Citrate transporters play a critical role in aluminium-stimulated citrate efflux in rice bean (Vigna umbellata) roots.
OBJECTIVE
Aluminium (Al) stimulates the efflux of citrate from apices of rice bean (Vigna umbellata) roots. This response is delayed at least 3 h when roots are exposed to 50 microm Al, indicating that some inducible processes leading to citrate efflux are involved. The physiological bases responsible for the delayed response were examined here.


METHODS
The effects of several antagonists of anion channels and citrate carriers, and of the protein synthesis inhibitor, cycloheximide (CHM) on Al-stimulated citrate efflux and/or citrate content were examined by high-pressure liquid chromatography (HPLC) or an enzymatic method.


RESULTS
Both anion channel inhibitors and citrate carrier inhibitors can inhibit Al-stimulated citrate efflux, with anthracene-9-carboxylic acid (A-9-C, an anion channel inhibitor) and phenylisothiocyanate (PI, a citrate carrier inhibitor) the most effective inhibitors. A 6 h pulse of 50 microm Al induced a significant increase of citrate content in root apices and release of citrate. However, the increase in citrate content preceded the efflux. Furthermore, the release of citrate stimulated by the pulse treatment was inhibited by both A-9-C and PI, indicating the importance of the citrate carrier on the mitochondrial membrane and the anion channel on the plasma membrane for the Al-stimulated citrate efflux. CHM (20 microm) also significantly inhibited Al-stimulated citrate efflux, confirming that de novo protein synthesis is required for Al-stimulated citrate efflux.


CONCLUSIONS
These results indicate that the activation of genes possibly encoding citrate transporters plays a critical role in Al-stimulated citrate efflux.
Yang
Jian Li
JL
College of Environmental and Resource Sciences, Zhejiang University, Hangzhou 310029, China.
Zhang
Lei
L
Li
Ya Ying
YY
You
Jiang Feng
JF
Wu
Ping
P
Zheng
Shao Jian
SJ
D000535:Aluminum / Q000633:toxicity*; D000838:Anions; D000873:Anthracenes / Q000494:pharmacology; D002352:Carrier Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism*; D019343:Citric Acid / Q000378:metabolism*; D007887:Fabaceae / Q000378:metabolism*; D007473:Ion Channels / Q000378:metabolism; D009544:Niflumic Acid / Q000494:pharmacology; D010658:Phenylglyoxal / Q000494:pharmacology; D018517:Plant Roots / Q000378:metabolism*; D014176:Protein Biosynthesis; D011500:Protein Synthesis Inhibitors / Q000494:pharmacology
D000838:Anions; D000873:Anthracenes; D002352:Carrier Proteins; D007473:Ion Channels; D011500:Protein Synthesis Inhibitors; C020723:citrate-binding transport protein; D019343:Citric Acid; D009544:Niflumic Acid; C010030:9-anthroic acid; D000535:Aluminum; D010658:Phenylglyoxal