CDNA-AFLP Analysis On Gene Expression In Response To Boron-Aluminum Interactions In Leaves Of Citrus Grandis

Aluminum (Al)-toxicity is the major factor limiting crops growth in many tropical and subtropical of acidic soils, causing a great harm for agricultural production. Increasing evidence shows that boron (B) supply can alleviate Al-toxicity in higher plants. However, little information is available on the molecular mechanisms for boron (B)-induced alleviation of plant Al-toxicity. In this study, Sour pummelo (Citrus grandis) seedlings were irrigated daily for 18 weeks with nutrient solution containing two B (2.5 or 20 μM H3BO3)×2 Al (0 or 1.2 mM AlC13·6H2O) levels. Thereafter, effects of B-Al interactions on growth and the content of B and Al in leaves were investigated. In addition, leaves for total RNA extraction were collected, then extracting total RNA, synthesizing cDNA and performing cDNA-AFLP (cDNA-Amplified fragment length polymorphic) analysis. The main results are as follows:1. Al-toxicity decreased plant growth. In non-Al-treated (-Al) seedlings, B supply did not significantly affect plant growth, while the dry weight (DW) of Al-treated (+Al) seedlings was significantly higher under 20 μM B than under 2.5 μM B. Al-toxicity increased leaf concentrations of Al and B. Al concentration in+Al leaves was higher under 2.5 μM B than under 20 μM B, while its concentration in -Al leaves was similar between the two B treatments. By contrast, leaf B concentration was higher under 20 μM B than under 2.5 μM B.2. Al-toxicity decreased leaf CO2 assimilation, stomatal conductance and transpiration rate, but increased intercellular CO2 concentration. Under Al-stress,20 μM B-treated leaves had higher leaf CO2 assimilation and lower intercellular CO2 concentration compared with 2.5 μM B-treated leaves, while B supply did not significantly affect leaf stomatal conductance and transpiration rate. All the four parameters in -Al leaves did not significantly differ between the two B treatments.3. We used cDNA-AFLP technique to detect 196 differentially expressed transcript-derived fragments (TDFs) from leaves responsive to B and Al interactions, among these TDFs,179 TDFs were sequenced successfully and 127 TDFs were identified. These genes were classified into following several categories:signal transduction, transport, carbohydrate and energy metabolism, nucleic acid metabolism, amino acid and protein metabolism, cell wall and cytoskeleton metabolism, lipid metabolism and stress response.4. The ameliorative mechanisms of B on leaf Al-toxicity might be related to the following several aspects:(a) trigging multiple signal transduction pathways such as Ca2+ signals, phosphorylation and dephosphorylation of proteins and others (i.e., impaired sucrose induction 1-like protein); (b) improving the expression levels of genes related to transport such as GTP-binding nuclear protein Ran-3-like and citrus sucrose transporter 1; (c) activating genes involved in energy production (i.e., ATP synthase CFO subunit 1 and NADH-plastoquinone oxidoreductase); and (d) improving the accumulation of amino acids (i.e., putative glutamine dumper 2 and asparagine synthetase [glutamine-hydrolyzing] 1-lik) and protein degradation (i.e., carboxyl-terminal peptidase, AMSH-like ubiquitin thioesterase 1-like and polyubiquitin 4-like). In addition, genes related to nucleic acid metabolism (i.e., pentatricopeptide repeat proteins, Copia-type pol polyprotein-like and PIF/Ping-Pong family of plant transposases) and stress response (i.e., ERD3 and RPMl-interacting protein 4-like) also played a role in B-induced alleviation of leaf Al-toxicity.