ALVES, V. M. C.

CAMPOS, L. J. M.

MAGALHAES, J. V. de

PITTA, G. V. E.

PRATES, H. T.

QUEIROZ, C. G. de S.

SCHAFFERT, R. E.

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Physiological Assessment of the Actual Role of AluminumExclusion Conferredby Citrate Release as an Aluminum Tolerance Mechanism in MaizePrevious  Top Next   LEONARDO J. M. CAMPOS1, HÉLIO T. PRATES2, CRISTINA G. DE S. QUEIROZ1,JURANDIR V. DE MAGALHÃES2, GILSON V. E. PITTA2, ROBERT E.SCHAFFERT2, VERA M. C. ALVES2   1Universidade Federal de Minas Gerais, Departamento de Botânica, CP 486, CEP 31270-901, Belo Horizonte, MG, Brazil, 2Embrapa Milho e Sorgo, CP 151, CEP 35701-970,Sete Lagoas, MG, Brazil. vera@cnpms.embrapa.br    Financial support: Mc Knight Foundation    INTRODUCTION  The exudation of organic acids in the rhizosphere is one of the mechanisms for Altolerance best supported by scientific evidence in several plant species. Severalexperiments conducted at Embrapa Maize and Sorghum have showed the presence ofthese compounds in the nutrient solution in which the roots have been immersed, but thisstrategy did not allow us to locate the exudation zone to the root apex or elsewhere alongthe root. However, the root apex is considered to be the main site for the toxic effect ofaluminum. Thus, knowledge of the spatial aspects of organic acid exudation along theroots is necessary to verify the effectiveness of this mechanism for Al tolerance in maize.The objective of this study was to identify and quantify organic acid exudation in responseto Al along the roots of three maize lines contrasting for tolerance to Al toxicity.   MATERIAL AND METHODS  Three maize lines with different levels of aluminum tolerance were used: Cateto –tolerant, L3 – intermediate tolerance and L53 – susceptible, in two experiments in whichthe effects of time of exposure to Al and Al concentration on the exudation of organicacids from the root apex were evaluated. Seedlings were subject to 40 µM Al+3 activityfor 0, 12, 24 and 48 hours, or to 48 hour-long treatments at 0, 20, 40 and 80 µM of Al+3activity. The seedlings of each treatment were transferred to trays containing germinationpaper moistened with a 500 µM CaCl2 basal solution and Al activities corresponding tothe 4 treatments. Four-milimiter Whatman chromatography paper discs were placed incontact with the root apex of the seedlings for collection of organic acids. The remainderof the roots was covered with germination paper. Four replications of 12 seedlings wereused for each treatment.  After two hours of contact with the roots, the chromatography paper discs weretransferred to plastic tubes containing 1 ml of mili Q water, agitated with a vortex andfiltrated. Organic acids were analyzed using a Shimadzu LC-10 model HPLC with aGROM-SIL 250 x 4 mm column, C-18, a mobile phase of 18mM KH2PO4 and a flow of0,4 ml per minute. The organic acids were identified and quantified at a wavelength  =215nm and compared with an external standard containing malic, citric, succinic, cis andtrans aconictic acid. Selected samples were analyzed for verification of possiblecontamination caused by microorganisms.  A second experiment was conducted to evaluate the sites of organic acid release along theroot - the root tip, 15mm, and 50 mm from the root tip - after 48 hours of exposure to40M Al activity. The methodologies for exudate collection and determination were thesame as described for the preceding experiment.  A third experiment was conduced to evaluate the aluminum concentration in root tipsafter 6, 12, 24 and 48 h of 40M Al activity. Aluminum concentration was quantified by acolorimetric assay.   RESULTS AND DISCUSSION  Citrate was the main organic acid released from maize roots. Citric acid exudationincreased with the exposure time and with increasing Al activities in the nutrient solutionfor the tolerant lines Cateto and L3 (Figures 1 and 2). The sensitive line L53 releasedsmaller amounts of citrate in comparison to the tolerant lines, and did not show anincrease in the exudation rate with time of exposure to Al. The sensitive line showed asimilar behavior to the tolerant lines, but with a significantly smaller increase in the rate ofexudation of citrate in response to increasing Al activities. The citric acid release washigher for the Al tolerant lines Cateto and L3 at the root tip than at 15 or 50 mm from theroot tip after 48 hour in presence of 40 M of Al activity (Figure 4). The enhancement ofcitric acid exudation in response to increasing Al concentrations and the greater exudationat the root apex were thus consistent with the role of organic acid exudation as analuminum tolerance mechanism in maize. On the other hand, the higher exudationobserved in L3 in relation to Cateto did not provide comparatively greater tolerance toAl. This can be verified by the greater reduction of seminal root growth of L3 after twodays of exposure to 40 M Al activity. (Figure 3). Even so, the higher exudation rate ofL3 relative to L53 and Cateto did provide greater Al exclusion, as L3 showed smaller Alaccumulation after 48 hours of exposure to Al (Figure 5). Our results suggest that otherfactors should be contributing to the high level of Al tolerance observed in Cateto besidesan exclusion mechanism based on citrate exudation.              

Physiological assessment of the actual role of aluminum exclusion conferred by citrate release as an aluminum tolerance mechanism in maize.

http://www.alice.cnptia.embrapa.br/alice/bitstream/doc/488093/1/Physiologicalassessment.pdf

Physiological assessment of the actual role of aluminum exclusion conferred by citrate release as an aluminum tolerance mechanism in maize.

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