I. Responses of plant to trace metallic elements excess
- A. Cadmium accumulation and tolerance in plants
Several strategies are being developed in our laboratory to identify genes involved in Cd tolerance and hyperaccumulation.
Screening of cadmium resistant yeasts
Growth of S. cerevisiae (strain BY474)
Comparison of Thlaspi caerulescens populations
Populations originating from heavy metal polluted or non polluted soils, have been studied in collaboration with Prof. A. Smith (University of Oxford). (Roosens et al. 2003)
Populations with contrasting Cd tolerance and accumulation capacities are being further studied.
Three populations originated from St Félix-de-Pallière, Puente Basadre and Prayon (from left to right) grown on high Cd levels.
Study of Cd hyperaccumulation in Arabidopsis halleri
cDNA-AFLP analysis and characterization of genes highly expressed in Cd tolerant genotypes
cDNA-AFLP analysis has been applied on extreme genotypes of the BC1 progeny for Cd tolerance (Craciun et al.2006)
“Transcript-derived-DNA-fragments” visualised in the cDNA-AFLP that correlate with Cd tolerance are studied. The assignment of a function is greatly facilitated by the high homology (95% identity at the DNA level are predicted) with Arabidopsis thaliana.
Click on the picture to enlarge transcript profile in Cd sensible and resistant plants
Overexpression of selected genes in Arabidopsis thaliana
Selected genes are overexpressed in Arabidopsis thaliana, which is the easiest transformable plant since transformation can be done by flower dipping. As A. thaliana is very sensitive to cadmium, effect of gene overexpression on Cd tolerance can be easily measured. If enhanced cadmium accumulation and/or tolerance is observed, application will be foreseen in cultivated species.
Overexpression of selected Thlaspi caerulescens cDNAs in Nicotiana plumbaginifolia
Selected genes are overexpressed in Nicotiana plumbaginifolia, which is a high biomass easy transformable species. Better growth on contamined soils of Brussels could be observed for some lines (Alban Heudiard).
- B. COPPER ACCUMULATION AND TOLERANCE IN PLANTS
A new research has been recently developed on plant responses to Cu excess.
(PhD thesis of Alfred Cubaka-Kagale, François Chipeng, Hélène Lequeux)
II. CONTRIBUTION TO THE STUDY OF MAGNESIUM DEFICIENCY IN PLANTS:
- Major elements nutrition
- Physiological characterization of Mg deficiency in plants
- Magnesium homeostasis in plants
(i) The first approach uses the variation of Mg concentration in Arabidopsis mutants and natural accessions as a source of diversity to find new genes and alleles controlling root and shoot Mg homeostasis.
(ii) Because the knowledge about the impact of Mg deficiency on physiological processes was scarce, we proceeded to transcriptome analyses to provide non-biased hints about targets of Mg starvation. We published a thorough description of the transcriptomic responses (within hours or days before the outbreak of visual symptoms) of Mg deprivation (New Phytol 187: 119-131; 187: 132-144). Mg starvation triggered a temporal difference in the early transcriptomic response in organs, with a higher number of genes being differentially regulated in the root after 8h and in young mature leaves after 28h. Unlike other mineral deficiencies, putative Mg transporters genes were not induced (post-transcriptional induction cannot be excluded). Interestingly, the rhythmic expression of circadian clock component genes was altered in roots, while abscisic acid signalling was triggered in leaves. Delayed responses after 7 days of Mg starvation impacted on the abundance of 33 % of the transcripts in leaves and less than 2% of the transcripts in roots. The analysis confirmed the visual observation that Mg starvation affected the shoot more than the root in Arabidopsis. Re-supply of Mg restored initial patterns of gene expression for one-fifth of the transcripts in the leaves and half in the roots. Further study of the circadian rhythm confirmed the altered amplitude of clock genes expression while the phase was still maintained. Higher expression was observed of genes in the ethylene biosynthetic pathway, in the reactive oxygen species detoxification and in the photoprotection of the photosynthetic apparatus. Higher production of ethylene and altered levels of antioxidants supported those transcriptomic data.
Effects of Mg deficiency on photosystems
Visual symptoms of Mg deficiency in sugar beet. Mg deficiency appears first on the most recently developped leaves as chlorosis between the veins, which remind green. Brown spottings and necroses appear when deficiency is severe or under high light intensity.
The behaviour of PSII and PSI was assessed using direct and modulated fluorescence measurement, near-infrared absorbance changes and 77K fluorescence emission spectroscopy. An early effect on the maximum PSI oxidation rate could be identified before any decrease in the rate of PSII maximum quantum yield of primary photochemistry, PSII electron transport or any chlorophyll degradation. Also, a decrease in the amplitude fluorescence emission peak at 735 nm has been observed, suggesting an early effect of Mg deficiency on PSI. Concomitantly, sucrose accumulates in source leaves upon Mg deficiency as an early response. High sucrose levels are known to down-regulate the chloroplast electron chain transport. The delay of photosynthesis regulation in Mg-deficient plants may provoke an unbalance between light and dark reactions, so inducing oxidative stress and further leading to necrogenesis on leaves.
Effects of Mg deficiency on sucrose partitioning
Sucrose pathway from production to consumption and storage organs. In sugar beet, sucrose loading from the apoplasm to companion cell is operated trough sucrose/H+ symporter.
Fig. from Hermans et al, 2006