Abstract
Cation/H+ exchangers encoded by CAX genes play an important role in the vacuolar accumulation of metals including Ca2+ and Mn2+. Arabidopsis thaliana CAX1 and CAX3 have been previously shown to differ phylogenetically from CAX2 but the physiological roles of these different transporters are still unclear. To examine the functions and the potential of redundancy between these three cation transporters, cax1/cax2 and cax2/cax3 double knockout mutants were generated and compared with wild type and cax single knockouts. These double mutants had equivalent metal stress responses to single cax mutants. Both cax1 and cax1/cax2 had increased tolerance to Mg stress, while cax2 and cax2/cax3 both had increased sensitivity to Mn stress. The cax1/cax2 and cax2/cax3 mutants did not exhibit the deleterious developmental phenotypes previously seen with the cax1/cax3 mutant. However, these new double mutants did show alterations in seed germination, specifically a delay in germination time. These alterations correlated with changes in nutrient content within the seeds of the mutants, particularly the cax1/cax2 mutant which had significantly higher seed content of Ca and Mn. This study indicates that the presence of these Arabidopsis CAX transporters is important for normal germination and infers a role for CAX proteins in metal homeostasis within the seed.
Highlights
Metal transporters play a major role in regulating metal homeostasis, in controlling the acquisition of essential metal nutrients into the plant, coordinating the distribution and partitioning of these nutrients to appropriate locations within the plant and within individual cells, and preventing or responding to metal toxicity [1]
To examine the potential phenotypes of double mutants generated between two phylogenetically distinct CAX genes; CAX2 with either CAX1 or CAX3, the homozygous cax1-1, cax2-2 and cax3-1 knockout mutants which we previously isolated and characterised [11,15,25] were crossed and cax1/cax2 and cax2/cax3 double mutants were selected in the F2 generation
The cax2 and cax3 mutants showed a moderate induction of other CAX genes, with CAX1 and CAX3 and CAX1 induced slightly relative to wild type, while cax1/cax2 and cax2/cax3 displayed substantial induction of CAX3 and CAX1 expression, respectively (Figure 1)
Summary
Metal transporters play a major role in regulating metal homeostasis, in controlling the acquisition of essential metal nutrients into the plant, coordinating the distribution and partitioning of these nutrients to appropriate locations within the plant and within individual cells, and preventing or responding to metal toxicity [1]. CAX (cation/H+ exchanger) genes encode a family of important vacuolar-localised metal transporters that function as cation/H+ exchangers [5]. They mediate the high capacity sequestration of cations into the vacuole and are energised by the counter flux of H+ and utilise the large proton electrochemical gradient which exists across the tonoplast. Impaired hormone and developmental responses or abiotic stress phenotypes following deletion of Arabidopsis CAX genes such as CAX1 and CAX3, indicate Ca2+ signalling roles for these vacuolar Ca2+ transporters [11,15,16,17]
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