JIA-2018-09

2013 CHEN Ke-qin et al. Journal of Integrative Agriculture 2018, 17(9): 2007–2014 results showed that all of the five NAC transcription factors, AtSND1 , AtNST1 , AtNST2 , AtVND6 and AtVND7 , were also greatly induced (Fig. 4-B). 4. Discussion According to the previous reports, 28 of 57 AtSND1 transgenic Arabidopsis lines exhibited a prominent visual phenotype that showed a small rosette size and stunted growth of leaves with severely upward-curling blades; the flowers of SND1 overexpression lines were often sterile and had severely shortened sepals, petals, stamens, and carpels (Zhong et al . 2006). These phenotypes were similar to the CpSND1 overexpression lines in this study (Fig. 3-B and C). In poplar, overexpression of wood-associated NAC domain transcription factors PtrWND2B and PtrWND6B produced plants with severely curled leaves, which could also complement the secondary wall defects in the Arabidopsis snd1/nst1 double mutant. Overexpression of PtrWND2B and PtrWND6B in Arabidopsis induced the expression of secondary wall-associated transcription factors and secondary wall biosynthetic genes and caused the ectopic deposition of cellulose, xylan, and lignin. PtrWND2B and PtrWND6B were able to activate the promoter activities of a number of poplar wood-associated transcription factors and wood biosynthetic genes (Zhong et al . 2010a). The PtVNS07 / PtrWND6A and PtVNS11 / PtrWND1B genes contribute to the formation of xylem tissue and phloem fibres. Overexpression of the PtVNS / PtrWND genes, which are similar to AtVND7 and AtNST3/SND1 , induced ectopic secondary wall thickening in Arabidopsis seedlings as well as in poplar. All of the PtVNS/ PtrWND genes of the VND and NST groups exhibit the ability to positively regulate secondary wall thickening (Zhong et al . 2010a). Wood formation in poplar is regulated by cooperative functions of the NAC domain proteins (Ohtani et al . 2011). Suppression of PtrWND1B expression led to specific inhibition of fibre in secondary wall thickening in xylem and phloem tissues, and transgenic poplars were unable to grow straight. Up-regulation of PtrWND1B in poplar exhibited a phenotype of small leaves that were curled upward or downward (Zhao et al . 2014). SND1, together with other secondary wall NACs, including NST1, NST2, VND6 and VND7, binds to the SNBE sites in the promoters of their direct targets, not only in downstream MYB transcription factors but also in a number of non-transcription factor genes involved in secondary wall biosynthesis that all contain multiple SNBE sites (Zhong et al . 2010b). In addition, the NAC-MYB regulation model was conserved in different species. In Arabidopsis , the SND1 protein can bind directly to its own promoter and is characterized by the critical nucleotides required for SND1 binding, which is TACXTTXXXXATGA (Zhong et al . 2010b). Promoters of AtVND6/7, AtNST1, and AtSND1 also contained the SMRE (secondary wall MYB-responsive element (ACC(A/T)A(A/C)(T/C)) binding site, which is recognized directly by MYB46 (Zhong and Ye 2014). In addition, we also found that the promoters of the AtNST1 , AtNST2 , AtSND1 , AtVND6 , AtVND7 , MdSND1 and MdNST1 genes contained several MYB-binding sites, such as M46RE ((A/G)(C/T)T(A/T)GGT(A/G)) and SMRE (ACC(A/T)A(A/C)(T/C)), that are related to secondary cell wall biosynthesis (unpublished data). This may be the reason why the expression levels of AtNST1 , AtNST2 , AtSND1 , AtVND6 and AtVND7 were increased over two-fold in the CpSND1 -overexpressing Arabidopsis lines (Fig. 4-B). 5. Conclusion CpSND1 transcription factor had similar function withAtSND1 for secondary wall synthesis in Arabidopsis . CpSND1 as the transcription activator can upregulate the expression of secondary wall biosynthesis genes and induce the formation of secondary wall in transgenic Arabidopsis . The function of SND1 is high conservative among different species. c c c c a a a a b a b a b b b b 0 50 100 150 200 A B AtMYB46 AtMYB83 AtMYB63 AtMYB58 Relative expression WT L1 L4 L5 d c c d c a a a b a b b a a ab c b b c b 0 5 10 15 20 25 30 AtSND1 AtNST1 AtNST2 AtVND6 AtVND7 Relative expression Fig. 4   The expression of MYB and NAC involved in the secondary cell wall formation in WT and CpSND1 - overexpressing Arabidopsis lines. A, the expression of MYB family members. AtMYB46 , AT5G12870; AtMYB83 , AT3G08500; AtMYB63 , AT1G79180; AtMYB58 , AT1G16490. B, the expression of NAC family members. AtSND1 , AT1G32770; AtNST1 , AT2G46770; AtNST2 , AT3G61910; AtVND6 , AT5G62380; AtVND7 , AT1G71930. Different letters mean significant different at the 0.05 probability level. The error bars mean SD.