decrease in MGDG LA content while those of PC and DGDG continued to increase. Variations in FAD gene expression, matched those in LA contents. In 1183, LA contents decreased in all lipid classes in response to water stress, as did FAD3 and FAD8 gene expression levels. Rehydration after a moderate water stress induced stimulation mostly in FAD3 gene expression in both cvs. LA contents were equivalent to control levels in EPACE-1. In 1183, they were back to control levels in PC shortly after rehydration but remained low in galactolipids. These results suggested that omega-3 FAD activities were involved in the increase in leaf membrane unsaturation, in the drought tolerant plants whereas the sensitive plants lost PUFAs in response to the treatment. The significance of this discrepancy between the two cvs. in terms of adaptation to drought is discussed.     

A patatin-like protein with galactolipase activity is induced by drought stress in Vigna unguiculata leaves

This paper reports the cloning of a cDNA (Vupat1) expressed in Vigna unguiculata leaves coding for a protein with 48% sequence homology to patatin, the major protein from potato tuber which has lipolytic acylhydrolase activity. Two cultivars differing in drought tolerance were examined in Northern-blot analyses. Expression of Vupat1 is stimulated by drought stress, especially in the drought-sensitive cultivar. Vupat1 was expressed in the baculovirus system as a fusion protein secreted in the culture medium. The recombinant protein displays lipolytic activity towards monogalactosyldiacylglycerol, digalactosyldiacylglycerol and sulphoquinovosyldiacylglycerols.     

A novel patatin-like gene stimulated by drought stress encodes a galactolipid acyl hydrolase

A cDNA (Vupat1) encoding a predicted 43 kDa protein was isolated from drought-stressed cowpea (Vigna unguiculata) leaves. It has homology with patatin, a potato tuber storage protein with lipolytic acyl hydrolase activity. The recombinant protein VUPAT1 expressed in the baculovirus system displays preferentially galactolipid acyl hydrolase activity. Phospholipids are very slowly hydrolyzed and apparently triacylglycerols are not deacylated. Vupat1 promoter contains putative drought-inducible sequences. Northern blots showed that gene expression is stimulated by drought stress and is more pronounced in a drought-sensitive cultivar than in a drought-tolerant one. An involvement in drought-induced galactolipid degradation is proposed for VUPAT1.     

Physiological induction of regulatory Qa-1-restricted CD8+ T cells triggered by endogenous CD4+ T cell responses

T cell-dependent autoimmune diseases are characterized by the expansion of T cell clones that recognize immunodominant epitopes on the target antigen. As a consequence, for a given autoimmune disorder, pathogenic T cell clones express T cell receptors with a limited number of variable regions that define antigenic specificity. Qa-1, a MHC class I-like molecule, presents peptides from the variable region of TCRs to Qa-1-restricted CD8+ T cells. The induction of Vß-specific CD8+ T cells has been harnessed in an immunotherapeutic strategy known as the “T cell vaccination” (TCV) that comprises the injection of activated and attenuated CD4+ T cell clones so as to induce protective CD8+ T cells. We hypothesized that Qa-1-restricted CD8+ regulatory T cells could also constitute a physiologic regulatory arm of lymphocyte responses upon expansion of endogenous CD4+ T cells, in the absence of deliberate exogenous T cell vaccination. We immunized mice with two types of antigenic challenges in order to sequentially expand antigen-specific endogenous CD4+ T cells with distinct antigenic specificities but characterized by a common Vß chain in their TCR. The first immunization was performed with a non-self antigen while the second challenge was performed with a myelin-derived peptide known to drive experimental autoimmune encephalomyelitis (EAE), a mouse model of multiple sclerosis. We show that regulatory Vß-specific Qa-1-restricted CD8+ T cells induced during the first endogenous CD4+ T cell responses are able to control the expansion of subsequently mobilized pathogenic autoreactive CD4+ T cells. In conclusion, apart from the immunotherapeutic TCV, Qa-1-restricted specialized CD8+ regulatory T cells can also be induced during endogenous CD4+ T cell responses. At variance with other regulatory T cell subsets, the action of these Qa-1-restricted T cells seems to be restricted to the immediate re-activation of CD4+ T cells.     

Belowground organism activities affect plant aboveground phenotype, inducing plant tolerance to parasites

Soil fauna activities are probably more important than currently acknowledged in determining individual plant response to stresses and overall plant diversity. Here we demonstrate that the positive effect of earthworms on rice could be the result of a systemic effect on plant physiology. Moreover, this effect could improve tolerance to stressors such as parasitic nematodes. In a controlled experiment, an 82% decrease in the production of infested plants was suppressed when earthworms were present. Earthworms had no direct effect on nematode population size. In their presence, however, root biomass was not affected by nematodes and the expected inhibition of photosynthesis was suppressed. In the leaves, the expression of three stress‐responsive genes (coding for lipoxygenase, phospholipase D and cysteine protease) was modulated by the presence of belowground invertebrate activities. We document conflicting systemic effects of parasitic nematodes and beneficial earthworms, although we cannot precisely identify the mechanism involved. These results reveal the importance of non‐trophic belowground/aboveground interactions for plant health and response to stresses.