Wood is formed through the activity of a specialized lateral meristem, the so called cambium. Although the cambium evolved after the occurrence of apical growth, regulators of stem cell activity and maintenance are partially conserved across the different types of meristems. Since different types of meristems form different organs and tissues, divergence of meristematic regulators can be expected at the level of cell differentiation. In order to unravel mechanisms specific to the differentiation of woody cells we are studying cambial functions of homologs of transcription factors with known roles during organogenesis in the shoot apical meristem. To this end we make use of the simple wood anatomy of the Arabidopsis hypocotyl and analyze differentiation of cambial derivatives in loss-of-function mutants. We are currently focusing on a pair of transcription factors, which is expressed across all the meristems. Interestingly, these transcription factors repress differentiation in the shoot apical meristem but promote the formation of secondary xylem in the hypocotyl.
In both the shoot apical meristem and the cambium, the plant hormone auxin is thought to be an important regulator of cell differentiation. A radial auxin gradient spanning wood-forming tissues has been suggested to be instructive for cell division, expansion and terminal differentiation of cambial derivatives. We recently discovered a first component of the auxin transport machinery, WAT1, which is directly involved in the differentiation of secondary cell walls. Defective wall differentiation in wat1 mutants can be completely rescued by local applications of auxin. The WAT1 protein, which we localized to the tonoplast, can facilitate the export of auxin from the vacuole to the cytoplasm. We are now studying how WAT1 interacts with other auxin transporters and to which extent it is involved in the generation of auxin gradients.
While for metazoan model systems a plethora of markers for consecutive steps of differentiation and cell types are available plant researchers have interpreted differentiation processes mainly on the basis of the position of a cell in a tissue/organ context, histologic features and activity of reporter genes. We think that chemical properties of the cell wall are typical for distinct stages of differentiation and for specific cell types. Therefore, we are currently building up a library of monoclonal antibodies against cell wall epitopes in order to obtain stable markers for cell differentiation in the Arabidopsis hypocotyl. Along with this initiative we implement state-of-the-art imaging analysis in order to obtain quantitative data of morphometric parameters and fluorescence signal.
Together with an industrial partner we are trying to transfer our knowledge about the differentiation of secondary cell walls into economically important poplar trees with the goal of tailoring wood properties to industrial needs.
- Jin X, Zimmermann J, Polle A and Fischer U 2015. Auxin is a long-range signal that acts independently of ethylene signaling on leaf abscission in Populus. Front. Plant Sci. 6:634. doi: 10.3389/fpls.2015.00634
- Liebsch D, Sunaryo W, Holmlund M, Norberg M, Zhang J, Hall HC, Helizon H, Jin X, Helariutta Y, Nilsson O, Polle A, Fischer U 2014. Class I KNOX transcription factors promote differentiation of cambial derivatives into xylem fibers in the Arabidopsis. Development 141, 4311-4319. doi: 10.1242/dev.111369. http://dx.doi.org/10.1242/dev.111369
- Bhalerao R, Fischer U 2014. Auxin gradients across wood – instructive or incidental? Physiologia Plantarum 151, 43-51.http://dx.doi.org/10.1111/ppl.12134
Ranocha P, Oana D, Reka N, Felten J, Corratge-Faille C, Novak O, Morreel K, Lacome B, Martinez Y, Pfrunder S, Jin X, Renou JP, Thibaud JB, Ljung K, Fischer U, Martinoia E, Boerjan W, Goffner D 2013. Arabidopsis WAT1 is a vacuolar auxin transport facilitator required for auxin homeostasis. Nature Communications, 4, 2525. http://dx.doi.org/10.1038/ncomms3625
- Hall HC, Fakhrzadeh A, Luengo Hendriks CL and Fischer U 2016. Precision automation of cell type classification and sub-cellular fluorescence quantification from laser scanning confocal images. Front. Plant Sci. 7:119. http://dx.doi.org/10.3389/fpls.2016.00119