Spatial and Temporal Expression of a Citrate Permease and Aluminium-induced Citrate Exudation in Common Bean
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: firstname.lastname@example.org 2YARA GmbH & Co. KG, Hanninghof 35, D-48249 Dülmen, Germany 3International Center For Tropical Agriculture (CIAT) AA 6713, Cali, ColombiaIntroductionAluminium (Al) toxicity is one of the major limiting factors for plant growth in acid soils especially in the tropics where common bean (Phaseolus vulgaris) isproduced. Al resistance in common bean is conferred by citrate exudation which detoxifies Al in the root apoplast. Citrate exudation starts after a considerablelag period subsequent to Al treatment indicating that the activation of resistance mechanism involves Al-induced gene expression (Rangel et al., 2010). Weidentified a putative citrate permease gene PvMATE (Phaseolus vulgaris Multidrug and Toxin Extrusion) which is highly up regulated by Al treatment (Eticha etal., 2010). Here, the spatial and temporal expression of PvMATE in the root tip in relation to citrate exudation is presented.Materials and methodsTwo contrasting common bean genotypes, Quimbaya (Al-resistant) and VAX 1 (Al-sensitive) were used to study gene expression as well as citrate exudation ofthe 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. This1 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 by3 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.