Mitteilungen der Gesellschaft für Pflanzenbauwissenschaften Band 23

Mitt. Ges. Pflanzenbauwiss. 23: 245 (2011)
Spatial and temporal expression of a citrate transporter gene and
aluminium-induced citrate exudation in common bean
Dejene Eticha and Walter J. Horst
Institute of Plant Nutrition, Leibniz Universität Hannover. E-Mail: eticha@pflern.uni-hannover.de
Introduction
Aluminium (Al) toxicity is one of the major limiting factors for plant growth in acid soils
especially in the tropics where common bean (Phaseolus vulgaris) is produced. Al
resistance in common bean is conferred by citrate exudation which detoxifies Al in
the root apoplast. Citrate exudation starts after a considerable lag period subsequent
to Al treatment indicating that the activation of resistance mechanism involves Alinduced
gene expression. We identified a putative citrate transporter gene PvMATE
(P. vulgaris Multidrug and Toxin Extrusion) the expression of which is highly
enhanced by Al treatment (Eticha et al., 2010). Here, the spatial and temporal
expression of PvMATE in relation to citrate exudation is presented.
Materials and Methods
Two contrasting bean genotypes, Quimbaya (Al-resistant) and VAX 1 (Al-sensitive)
were used to study gene expression as well as citrate exudation of the different
apical root zones (0-3 mm, 3-6mm, and 6-10 mm).
Result and Discussion
PvMATE gene is more strongly expressed at the root tip compared to the elongation
zone of Al-treated plants in agreement with higher citrate exudation at the root tip.
Citrate exudation in the elongation zone was also elevated indicating that in common
bean not only the transition but also the elongation zone must be protected from Al
injury (Rangel et. al., 2007). The temporal pattern of citrate exudation and gene
expression of PvMATE and a transcription factor, STOP1, regulating the expression
of PvMATE was studied. Significant citrate exudation started after a delay of about 4
h. It continued to increase in the Al-resistant genotype but dropped shortly afterwards
in the sensitive genotype. Enhanced expression of PvMATE was observed about an
hour earlier than the beginning of citrate exudation. The expression of the PvMATE
gene continued to increase but citrate exudation decreased after extended treatment
period indicating that in long term Al treatment, not the transporter but rather the
synthesis of citrate is limiting. Furthermore, enhanced expression of STOP1 was
observed already after 1 h Al treatment and increased up to 3 h in both genotypes
indicating that this transcription factor is indeed involved in activating the expression
of PvMATE in common bean. In conclusion, in common bean, citrate exudation
involves a sequential cascade of events which include the reception of an Al signal,
activation of the STOP1 transcription factor, enhancement of citrate transporter
(pvMATE) gene expression and subsequent citrate exudation resulting in resumption
of root growth under Al stress.
References
Eticha D. et al. 2010: Annals of Botany 105:1119-1128.
Rangel et al. 2007: Journal of Experimental Botany 58:3895-3904.