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 × Col recombinant inbred (RI) population, B efficiency was evaluated in the original parents (Ler and Col-4) and two F8 lines (1938 and 1961), both of which were selected on the basis of phenotype and genotype of the RI population. The parent Ler and F8 progeny 1938 had higher BEC and B utilization efficiency (BUE) values than those calculated for parent Col-4 and F8 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 F8 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 μM 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 μM 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.