Brassica napus responses to short-term excessive copper treatment with decrease of photosynthetic pigments, differential expression of heavy metal homeostasis genes including activation of gene NRAMP4 involved in photosystem II stabilization.

In the present study, the influence of 50 and 100 µM CuSO4 was investigated starting from 3 h till 72 h treatment of 4-weeks Brassica napus plants. High CuSO4 concentrations in nutrient medium resulted in the rapid copper accumulation in plants, especially in roots, much slower and to lower degree in leaves. Copper excess induced early decrease in the leaf water content and temporary leaf wilting. The decrease in content of photosynthetic pigments became significant to 24 h of excessive copper treatments and reached 35 % decrease to 72 h, but there were no significant changes in maximum quantum efficiency of photosystem II photochemistry. The copper excess affected the expression of ten genes involved in heavy metal homeostasis and copper detoxification. The results showed the differential and organ-specific expression of most genes. The potential roles of copper-activated genes encoding heavy metal transporters (ZIP5, NRAMP4, YSL2, and MRP1), metallothioneins (MT1a and MT2b), low-molecular chelator synthesis enzymes (PCS1 and NAS2), and metallochaperones (CCS and HIPP06) in heavy metal homeostasis and copper ion detoxification were discussed. The highest increase in gene expression was shown for NRAMP4 in leaves in spite of relatively moderate Cu accumulation there. The opinion was advanced that the NRAMP4 activation can be considered among the early reactions in the defense of the photosystem II against copper excess.