Transgenic Research 9: 383–393, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 383 Review Plant tubulins: a melting pot for basic questions and promising applications Diego Breviario 1 & Peter Nick 2 1 Istituto Biosintesi Vegetali CNR, Via Bassini 15, 20133 Milano, Italy (E-mail: diego@icm.mi.cnr.it) 2 Institut fur Biologie II, Schanzlestr. 1, 79104 Freiburg, Germany (E-mail: pnick@sun2.ruf.uni-freiburg.de) Received 23 March 2000; revised 23 May 2000; accepted 23 May 2000 Key words: microtubules, tubulin, plant biotechnology Why it is worth considering microtubules Two recent results emerging from very different ex- periences exemplify the great importance of the plant cytoskeleton and microtubules (MTs). Attempts to isolate morphogenetic genes by screening for em- bryogenesis mutants in Arabidopsis thaliana did not uncover the expected homeotic genes but rather genes that are related to the formation of the cell plate or the symmetry of cell division (Mayer et al., 1993; Shevell et al., 1994; Lukowitz et al., 1996). Decades of continuos application of dinitroaniline herbicides used against weeds in cotton, soybean, wheat and oilseed crops has resulted in the selection of resistance against these compounds. In one of these weeds, goosegrass, the molecular cause of this resistance has been un- covered recently and shown to be linked to a mutation in the coding sequence of α-tubulin (Yamamoto et al., 1998; Anthony & Hussey, 1999a). So, these two apparently distant findings stress the same point – MTs are fundamental structures deeply involved in plant growth and development. Despite this, so far plant MTs have been substantially neglected as tools for applications in modern plant biotechnology. This nonchalance is both undeserved and astonishing since MTs represent versatile tools for the manipulation of plant morphogenesis and development: 1. They control numerous aspects of plant morpho- genesis and adaptation to the environment. 2. Their major components, the tubulins, comprise a family of different members with specific patterns of regulation and different molecular properties. In other words, plant tubulins are ideal targets for approaches that are both subtle and specific. 3. The microtubular cytoskeleton of plants is func- tionally distinct from that of animals, and this allows for the design of chemical agents, such as herbicides or fungicides, that are toxic for plants but safe for animals. The functions of plant MTs are numerous, and not confined to the establishment and movement of the division spindle. Plants have evolved specific microtu- bule arrays that serve to control cell shape in response to external signals such as light, gravity or mechan- ical strains, or to internal signals such as hormones or developmental state (for review see Nick, 1998). This microtubular response represents a key step in the flexibility of morphogenesis that is characteristic for plants. In interphase cells, the cortical microtubules control the direction that new cellulose microfibrils are deposited (Figure 1(a)) and, thus, the mechanic prop- erties of the expanding cell wall (Giddings & Staeh- elin, 1991). It is possible to change cell shape merely by manipulation of cortical microtubules. In dividing plant cells it is a band of microtubules, the preprophase band, that marks the axis and symmetry of cell divi- sion (Figure 1(b)), and a different microtubule array, the phragmoplast, organizes the formation of the new cell plate following nuclear division (Figure 1(c)). In addition to the functions essential for cellular morphogenesis, MTs participate in the response to abiotic and biotic stresses. They can disassemble in response to low temperature (Figure 1(d)) modulat- ing the sensitivity of cold-sensitive calcium channels (Mazars et al., 1997). They are essential for an ef- fective defence against fungal pathogens (Kobayashi et al., 1997), probably by guiding secretion towards the penetration site (Figure 1(e)). On the other hand,