Abstract
The mechanisms underlying tolerance to B-toxicity in plants are still controversial. Our previous studies indicated that B-toxicity is mainly limited to leaves in Citrus and that alternations of cell-wall structure in vascular bundles are involved in tolerance to B-toxicity. Here, miRNAs and their expression patterns were first identified in B-treated Citrus sinensis (tolerant) and C. grandis (intolerant) leaves via high-throughput sequencing. Candidate miRNAs were then verified with molecular and anatomical approaches. The results showed that 51 miRNAs in C. grandis and 20 miRNAs in C. sinensis were differentially expressed after B-toxic treatment. MiR395a and miR397a were the most significantly up-regulated miRNAs in B-toxic C. grandis leaves, but both were down-regulated in B-toxic C. sinensis leaves. Four auxin response factor genes and two laccase (LAC) genes were confirmed through 5′-RACE to be real targets of miR160a and miR397a, respectively. Up-regulation of LAC4 resulted in secondary deposition of cell-wall polysaccharides in vessel elements of C. sinensis, whereas down-regulation of both LAC17 and LAC4, led to poorly developed vessel elements in C. grandis. Our findings demonstrated that miR397a plays a pivotal role in woody Citrus tolerance to B-toxicity by targeting LAC17 and LAC4, both of which are responsible for secondary cell-wall synthesis.
Highlights
Boron (B), an essential micronutrient, plays a crucial role in plant growth and development as well as sexual reproduction[1,2]
In C. grandis, B-toxicity leads to progressive basal-to-top development of toxic symptoms in the leaves, beginning with tip yellowing, followed by marginal and interveinal chlorosis, and senescence and abscission
No visible symptoms were observed in C. sinensis leaves under the same conditions, suggesting that C. sinensis is more tolerant to B-toxicity than C. grandis
Summary
Boron (B), an essential micronutrient, plays a crucial role in plant growth and development as well as sexual reproduction[1,2]. It is involved in the formation of the primary cell wall[3], membrane integrity[4,5] and cell-wall structure[6] and participates in diverse physiological and biochemical processes, including cell-to-cell signalling[7], secondary metabolism[8], and gene expression[9], in higher plants. The molecular basis of B-toxicity in Arabidopsis thaliana was not reported until recently, by Aquea et al.[16] These authors found that B-toxicity induced the expression of genes related to ABA signalling but www.nature.com/scientificreports/. Our previous studies suggested that other mechanisms of tolerance to B-toxicity might exist in woody Citrus plants
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