Why to Spend Tax Money on Plant Microtubules?

Plant microtubules have evolved into a versatile tool to link environmental signals into flexible morphogenesis. Cortical microtubules define the axiality of cell expansion by control of cellulose orientation. Plant-specific microtubule structures such as preprophase band and phragmoplast determine symmetry and axiality of cell divisions. In addition, microtubules act as sensors and integrators for stimuli such as mechanic load and gravity but also osmotic stress, cold, and pathogen attack. Many of these functions are specific for plants and involve unique proteins or the recruitment of proteins to new functions. The review aims to ventilate the potential of microtubule-based strategies for biotechnological application by highlighting representative case studies. These include reorientation of cortical microtubules to increase lodging resistance, control of microtubule dynamics to alter the gravity-dependent orientation of leaves, the use of microtubules as sensitive thermometers to improve adaptive cold tolerance of chilling and freezing sensitive plants, the reduction of microtubule treadmilling to inhibit cell-to-cell transport of plant viruses, or the modulation of plant defence genes by pharmacological manipulation of microtubules. The specificity of these responses is controlled by a great variety of specific associated proteins opening a wide field for biotechnological manipulation of plant architecture and stress tolerance. 1 Motivation: Plant Architecture Defines Yield Plant architecture represents a target with high potential for plant biotechnology. When leaf angles can be manipulated, this will allow the sunlight to penetrate deeper into the canopy (Zheng et al. 2008). When internodes can be shortened, this will increase lodging resistance. When the formation of new tillers is suppressed, P. Nick (*) Molecular Cell Biology, Botanical Institute, Karlsruhe Institute of Technology, Kaiserstr. 2, Karlsruhe 76128, Germany e-mail: [email protected] P. Nick and Z. Opatrný (eds.), Applied Plant Cell Biology, Plant Cell Monographs 22, DOI 10.1007/978-3-642-41787-0_2, © Springer-Verlag Berlin Heidelberg 2014 39 this will promote the filling of grains on the main ear (Sakamoto and Matsuoka 2004). These important traits for breeding of high-yielding cultivars have been modelled numerically to assess the impact of genetic traits on plant morphology and expected yield on a quantitative base (Xu et al. 2011). Plants with ‘ideal architecture’ would show reduced shoot length, reduced tiller number, and increased grain weight as traits (Jiao et al. 2010). Microtubules, as central regulators of plant growth and development, provide an important target for biotechnological applications aiming to change plant architecture. However, the potential of microtubules for plant biotechnology, so far, is still to be exploited motivating the current chapter. Yield losses by lodging and windbreak are considerable: In rice, for example, they are estimated to range up to 40 % (Nishiyama 1986). Control of plant height has therefore been a major topic in cereal crops, because the resistance of a plant to lodging and windbreak is inversely related to the square of plant height (Oda et al. 1966). This means that a reduction of internode elongation by 50 % will reduce lodging to 25 %. Thus, the agronomic importance of reduced shoot length cannot be overestimated, shifting the control of plant height into the centre of interest, especially for cereal crops. In fact, a central factor in the green revolution of cereals has been a mutation in the so-called DELLA regulators of gibberellindependent shoot elongation (Peng et al. 1999), and recently it was uncovered that during the domestication of japonica rice a semidwarf mutation linked to gibberellin synthesis had been selected (Asano et al. 2011). However, the impact of plant architecture is not confined to lodging resistance. For instance, the resistance of crop plants to wind depends on the angle between main and branch roots (Stokes et al. 1995), and the marketable yield very often depends on the partitioning of biomass. As already pointed out in the 1920s as ‘Law of Homologous Series’ established by the Russian geneticist Vavilov (Vavilov 1922) for the domestication of crop plants, apical dominance represented a central factor. Instead of numerous, small axes bearing numerous, but smaller fruits, one main axis was established during domestication of many crops. This change of architecture is impressively illustrated by the transition from the ancestral teosinte to modern maize linked to mainly one locus controlling the formation of side branches (Doebley et al. 1995). Production of fewer but larger fruits, tubers, or grains facilitates processing, whereas in other cases, such as breeding of potatoes or tulips, the advantage might be on the side of more but smaller structures. The morphogenetic events involved in the formation of tubers, fruits, or side branches might thus be manipulated to recruit biomass optimally between product quantity, size, and quality, without the need to interfere with source–sink relations or photosynthetic efficiency in general. Microtubules control plant architecture at two levels: (1) They act downstream as effectors. Microtubules control the axis of cell division and cell expansion and therefore link the output of signalling triggered by environment and development to a response of plant architecture. (2) They act upstream as sensors. Microtubules can integrate the mechanic load resulting from growth and architecture and feed this 40 P. Nick