Magnesium Deficiency Phenotypes Upon Multiple Knockout of Arabidopsis thaliana MRS2 Clade B Genes Can be Ameliorated by Concomitantly Reduced Calcium Supply

Plant MRS2 membrane protein family members have been shown to play important roles in magnesium uptake and homeostasis. Single and double knockouts for two Arabidopsis thaliana genes, AtMRS2-1 and AtMRS2-5, have previously not shown significant phenotypes even under limiting Mg2+ supply although both are strongly expressed already in early seedlings. Together with AtMRS2-10, these genes form clade B of the AtMRS2 gene family. We now succeeded in obtaining homozygous AtMRS2-1/10 double and AtMRS2-1/5/10 triple knockout lines after selection under increased magnesium supply. Although wilting early, both new mutant lines develop fully and are also fertile under standard magnesium supply, but show severe developmental retardation under limiting Mg2+ concentrations. To investigate nutrient dependency of germination and seedling development under various conditions, including variable supplies of Mg2+, Ca2+, Zn2+, Mn2+, Co2+, Cd2+ and Cu2+, in a reproducible and economical way, we employed a small-scale liquid culturing system in 24-well plate set-ups. This allowed the growth and monitoring of individual plantlets of different mutant lines under several nutritional conditions in parallel, and the scoring and statistical evaluation of developmental stages and biomass accumulation. Detrimental effects of higher concentrations of these elements were similar in mutants and the wild type. However, growth retardation phenotypes seen upon hydroponic cultivation under low Mg2+ could be ameliorated when Ca2+ concentrations were concomitantly lowered, supporting indications for an important interplay of these two most abundant divalent cations in the nutrient homeostasis of plants.