Abstract
Boron (B) toxicity is observed in some citrus orchards in China. However, limited data are available on the molecular mechanisms of citrus B-toxicity and B-tolerance. Using cDNA-AFLP, we identified 20 up- and 52 down-regulated genes, and 44 up- and 66 down-regulated genes from excess B-treated Citrus sinensis and Citrus grandis roots, respectively, thereby demonstrating that gene expression profiles were more affected in the latter. In addition, phosphorus and total soluble protein concentrations were lowered only in excess B-treated C. grandis roots. Apparently, C. sinensis had higher B-tolerance than C. grandis. Our results suggested that the following several aspects were responsible for the difference in the B-tolerance between the two citrus species including: (a) B-excess induced Root Hair Defective 3 expression in C. sinensis roots, and repressed villin4 expression in C. grandis roots; accordingly, root growth was less inhibited by B-excess in the former; (b) antioxidant systems were impaired in excess B-treated C. grandis roots, hence accelerating root senescence; (c) genes related to Ca2+ signals were inhibited (induced) by B-excess in C. grandis (C. sinensis) roots. B-excess-responsive genes related to energy (i.e., alternative oxidase and cytochrome P450), lipid (i.e., Glycerol-3-phosphate acyltransferase 9 and citrus dioxygenase), and nucleic acid (i.e., HDA19, histone 4, and ribonucleotide reductase RNR1 like protein) metabolisms also possibly accounted for the difference in the B-tolerance between the two citrus species. These data increased our understanding of the mechanisms on citrus B-toxicity and B-tolerance at transcriptional level.
Highlights
Boron (B), an essential element for normal growth and development of higher plants, is taken up by roots in the form of boric acid from soil solution
We further examined long-term B-excess-induced alterations of gene profiles revealed by cDNA-AFLP in B-tolerant C. sinensis and B-intolerant C. grandis roots (Guo et al, 2014; Huang et al, 2014; Sang et al, 2015)
Root B concentration was higher in excess B-treated C. grandis and C. sinensis roots than in controls
Summary
Boron (B), an essential element for normal growth and development of higher plants, is taken up by roots in the form of boric acid from soil solution. In addition to restricting B accumulation in plant tissues via enhancing the expression of efflux transporters similar to barely Bot, genes related to stress/defense, protein synthesis, cellular organization, and signal transduction had a positive role in the B-tolerance of P. distans. Aquea et al (2012) obtained 240 up-regulated genes, which mainly function in ABA signaling, ABA response, or cell wall modifications, and 211 down-regulated genes, which are mainly related to glucosinolate biosynthesis, water, or nutrient transport, suggesting that B-toxicity-induced water stress was responsible for root growth inhibition under B-stress. Tombuloglu et al (2015) used high-throughput RNA-Seq to examine B-toxicity-induced alterations of gene profiles in barely and observed a total of 14,385 and 11,472 differentially expressed transcripts from high-B-treated roots and leaves, respectively, and concluded that genes involved in cell wall and cytoskeleton, plasma membrane, stress response, and signal transduction played key roles in the B-tolerance of barely. The transcriptional responses of herbaceous plants to B-toxicity have been investigated in some details, such data are very limited in woody plants
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