Functional identification of an EXPA gene (NcEXPA8) isolated from the tree Neolamarckia cadamba

As a class of important cell growth regulators, expansins have been studied for over 20 years. Since Neolamarckia cadamba (Roxb.) Bosser was praised as a ‘miraculous tree’ at the World Forestry Congress in 1972 due to its rapid growth. A lot of research has been carried out to uncover the underlying mechanisms of this rapid growth. Based on previous findings and our research, we hypothesized that expansins may play an important role in such growth. Quantitative real-time polymerase chain reaction (qRT-PCR) analysis revealed that the N. cadamba expansin family member NcEXPA8 is highly expressed in all four young tissues, particularly in the cambium region, suggesting that NcEXPA8 acts as a key regulator of secondary growth in this gene family. Overexpression of NcEXPA8 in Arabidopsis thaliana increased the diameter and height of the main stem. It also promoted interfascicular fiber cell elongation and cell-wall thickness but did not alter the cellulose content in the cell wall. These results suggested that expansins act as activators during secondary fiber cell elongation during tip growth to promote plant growth.