The ABCC subfamily of the ATP binding cassette (ABC) transporters, which were 
formerly known as multidrug resistance-related proteins (MRPs), consists of 
closely related members found in all eukaryotic organisms. Although more than a 
decade of intensive research has elapsed since the first MRP protein was 
functionally characterised in Arabidopsis thaliana, knowledge of this particular 
transporter family is still limited in plants. Although ABCC proteins were 
originally defined as vacuolar pumps of glutathione-S (GS) conjugates, evidence, 
as well as speculation, on their endogenous functions inside the cell ranges 
from detoxification and heavy metal sequestration, to chlorophyll catabolite 
transport and ion channel regulation. The characterisation of knockout mutants 
in Arabidopsis has been pivotal for elucidation of different roles of ABCC 
transporters. However, a functional annotation for the majority of these 
transport proteins is still lacking, even in this model plant. On the one hand, 
this problem seems to be caused by functional redundancy between family members, 
which might lead to physiological complementation by a highly homologous gene in 
the mutant lines. On the other hand, there is growing evidence that the 
functional diversity of ABCC genes in Arabidopsis and other plants is far 
greater than previously assumed. For example, analysis of microarray expression 
data supports involvement of ABCC transporters in the response to biotic stress: 
particular changes in ABCC transcript levels are found, which are 
pathogen-specific and evoke distinct signalling cascades. Current knowledge 
about plant ABCC transporters indicates that novel and unexpected functions and 
substrates of these proteins are still waiting to be elucidated.