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Both deficiency and toxicity of the essential micronutrient boron (B) lead to reduced crop yield in agriculture. However, our understanding of the molecular responses of plants under B stresses to tackle the yield loss is limited. Therefore, in the present study, transcriptional alterations in sensitive and tolerant barley cultivars under B deficiency and toxicity were investigated in order to reveal the molecular responses. Transcriptomes were monitored at seedling stage by global expression profiling using oligonucleotide microarrays. In the context of the study, we have determined that response to B toxicity in barley involved jasmonic acid and various components of biotic stress responses. Examination of expression profiles indicated that B toxicity and deficiency resulted in significant global changes in the transcriptomes of leaf and root tissues, respectively. Inter-varietal comparison of sensitive and tolerant genotypes of barley revealed that a combinatorial effect of transcription factors on regulation could alter the gene expression patterns in tolerant cultivar and provide B toxicity tolerance. Furthermore, mechanisms of vacuolar sorting or efflux by transporters and aquaporins might be contributing to the tolerance to B stresses in barley according to the results of this study. Additionally, we have identified and cloned the HvBor1a gene encoding a putative B transporter in barley using candidate gene approach and functionally characterized its roles in the tolerance to B stresses. The full length coding sequence and also the non-coding regions of the gene were identified. It was demonstrated that the protein product of HvBor1a was localized to the plasma membrane and it displayed B transporter activity. High transcript abundances in leaf tissues of barley suggested a role for HvBor1a in re-distribution of B within the plant tissues. Interestingly, examination of last intron of HvBor1a has led to the identification of an alternatively spliced variant in certain cultivars of barley. Furthermore, interval mapping and positional cloning was performed to locate the HvBor1a on 3H B tolerance QTL and a novel CAPS marker was developed to narrow the genetic distances at the locus. As a conclusion, this work presents, for the first time, the transcriptome profiling of a member of Triticeae under B toxicity and deficiency. The data generated should enlighten succeeding studies to unravel molecular mechanisms and signaling networks of tolerance to B stresses especially in crops like barley and wheat. The results of the study will provide novel tools and genes for conventional and biotechnological approaches for the reduction of yield loss due to B toxicity or deficiency.

The incidences of suspected Boron (B) deficiency have increased recently in Kentucky tobacco fields, potentially due to recent changes in management practices. The symptoms observed in the field include; hollow stalk, stunted growth, deformed or no bud formation, small slits on the lower leaf midrib and uncontrollable breaking of the midrib approximately two inches from the stalk. B is a micronutrient tobacco needs in minute amounts, however excessive additions of B could cause toxicity. The objectives of this work were to 1) establish critical points for B sufficiency, 2) describe and define B deficiency and toxicity symptoms and 3) develop field strategies to aid in the mitigation of B deficiency. A general nutrient response curve was generated utilizing solution culture experiments. The peak of the response curve appeared to occur at solution B concentration of 100 µM. Trace-levels of B contamination and small plant size in the solution culture limited the development of deficiency symptoms. Toxicity was observed at solution concentrations of 400 µM and above. Despite choosing sites with a history of B deficiency, deficiency symptoms were not observed during this study. Toxicity was observed when 0.56 kg B/ha or greater was applied as simulated transplant water treatments. No toxicity was observed when B was applied as a soil broadcast or foliar application. Recommendations are to apply B with caution as a broadcast application to avoid potential problems with toxicity. Additional research is required to refine the nutrient response curve and better understand B deficiency.