ObjectiveChalcone synthase (CHS) is a key enzyme in the biosynthesis pathway of flavonoids. Overexpression of CHS promotes an accumulation of flavonoids and enhances the ability of plants to resist abiotic stresses such as salinity and drought. The objective of the study is to elucidate the function of BpCHS3 to salt tolerance.

MethodOur previously obtained transgenic birch plants of BpCHS3 were used in this study. The content of anthocyanins in leaves, salt stress index, chlorophyll fluorescence and photosynthetic parameters under NaCl stress were determined. qRT-PCR and Northern Blot were used to determine the relative expression levels of five key enzyme genes of downstream of CHS and BpCHS family members in the flavonoid metabolic pathway.

ResultThe wild type (WT) had a salt damage index of 83% in the 25th day under 0.3% NaCl stress, whereas transgenic birch was only 39%. The chlorophyll fluorescence and photosynthetic parameters of transgenic Betula platyphylla seedlings under 0.4% NaCl stress showed that the maximum light energy conversion efficiency (F_v/F_m) for most transgenic lines was still normal in the 9th day, while F_v/F_m was reduced to 0.66 and the actual photochemical efficiency (Φ_PSII) and photochemical quenching coefficient (qP) showed a decreasing trend for WT. The WT strain had a higher decline than the transgenic lines. The mean value of net photosynthetic rate (P_n) for five transgenic lines was still higher than 43.47% of WT under salt stress in 6 days. It is believed that the overexpression of BpCHS3 gene in Betula platyphylla under salt stress can maintain its higher photoelectron transport activity, increase the primary light energy conversion efficiency of PSII reaction center, and maintain a higher net photosynthetic efficiency. qRT-PCR and Northern Blot showed that the introduced target gene BpCHS3 was expressed in BpCHS3 transgenic lines. All of them showed up-regulated expression in different levels. Only BpCHS3 expression was significantly up-regulated in BpCHS family members. Both BpCHS1 and BpCHS2 were down-regulated or not significantly different from WT lines. Anthocyanin content of BpCHS3 overexpression lines was significantly lower than that of WT lines. It is speculated that overexpression of BpCHS3 results in co-suppression of two other CHS family members affects the synthesis of anthocyanins.

ConclusionThe salt tolerance of BpCHS3 transgenic birch was increased, which was not related to the content of anthocyanin. The overexpression of BpCHS3 may promote the accumulation of other flavonoids, thus enhancing the salt tolerance of Betula platyphylla.