TOLERÂNCIA AO ALUMÍNIO EM MILHO TROPICAL: HERANÇA GENÉTICA, MAPEAMENTO DE QTLs E EXPRESSÃO GÊNICA

Abstract

The objectives of this work were to determine the genetic inheritance, map QTL Al tolerance and quantify the differential gene expression of Al tolerance genes in tropical origin maize. To determine the genetic control of Al tolerance were evaluated diallel crosses between contrasting genotypes in the hybrid maize germplasm and maize landrace germplasm. Affiliates generations of diallel crosses and parental of respective crosses were evaluated for Al tolerance through minimal solution methodology. The DIF data (root growth difference) were analyzed by diallel Griffing model (1956) Method 2 (parental, hybrid and reciprocal). Later was determined the tolerance inheritance from average analysis of segregating generations. The landrace genotypes showed overall higher DIF average to hybrid germplasm, confirming the greater tolerance in this group. The diallel crosses involving the V 06 landrace stood out by the high specific and general combining ability for Al tolerance. The results of the generation average analysis indicated, for all families studied, quantitative inheritance of Al tolerance with predominance of genetic variance explained by additive effects. The heritability in the narrow sense was of intermediate magnitude, indicating the possibility of genetic gains to artificial selection of Al tolerant genotypes in F2 generation. The QTL mapping was performed based on phenotypic analysis of 114 F2:3 progenies, evaluated at minimum solution in presence of 4 mg L-1 Al. Microsatellite and AFLPs polymorphic loci between the parental lines of the mapping population were used for construction of the linkage map with the help of MAPMAKER program version 3.0 and QTL detection performed by the composite interval mapping. Nine tolerance QTLs were mapped in eight linkage groups (chromosomes 2, 4, 5, 6, 7, 8, 9 and 10), which explained 70.3% of phenotypic variation in Al tolerance. The results confirmed the three major QTL (bins 6.00, 8.05 and 10.01) described in the literature for Al tolerance, which were responsible for the most of phenotypic variation (40.3%). The high number of mapped QTLs in F2:3 segregating population confirms the quantitative inheritance of Al tolerance in the tropical maize germplasm. The molecular screening of ZmMATE1 and ZmMATE2 genes in hybrid and landraces maize germplasm showed no association of the amplified fragments with Al tolerance index. For studies of gene expression, the tolerant (H 44 and V 18) and sensitive genotypes (V H 22 and 25) were subjected to minimal solution for different exposure periods (0, 1, 3, 6, 9, 12, 24 and 48 h). After total RNA extraction from each genotype and the synthesis of cDNAs from each sample, were performed qRT-PCR assays to quantitate the expression of ZmMATE1, ZmMATE2 and ZmNrat1 genes. The results showed the highest differential expression of tolerant landrace V 18 for ZmMATE1 gene, indicating that, possibly, the exudation of citrate should be the main mechanism of Al tolerance in this genotype. The results also demonstrated the possibility of another type Al tolerance mechanism for the tolerant hybrid H 44, since showed differential expression only for ZmNrat1 gene in initial phase of exposure (1 to 3 h). The same genotypes used in the gene expression quantification were evaluated for roots differential staining with hematoxylin. The genotypes were submitted to minimal solution with Al for 6, 12, 24 and 48 h exposure times, evaluating the differential staining of roots in the respective periods. It was not possible to observe differential staining with hematoxylin among maize tolerant and sensitive genotypes to Al.