Physiological and molecular analysis of the interaction between aluminium toxicity and drought stress in common bean (Phaseolus vulgaris)
Zhong-Bao Yang
2
Dejene Eticha
2
Alfonso Albacete
1
Idupulapati Madhusudana Rao
0
Thomas Roitsch
1
Walter Johannes Horst
2
0
International Center for Tropical Agriculture (CIAT)
, AA 6713, Cali,
Colombia
1
Institute of Plant Science, Karl-Franzens-Universita t Graz
, Schubertstrasse 51, A-8010 Graz,
Austria
2
Institute of Plant Nutrition, Leibniz Universita t Hannover
, Herrenhaeuser Str. 2, D-30419 Hannover,
Germany
Aluminium (Al) toxicity and drought are two major factors limiting common bean (Phaseolus vulgaris) production in the tropics. Short-term effects of Al toxicity and drought stress on root growth in acid, Al-toxic soil were studied, with special emphasis on Al-drought interaction in the root apex. Root elongation was inhibited by both Al and drought. Combined stresses resulted in a more severe inhibition of root elongation than either stress alone. This result was different from the alleviation of Al toxicity by osmotic stress (-0.60 MPa polyethylene glycol) in hydroponics. However, drought reduced the impact of Al on the root tip, as indicated by the reduction of Al-induced callose formation and MATE expression. Combined Al and drought stress enhanced up-regulation of ACCO expression and synthesis of zeatin riboside, reduced drought-enhanced abscisic acid (ABA) concentration, and expression of NCED involved in ABA biosynthesis and the transcription factors bZIP and MYB, thus affecting the regulation of ABA-dependent genes (SUS, PvLEA18, KS-DHN, and LTP) in root tips. The results provide circumstantial evidence that in soil, drought alleviates Al injury, but Al renders the root apex more drought-sensitive, particularly by impacting the gene regulatory network involved in ABA signal transduction and cross-talk with other phytohormones necessary for maintaining root growth under drought.
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Common bean (Phaseolus vlugaris L.) is the major food
legume for human nutrition in the world, and a major
source of calories and protein, particularly in many
developing Latin American and African countries (Graham,
1978; Rao, 2001). Common bean production in the tropics
is severely limited by two major abiotic stresses, drought
and aluminium (Al) toxicity (Goldman et al., 1989; Ishitani
et al., 2004). Generally, common bean has been regarded as
an Al- and drought-sensitive crop (Rao, 2001; Beebe et al.,
2008). In many regions of the developing world, drought
and Al toxicity overlap (Wortmann et al., 1998; Thung and
Rao, 1999; Beebe et al., 2011). Furthermore, on many
acid soils, intermittent drought stress during the growing
period could cause yield reduction of 3060% (CIAT,
1992; Wortmann et al., 1998).
The root apex is the most Al-sensitive root zone (Horst
et al., 1992; Delhaize and Ryan, 1995). In common bean,
the transition zone (12 mm) and the elongation zone are
targets of Al injury (Rangel et al., 2007). Excess Al will
result in a rapid inhibition of root elongation and enhanced
callose synthesis in the root tips; both are sensitive indicators
of Al injury in roots (Delhaize and Ryan, 1995; Sta and
Horst, 2009). Applying the pressure probe technique to 5 cm
root tips of an Al-sensitive maize cultivar, Gunse et al. (1997)
found that Al treatment decreased both the cellular and
whole root hydraulic conductivities and cell wall extensibility.
However, by application of Al only to the 1 cm root apex,
Sivaguru et al. (2006) did not find any impairment of xylem
water flow.
Al resistance in common bean is related to lower Al
accumulation in the root tips (Rangel et al., 2007). Lower
Al accumulation and thus the detoxification of Al in the
apoplast through root exudates, such as Al-activated
exudation of citrate from root tips, play a key role in Al
resistance in common bean (Miyasaka et al., 1991; Rangel
et al., 2009, 2010; Horst et al., 2010). Eticha et al. (2010)
showed that the Al-induced expression of a MATE
(multidrug and toxin extrusion family protein) gene in root apices
is a prerequisite for citrate exudation and Al resistance in
common bean. In addition Al-induced inhibition of root
elongation was positively correlated with the expression of
an ACCO (1-aminocyclopropane-1-carboxylic acid oxidase)
gene in the root apex (Eticha et al., 2010). The expression of
MATE and ACCO has been used as a sensitive indicator of
Al impact on the root apex in common bean (Yang et al.,
2011).
Drought strongly affects the root apex, leading to
inhibition of root elongation (Sharp et al., 2004). The
maintenance of root growth during water deficit is a prerequisite
for water uptake from the subsoil (Sponchiado et al., 1989;
Serraj and Sinclair, 2002). In maize, three mechanisms
involved in primary root growth maintenance under water
deficit have been proposed: osmotic adjustment; modification
of cell wall (CW) extension properties; and the role of
abscisic acid (ABA) accumulation (Sharp et al., 2004;
Yamaguchi et al., 2010).
To the authors knowledge the interaction of drought and
Al at the level of the root apex (...truncated)