Regulation of secondary growth initiation in Arabidopsis

The functional dynamics of plants is, to a large extent, based on the properties of their vascular system which plays a crucial role in long-distance transport and the provision of mechanical support for the growing plant body. In this respect, especially lateral growth of shoot axes by the production of additional vascular tissue is essential for the establishment of extended shoot systems. Lateral growth, also known as secondary growth, is based on the tissue forming properties of a lateral meristem called the cambium the activity of which leads to the production of secondary vascular tissue (wood and bast) at the periphery of the shoot. Considering its function as a stem cell niche which is essential for the constant production of new tissues and its post-embryonic establishment in a differentiated environment, the cambium represents an ideal object to address questions concerning the regulation of cell identity and the establishment of meristematic activity. Interestingly, in contrast to our understanding of apical meristems and despite its fundamental role in many aspects of plant growth and the production of biomass, knowledge about the molecular control of cambium activity is very limited. Therefore, in this project we propose to elucidate molecular mechanisms essential for the induction of secondary growth. Due to the exceptional availability of genetic and molecular tools, Arabidopsis thaliana is an efficient model for analysing secondary growth in plants. In this project, we will focus on the initiation of the interfascicular cambium in the main Arabidopsis shoot which, histologically, is a very defined process and easy to monitor due to characteristic cell divisions. We propose to unravel molecular mechanisms involved in the regulation of vascular cambium activity by histological analysis of auxin signalling and transport, tissue-specific transcriptional profiling and the performance of a forward mutagenesis screen. Furthermore, we will transfer our results to the situation in Populus, a tree where cambium activity is being extensively studied. In summary, we propose a comprehensive analysis of secondary growth initiation in Arabidopsis, and a subsequent transfer to a woody species. Following this strategy, the project has the potential to address basic questions in developmental biology by identifying key factors important for the establishment of cell identity, in particular, meristematic identity. It will also possibly reveal phylogenetic relationships of molecular control mechanisms of an important developmental process in plants. Beyond these aspects, it will contribute significantly to our understanding of wood production, one of the major processes essential for the generation of renewable energy sources and basic materials on our planet.

In contrast to animals, plants have the capacity to grow and form new organs and tissues throughout their entire life cycle, thereby building up some of the largest organisms on earth. This remarkable capacity is based on the activity of stem cell-like tissues - the meristems - located at shoot and root apices and, in a large repertoire of species, in lateral positions at the flanks of growth axes. In comparison to apical meristems, our knowledge of the molecular mechanisms controlling the activity of lateral meristems like the cambium is very limited. This is despite the fact that the cambium is responsible for wood formation, and thus for the accumulation of large amounts of terrestrial biomass, and for fixation of atmospheric CO 2 . During this project, an in vitro system was established by which cambium initiation can be stimulated under controlled conditions in stems of the reference plant Arabidopsis thaliana. By exploiting the benefits of this system, genome-wide and tissue-specific alterations in transcript accumulation during cambium initiation were revealed. Based on these results, two novel receptor-like kinases, namely MOL1 and RUL1, were identified as opposing cambium regulators. In addition, a forward mutagenesis screen performed in Arabidopsis thaliana led to the isolation of a series of novel mutants displaying ectopic cambium formation, which represent the first mutants of this class. Remarkably, a mutant, designated as cambium islands1 (cbi1), is characterized by the stochastic initiation of cambium identity along the stem, suggesting that the CBI1 gene represents a novel repressor of cambium initiation. The analysis of the molecular role of CBI1 will be highly informative of how cambium initiation is regulated, and provide another viewpoint for studying lateral plant growth. In summary, these findings demonstrated that valuable tools for studying cambium regulation were established and that opportunities for dissecting lateral growth in plants from novel perspectives were generated. In particular, the project provided starting points for analyzing the process of wood formation in trees, organisms that are difficult to address by standard genetic approaches.