Dehydrin defines a complex family of intrinsically disordered proteins with potential adaptive value with regard to freeze-induced cell dehydration. Search within an expressed sequence tags library from cDNAs of cold-acclimated crowns of alfalfa (Medicago sativa spp. sativa L.) identified transcripts putatively encoding K(3)-type dehydrins. Analysis of full-length coding sequences unveiled two highly homologous sequence variants, K(3)-A and K(3)-B. An increase in the frequency of genotypes yielding positive genomic amplification of the K(3)-dehydrin variants in response to selection for superior tolerance to freezing and the induction of their expression at low temperature strongly support a link with cold adaptation. The presence of multiple allelic forms within single genotypes and independent segregation indicate that the two K(3) dehydrin variants are encoded by distinct genes located at unlinked loci. The co-inheritance of the K(3)-A dehydrin with a Y(2)K(4) dehydrin restriction fragment length polymorphism with a demonstrated impact on freezing tolerance suggests the presence of a genome domain where these functionally related genes are located. These results provide additional evidence that dehydrin play important roles with regard to tolerance to subfreezing temperatures. They also underscore the value of recurrent selection to help identify variants within a large multigene family in allopolyploid species like alfalfa.
Reverse transcription quantitative PCR (RT-qPCR) is a highly sensitive technique that has become the standard for the analysis of differences in gene expression in response to experimental treatments or among genetic sources. The accuracy of the RT-qPCR results can be significantly affected by uncontrolled sources of variation that can be accounted for normalization with so-called reference genes stably expressed under various conditions. In this study we assessed the stability of 21 reference gene candidates in crowns of two alfalfa cultivars (Apica and Evolution) exposed to various environmental conditions (cold, water stress and photoperiod) and from above ground biomass of the cultivar Orca sampled at three developmental stages (vegetative, full bloom and mature pods). Candidates were selected based on their previous identification in other plant species or their stable expression in a differential hybridization of alfalfa ESTs with cDNA from nonacclimated and cold-acclimated alfalfa. Genes encoding ubiquitin protein ligase 2a (UBL-2a), actin depolymerizing factor (ADF) and retention in endoplasmic reticulum 1 protein (Rer1) were the most stable across experimental conditions. Conversely β-actin (Act), α-tubulin (Tub) and glyceraldehyde 3-phosphate dehydrogenase (GAPDH) frequently used as "housekeeping genes" in gene expression studies showed poor stability. No more than two reference genes were required to normalize the gene expression data under each condition. Normalization of the expression of genes of interest with unstable reference genes led to observations that were conflicting with those made with validated reference genes and that were in some cases inconsistent with the current knowledge of the trait. The reference genes identified in this study are strong candidates for normalization of gene expression in cultivated alfalfa.
Cowpea (Vigna unguiculata (L.) Walp.) is an important legume crop in subsaharian Africa. However, its cultivation is threatened by the parasitic weed Striga gesnerioides (Willd.) Vatke, which causes considerable yield losses. Striga gesnerioides is an obligate parasite with low photosynthetic activity. Its seeds germinate in response to specific germination stimulants exuded by host roots. After germination, an haustorium is formed through differentiation of the radicular apex. A vascular connection is subsequently established, allowing the parasitic weed to absorb water and nutrients that are essential for its development. Control of S. gesnerioides is difficult to achieve because of the intimate association between the parasitic weed and its host. The use of resistant cowpea cultivars, in combination with appropriate cultural practices, is probably the easiest and most effective method to control S. gesnerioides and achieve long term reduction of its seed bank in the soil.Key words: germination, interaction, control methods, cultivar resistance, Striga gesnerioides, Vigna unguiculata.
Dube M‐P & Belzile FJ (2010). Low genetic variability of Striga gesnerioides populations parasitic on cowpea might be explained by a recent origin. Weed Research50, 493–502. Summary Striga gesnerioides is an obligate root hemiparasitic plant that causes considerable yield losses to cowpea, an important crop legume of Sub‐Saharan Africa. The use of resistant cultivars is the easiest and most effective method to control the parasite. Several cowpea cultivars exhibiting resistance have been identified during the last decades. However, most resistant cultivars show a differential response when grown in different countries across West Africa, suggesting that there are different races of S. gesnerioides. In this study, we investigated the genetic variability within and between 43 populations of five of the previously recognised races of the parasite present in West Africa. Amplified fragment length polymorphism (AFLP) markers were used on up to 10 individuals from each population. These markers showed almost no genetic variability within populations. The variability between the populations was also extremely low and did not allow discrimination of the five races. There was a certain geographical structure, but no ‘racial’ clustering could be seen. Even AFLP markers previously reported to be race‐specific on another set of Striga populations proved unable to discriminate between races in this collection of populations. Possible causes of the low level of genetic variability include the hypothesis that this strain has only quite recently arisen. Such a low level of variability and the absence of specific markers for the virulence will have consequences on the evolution of the parasite and on the development of adequate control methods.
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