Physiological basis of QTLs for boron efficiency in Arabidopsis thaliana

Boron (B) is an essential micronutrient for higher plants, but the adaptability of plants to B deficiency varies widely both between and within species. On the basis of quantitative trait loci (QTL) analysis of the B efficiency coefficient (BEC) detected in an Arabidopsis thaliana Ler x Col recombinant inbred (RI) population, B efficiency was evaluated in the original parents (Ler and Col-4) and two F-8 lines (1938 and 1961), both of which were selected on the basis of phenotype and genotype of the RI population. The parent Ler and F-8 progeny 1938 had higher BEC and B utilization efficiency (BUE) values than those calculated for parent Col-4 and F-8 progeny 1961, respectively, when grown in nutrient solutions containing three different concentrations of B. The magnitude of the BEC and BUE-values was correlated closely with the combined phenotypic effect of the corresponding QTLs among the four genotypes. The F-8 line, 1938, inherited all four B-efficient QTLs, AtBE1-1, AtBE1-2, AtBE2 and AtBE5, from its two original parents. The four QTLs accounted for 65.2% of the total variation in BEC and 1938 showed the highest BEC (0.74) and BUE (10.5) values among the four genotypes when grown in nutrient solution that contained 0.324 mu M B. Only one minor-effect QTL (AtBE1-1) was found in the parent, Col-4. This QTL accounted only for 8.8% of total BEC variation and resulted in the lowest BEC (0.39) and BUE (0.76) in Col-4 when it was grown in nutrient solution that contained 0.324 mu M B. Phenotypic profile analysis showed that 1938 not only inherited the B utilization and distribution characteristics found in the silique of Ler, but also acquired the low-B requirement for root and shoot growth from Col-4. As a result, this genotype displayed the strongest tolerance to B deficiency. In addition, both B-efficient genotypes, 1938 and Ler, possessed the QTL (AtBE1-2) and both plants had high-seed yields and high-B distributions in their siliques. Therefore, we hypothesize that QTL AtBE1-2 plays a role in the utilization and/or the distribution of B to the silique when plants suffer from B deficiency. A close correlation between the B-efficient phenotype and the corresponding QTLs indicated that phenotypic differences depend on the genetic variation.