Global warming due to the rise of CO2 levels has profound impact on human society, economy and ecology. Therefore, sequestering of carbon is important to fight against warming. In nature, trees play essential roles in carbon sequestration by storing carbon in plant cell walls as wood. It is estimated that half of the dry mass of a tree is carbon. In theory, the thicker of a tree, the more carbon will be fixed. Thus, it is of great economic and ecological values to study how tree thickening (i.e. secondary growth) takes place. Due to the difficulties of genetic manipulation of trees, researchers use Arabidopsis root as a model, which shows similar wood structure to a tree trunk. The knowledge gained from Arabidopsis root study can be transferred to tree breeding.
It has been known almost for two decades that cytokinin, a key plant hormone that plays essential roles in many developmental processes, is a master regulator of secondary growth. A mutant lacking key cytokinin biosynthesis genes fails to initiate secondary growth. Conversely, cytokinin treatment, or over-expression of a cytokinin biosynthesis gene, leads to increased secondary growth. This occurs not only in Arabidopsis, but also in trees. However, the factors that operate downstream of cytokinin signaling during cambium development have remained unknown.
In the latest study, Lingling Ye, a post doc in Mähönen group, identified a group of LOB DOMAIN (LBD) transcription factors as direct targets of cytokinin signaling, and mediating secondary growth in Arabidopsis root. Interestingly, when overexpressing LBDs, cell divisions and cell growth were accelerated. Consistent with this, lbd knockout mutants showed reduced secondary growth. This work suggests that LBD regulate secondary growth by controlling cell growth and perhaps also cell divisions. Yet et al. also found that LBD negatively feedback into CK signaling, imposing a balance of secondary growth control.
In future, it would be interesting to see if LBDs play same roles in trees and in storage organs of plants.