Multiple mechanisms of heavy metal tolerance are differentially expressed in ecotypes of Artemisia fragrans.

Artemisia fragrans is a plant species with ability of growing on heavy metal-polluted soils. Ecotypes of this species naturally growing in polluted areas can accumulate and tolerate different amounts of heavy metals (HM), depending on soil contamination level at their origin. Heavy metal tolerance of various ecotypes collected from contaminated (AP, SP) and non-contaminated (BG) sites was compared by cultivation on a highly HM-contaminated river sediment and a non-contaminated agricultural control soil. Tissue-specific HM distribution was analyzed by laser ablation-inductively-coupled plasma-mass spectroscopy (LA-ICP-MS) and photosynthetic activity by non-invasive monitoring of chlorophyll fluorescence. Plant-mineral analysis did not reveal ecotype-differences in concentrations of Cd, Zn, Cu in shoots of Artemisia plants, suggesting no differential expression of root uptake or root to shoot translocation of HM. There was also no detectable rhizosphere effect on HM concentrations on the contaminated soil. However, despite high soil contaminations, all ecotypes accumulated Zn only in the concentration range of generally reported for normal growth of plants, while Cu and Cd concentrations were close to or even higher than the toxicity level for most plants. As a visible symptom of differences in HM tolerance, only the AP ecotype was able to enter the generative phase to complete its life cycle. Analysis of tissue-specific metal distribution revealed significantly lower concentrations of Cd in the leaf mesophyll of this ecotype, accumulating Cd mainly in the leaf petioles. A similar mesophyll exclusion was detectable also for Cu, although not associated with preferential accumulation in the leaf petioles. However, high mesophyll concentrations of Cd and Cu in the SP and BG ecotypes were associated with disturbances of the photosynthetic activity. The findings demonstrate differential expression of HM exclusion strategies in Artemisia ecotypes and suggest Cd and Cu exclusion from the photosynthetically active tissues as a major tolerance mechanism of the AP ecotype.

Flavodiiron protein Flv2/Flv4-related photoprotective mechanism dissipates excitation pressure of PSII in cooperation with phycobilisomes in Cyanobacteria.

Oxygenic photosynthesis evolved with cyanobacteria, the ancestors of plant chloroplasts. The highly oxidizing chemistry of water splitting required concomitant evolution of efficient photoprotection mechanisms to safeguard the photosynthetic machinery. The role of flavodiiron proteins (FDPs), originally called A-type flavoproteins or Flvs, in this context has only recently been appreciated. Cyanobacterial FDPs constitute a specific protein group that evolved to protect oxygenic photosynthesis. There are four FDPs in Synechocystis sp. PCC 6803 (Flv1 to Flv4). Two of them, Flv2 and Flv4, are encoded by an operon together with a Sll0218 protein. Their expression, tightly regulated by CO2 levels, is also influenced by changes in light intensity. Here we describe the overexpression of the flv4-2 operon in Synechocystis sp. PCC 6803 and demonstrate that it results in improved photochemistry of PSII. The flv4-2/OE mutant is more resistant to photoinhibition of PSII and exhibits a more oxidized state of the plastoquinone pool and reduced production of singlet oxygen compared with control strains. Results of biophysical measurements indicate that the flv4-2 operon functions in an alternative electron transfer pathway from PSII, and thus alleviates PSII excitation pressure by channeling up to 30% of PSII-originated electrons. Furthermore, intact phycobilisomes are required for stable expression of the flv4-2 operon genes and for the Flv2/Flv4 heterodimer-mediated electron transfer mechanism. The latter operates in photoprotection in a complementary way with the orange carotenoid protein-related nonphotochemical quenching. Expression of the flv4-2 operon and exchange of the D1 forms in PSII centers upon light stress, on the contrary, are mutually exclusive photoprotection strategies among cyanobacteria.