1) The document examines the spatial and temporal expression of a citrate permease gene (PvMATE) and aluminum-induced citrate exudation in common bean roots. 2) Results show that PvMATE is strongly expressed at the root tip, correlating with high citrate exudation in this region. Citrate exudation also remains substantial in the elongation zone. 3) Aluminum treatment induces expression of the STOP1 transcription factor before enhanced expression of PvMATE, indicating STOP1 regulates PvMATE. Citrate exudation follows a cascade of events involving Al signal reception, STOP1 activation, and PvMATE expression to maintain root growth under aluminum

1. Spatial and Temporal Expression of a Citrate Permease and
Aluminium-induced Citrate Exudation in Common Bean
Dejene Eticha1, Andrés F. Rangel1,2, Idupulapati M. Rao3 and Walter J. Horst1
1Institute of Plant Nutrition, Leibniz University Hannover, Herrenhaeuser Str. 2, D-30419 Hannover, Germany; e-Mail: dejene.eticha@pflern.uni-hannover.de
2YARA GmbH & Co. KG, Hanninghof 35, D-48249 Dülmen, Germany
3International Center For Tropical Agriculture (CIAT) AA 6713, Cali, Colombia
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 Al-induced gene expression (Rangel et al., 2010). We
identified a putative citrate permease gene PvMATE (Phaseolus vulgaris Multidrug and Toxin Extrusion) which is highly up regulated by Al treatment (Eticha et
al., 2010). Here, the spatial and temporal expression of PvMATE in the root tip in relation to citrate exudation is presented.
Materials and methods
Two contrasting common 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.
Results
Root growth of both genotypes is equally inhibited in short-term Al treatment PvMATE is strongly expressed at the root tip compared to the elongation zone. This
1 2 correlates with the high citrate exudation at the root apex. Citrate exudation in the
but genotypic difference is observed in medium- and long-term treatment.
elongation zone is still substantially high indicating that in common bean, not only
the transition but also the elongation zone must be protected.
Relative expression level
4000
Relative root elongation (%)
100 Quimbaya (R) Control
Vax-1 (S) Al [20 µM]
3000
80 PvMATE expression
2000
60 1000
0
40
100
[pmol (mm root)-1h-1]
Citrate exudation rate
20 80
60
Citrate exudation
0 40
1 2 3 4 5 6 7 8 9 10 12 16 20 24 20
Al treatment time (h) 0
0-3 3-6 6-10
Distance from the root apex (mm)
Fig. 1. Root growth of two contrasting common bean genotypes Fig. 2. Expression of citrate permease gene (PvMATE) and citrate exudation
treated with Al for up to 24h. at different root zones of Al-resistant bean genotype Qimbaya.
Al-induced expression of STOP1 transcription factor precedes the enhanced Citrate exudation began after a delay of about 4 h of Al treatment. It is preceded by
3 expression of PvMATE indicating that STOP1 regulates the PvMATE gene. 4 enhanced PvMATE expression. Citrate exudation continued to increase in the
resistant genotype but dropped shortly afterwards in the sensitive genotype.
12
Relative expression level
Quimbaya (R)
10 Vax-1 (S) 5
Quimbaya (R)
[nmol (10-mm root tip)-1 h-1]
8
STOP1 VAX-1 (S)
Citrate exudation rate
6 4
4
2 3
0
1000
Relative expression level
2
800
600
1
150 PvMATE
100
0
50
0
control 0-2 2-4 4-6 6-8 8-10
0 1 2 3 4 Al treatment time & citrate collection period (h)
Al treatment time (h)
Fig. 3. Gene expression of STOP1 and PvMATE at different Fig. 4. Citrate exudation of contrasting common bean genotypes at different
durations of Al treatment. durations of Al treatment.
Conclusion
In common bean, citrate exudation involves a cascade of events which include the reception of an Al
signal, activation of the STOP1 transcription factor, enhancement of citrate permease (pvMATE) gene
expression and subsequent citrate exudation resulting in the maintenance of root growth under Al stress.
Reference: Eticha et al. (2010) Annals of Botany, 105: 1119-1128. Rangel et al. (2010) Physiologia Plantarum, 38: 176-190.
The financial support of the German Research Foundation (DFG) is highly acknowledged.