Meristematic cells give rise to various organs of the plant and keep the plant growing. Among meristems, the vascular cambium plays a fundamental role in growth, because it promotes the thickening of stems and roots conferring girth to the plants. The vascular cambium continuously produces phloem towards the outside of stems and roots and xylem (or wood) towards the inside of stems and roots. This radial growth confers the structural support for other organs, such as leaves, flowers and fruits and provides a network of transport of water and nutrients and photo-assimilates throughout the plant body. This growth in girth (also named secondary growth) is crucial for plant biology. In addition, in trees, the cambium-mediated growth brings about large amounts of wood. Therefore, understanding the regulation of cambium activity is not only crucial to understand plant biology but also to enhance our possibilities for biomass formation. Yet, little is known about the genetic control of the cambium activity. With the goal of understanding of the genetics of secondary growth in plants, in WOODSofCHANGE we used advanced genomic technology to discover new genes involved in the regulation of the process. In concrete, we used an approach based on 166 natural strains of the model plant Arabidopsis thaliana collected from different locations of the planet and, therefore genetically adapted to specific environmental conditions. Such genetic adaptation is not only reflected in the genomes of the strains (which are different to one another) but also in the anatomical and morphological characteristics. Based on the rationale that the genetic differences between the strains should associate with the morphological and anatomical differences that we can observe among them, we used publicly available software to associate the differences between the strains in terms of secondary growth with variations in their genomes, to identify genomic regions containing genes that control secondary growth. This is an approach that is called Genome Wide Association Study (GWAS) that has been proved as a great gene discovery tool.
We assessed secondary growth by quantifying the amount of xylem production at the hypocotyl, which is located between the main stem and the root and is the most analogous organ in Arabidopsis to the woody stem of trees (Figure). By associating differences in terms of xylem development with the differences within the genomes of the analysed strains, we found a region of the genome significantly associated with cambial activity. Using further experimentation, we tested all the genes that were contained in the identified genomic region and identified one as a new regulator of secondary growth. To learn more about the specific activity of the new regulator of secondary growth, we performed experimentation that allowed us to determine that such new gene negatively regulates the production of one specific cell-type within the xylem (namely, the fibres) by preventing its precocious formation, thereby ensuring the correct pace of xylem development. In this way, mutants for the gene initiate the formation of fibres earlier in the development than normal plants (the latter usually called wild-type plants). Furthermore, our results suggest that this gene acts through transcriptional repression of previously described xylem fiber differentiation identity genes. The results from WOODSofCHANGE have advanced our understanding about the complex regulation of plant development in general and the genetic control of secondary growth in particular.
