Arnould Savouré

Proline: a multifunctional amino acid

Proline accumulates in many plant species in response to environmental stress. Although much is now known about proline metabolism, some aspects of its biological functions are still unclear. Here, we discuss the compartmentalization of proline biosynthesis, accumulation and degradation in the cytosol, chloroplast and mitochondria. We also describe the role of proline in cellular homeostasis, including redox balance and energy status. Proline can act as a signaling molecule to modulate mitochondrial functions, influence cell proliferation or cell death and trigger specific gene expression, which can be essential for plant recovery from stress. Although the regulation and function of proline accumulation are not yet completely understood, the engineering of proline metabolism could lead to new opportunities to improve plant tolerance of environmental stresses.

Plant cyclins : a unified nomenclature for plant A-, B- and D-type cyclins based on sequence organization

The comparative analysis of a large number of plant cyclins of the A/B family has recently revealed that plants possess two distinct B-type groups and three distinct A-type groups of cyclins [1]. Despite earlier uncertainties, this large-scale comparative analysis has allowed an unequivocal definition of plant cyclins into either A or B classes. We present here the most important results obtained in this study, and extend them to the case of plant D-type cyclins, in which three groups are identified. For each of the plant cyclin groups, consensus sequences have been established and a new, rational, plant-wide naming system is proposed in accordance with the guidelines of the Commission on Plant Gene Nomenclature. This nomenclature is based on the animal system indicating cyclin classes by an upper-case roman letter, and distinct groups within these classes by an arabic numeral suffix. The naming of plant cyclin classes is chosen to indicate homology to their closest animal class. The revised nomenclature of all described plant cyclins is presented, with their classification into groups CycA1, CycA2, CycA3, CycB1, CycB2, CycD1, CycD2 and CycD3.

Isolation, characterization, and chromosomal location of a gene encoding the Δ1-pyrroline-5-carboxylate synthetase in Arabidopsis thaliana

A full‐length cDNA and the corresponding At‐P5S gene encoding the first enzyme of the proline biosynthetic pathway, the Δ 1‐pyrroline‐5‐carboxylate (P5C) synthetase, were isolated in Arabidopsis thaliana. The At‐P5S cDNA encodes a protein of 717 amino acids showing high identity with the P5C synthetase of Vigna aconitifolia. Strong homology is also found at the N‐terminus to bacterial and yeast γ‐glutamyl kinase and at the C‐terminus to bacterial γ‐glutamyl phosphate reductase. Putative ATP‐ and NAD(P)H‐binding sites are suggested in the At‐P5S protein. The transcribed region of the At‐P5S gene is 4.8 kb long and contains 20 exons. Southern analysis suggests the presence of only one At‐P5S gene in the A. thaliana genome mapped at the bottom of the chromosome two. Expression analysis of At‐P5S in different organs reveals abundant At‐P5S transcripts in mature flowering plant. Rapid induction of the At‐P5S gene followed by accumulation of proline was observed in NaCl‐treated seedlings suggesting that At‐P5S is osmoregulated.

Abscisic acid-independent and abscisic acid-dependent regulation of proline biosynthesis following cold and osmotic stresses in Arabidopsis thaliana.

Abstract The role of the phytohormone abscisic acid (ABA) in the regulation of proline synthesis was investigated by following the expression of the At-P5S and At-P5R proline biosynthesis genes in Arabidopsis thaliana wild type, in an ABA-deficient aba1-1 mutant as well as in ABA-insensitive abi1-1 and abi2-1 mutants after ABA, cold and osmotic stress treatments. In wild-type and in ABA mutant seedlings, 50 μM ABA or osmotic stress treatment triggered expression of At-P5S, whereas At-P5R accumulation was scarcely detectable. Expression of either gene was mediated by endogenous ABA since transcript levels were similar in wild-type and in ABA-deficient mutant plants. Proline accumulated to a greater extent after osmotic stress than upon ABA or cold treatment. Thus, ABA-treated abi1-1 mutant plants accumulated less proline than the ABA-treated wild type. Upon salt stress, proline accumulated to a lesser extent in aba1-1 and abi1-1 mutant plants, suggesting an indirect role of ABA on proline accumulation during salt adaptation of the plant. These results indicate that the expression of the genes of the proline biosynthetic pathway is ABA independent upon cold and osmotic treatments, although their expression can be triggered by exogenously applied ABA. However, the endogenous ABA content may affect proline accumulation upon salt stress, suggesting post-transcriptional control of proline biosynthesis in response to NaCl.

Expression of antioxidant enzymes in response to abscisic acid and high osmoticum in tobacco BY 2 cell cultures

We have monitored the changes in antioxidative enzyme activities and their mRNA levels in tobacco (Nicotiana tabacum L. cv.) Bright Yellow 2 (BY-2) cell cultures subjected to salt stress, polyethylene glycol (PEG)-induced drought stress, and exogenous abscisic acid (ABA). Both NaCl and PEG treatments led to an increase in the total superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX) activities, whereas glutathione reductase (GR) activity remained unchanged. Increase in total SOD and APX activities was observed in cells grown in the presence of ABA. CAT activity remained constant whereas GR activity was reduced after the ABA treatment. All osmotic stress and ABA treatments had a differential effect on the accumulation of transcripts of the tested antioxidant genes. Our results show that osmotic stress alters the enzymatic defense reactions in tobacco BY-2 cell suspensions and suggest that ABA is not a secondary messenger in the induction of all antioxidant genes upon osmotic stress.

Activation of the cell cycle machinery and the isoflavonoid biosynthesis pathway by active Rhizobium meliloti Nod signal molecules in Medicago microcallus suspensions

We have shown that treatment of Medicago microcallus suspensions with the cognate Rhizobium meliloti Nod signal molecule NodRm‐IV(C16:2,S) can modify gene expression both qualitatively and quantitatively. At concentrations of 10(‐6) ‐ 10(‐9) M, this host specific plant morphogen but not the inactive non‐sulfated molecule stimulated cell cycle progression as indicated by the significantly enhanced thymidine incorporation, elevated number of S phase cells, increase in kinase activity of the p34cdc2‐related complexes and enhancement of the level of expression of several cell cycle marker genes, the histone H3‐1, the cdc2Ms and the cyclin cycMs2. The presented data suggest that at least part of the physiological role of the Nod factor may be linked to molecular events involved in the control of the plant cell division cycle. In situ hybridization experiments with antisense H3‐1 RNA probe indicated that only certain cells of the calli were able to respond to the Nod factor. High (10(‐6) M) but not low (10(‐9) M) concentrations of the active Nod factors induced the expression of the isoflavone reductase gene (IFR), a marker gene of the isoflavonoid biosynthesis pathway in most callus cells. Our results indicate that Medicago cell responses to the Nod signal molecules can be investigated in suspension cultures.

Identification of two alfalfa early nodulin genes with homology to members of the pea Enod12 gene family

In a search for plant genes expressed during early symbiotic interactions between Medicago sativa and Rhizobium meliloti, we have isolated and characterized two alfalfa genes which have strong sequence similarity to members of the Enod12 gene family of Pisum sativum. The M. sativa genes, MsEnod12A and B, encode putative protein products of 8066 Da and 12849 Da, respectively, each with a signal sequence at the N-terminus followed by a repetitive proline-rich region. Based on their expression during the initial period of nodule development, MsEnod12A and B are alfalfa early nodulin genes.

Isolation of a full-length mitotic cyclin cDNA clone CycIIIMs from Medicago sativa: Chromosomal mapping and expression

Cyclins in association with the protein kinase p34cdc2and related cyclin-dependent protein kinases (cdks) are key regulatory elements in controlling the cell division cycle. Here, we describe the identification and characterization of a full-length cDNA clone of alfalfa mitotic cyclin, termed CycIIIMs. Computer analysis of known plant cyclin gene sequences revealed that this cyclin belongs to the same structural group as the other known partial alfalfa cyclin sequences. Genetic segregation analysis based on DNA-DNA hybridization data showed that the CycIIIMs gene(s) locates in a single chromosomal region on linkage group 5 of the alfalfa genetic map between RFLP markers UO89A and CG13. The assignment of this cyclin to the mitotic cyclin class was based on its cDNA-derived sequence and its differential expression during G2/M cell cycle phase transition of a partially synchronized alfalfa cell culture. Sequence analysis indicated common motifs with both the A- and B-types of mitotic cyclins similarly to the newly described B3-type of animal cyclins.

Control of Nodule Induction and Plant Cell Growth by Nod Factors

We have shown that R. meliloti cells induced for their nod genes excrete a family of structurally related lipo-oligosaccharides, the Nod factors. These molecules evoke root hair deformation, cortical cell division, nodule induction and expression of an early nodulin gene, Nms-8b, to different extents. Using various approaches we have demonstrated that the NodRmIV(S) factor can act as plant growth regulator not only on Medicago root cells but also on cells grown in suspension. The factor was shown to stimulate the cell cycle; increased transition of cells from G1 to G2 phase was observed. In addition, perturbation of the hormonal balance in the plant cells was found to affect nodulation drastically, suggesting that the Nod factors may act in conjunction or via plant hormones to reprogram cells for nodule organogenesis.

Effect of nitrogen deficiency, salinity and drought on proline metabolism in Sesuvium portulacastrum

Drought and high salinity are responsible for large decreases in crop productivity all over theworld [1]. These losses of crop yield result from limitations of plant development through excessive ion accumulation, water deficit and mineral deficiencies [2]. Under these prevalent stresses, tolerant plants adopt various strategies with a wide range of biochemical to physiological and morphological adaptations [3]. Morphological ones include modifications in growth and allocation of assimilates towards roots for an efficient exploitation of soil nutrients [4]. The physiological strategy is represented by a higher selectivity for K+ over Na+ [5], an increase in K+-use efficiency [6], and the synthesis of organic osmolytes, with low molecular weight, for osmo-protection [7]. These osmolytes are sugars, polyols, amino acids, tertiary and quarternary ammonium, and tertiary sulphonium compounds [8].