The current thesis deals with the characterization of a family of three potential Ca2+ signal modulators from Arabidopsis thaliana referred to as DROPs (DMI1-like RCK Domain Proteins). Confocal microscopy of transiently transformed mesophyll protoplasts revealed that the DROP1-EYFP fusion protein localizes to the ER, while DROP2-EYFP and DROP3-EYFP fusion proteins localize to chloroplasts. As determined by analysis of microarray data and promoter-GUS studies, DROP1 seems to be generally expressed at a very low level only. In contrast, expression of DROP2 and DROP3 could be detected in seedlings, roots, leaves, siliques, and flowers, particularly in stigmata and pollen. Phenotypic studies of drop2 and drop3 knockout mutants reveal diverse roles of those genes in fertilization, root growth, and the plant immune response.