In contrast to TPX members of the CPC Figure B

In butein to TPX2, members of the CPC (Figure 1B), such as survivin, borealin, and INCEN-P, which associate with Aurora B/C in animals, do not appear to have obvious homologs in plants. Recently, however, a putative INCEN-P homolog, termed WYRD, has been identified [34]. Sequence comparison between animal INCEN-P and plant WYRD shows that the overall conservation is extremely poor and that the plant homolog is twofold larger than its animal counterpart [34]. Nevertheless, WYRD contains a characteristic C-terminal coiled-coil domain and an Aurora-binding IN-Box domain that is conserved throughout the Kingdoms [35]. Homozygous wyrd mutants are gametophytically lethal, and heterozygous plants show distinct developmental phenotypes such as impaired mitotic divisions in the male gametophyte and endosperm. In addition, heterozygous mutants of WYRD show aberrant embryonic division planes [34], but whether this represents a dosage or a dominant-negative effect remains to be determined. So far WYRD has not been linked to any of the three plant Aurora kinases. Establishing whether WYRD is a substrate and functions as part of a putative plant CPC represents a future challenge.
Functions of Aurora Kinases in Plants
Concluding Remarks and Future Outlook The fact that aurora mutants in Arabidopsis have very distinct developmental phenotypes makes it very interesting to search for (plant-specific) substrates, and this represents one of the major challenges for the field. Differential phosphoproteomics approaches comparing wild-type and Aurora knockdown mutants could potentially solve this. The identification of novel plant Aurora substrates will also shed light on the question if the phosphorylation consensus motif is conserved for plant Aurora kinases. To date, mutants in plant Aurora kinases are either leaky, lethal, or not specific for one of the three Aurora kinases. A powerful tool to study kinases is the so-called ‘magic bullet or bump and hole approach’ which targets small molecules such as ATP analogs to the ATP-binding pockets of the kinase to selectively knock down kinase activity. The ATP-binding pockets of Ark1+ and Ipl1p have been modified to create an analog-sensitive kinase allele by the broad-spectrum kinase inhibitor 4-anilinoquinazoline or the bulky ATP analog 1-NA-PP1, respectively 55, 56. However, one of the mutations that has been introduced into Ipl1p (T244A) is already present in all three Arabidopsis Aurora kinases (Figure 1C). Whether this represents an advantage for the use of this type of chemical inhibitors for plant Aurora kinases has to be investigated, as no crystal structure of the ATP-binding pocket of plant Aurora kinases is available to date. Also, a potential way of testing the requirements of Aurora kinase activity throughout plant development would be to use a conditional mutant complementation approach based on the N-end rule temperature-dependent degradation pathway [57]. Division-plane determination defects in aurora mutants hint toward a role for Aurora kinases in asymmetry [19]. Aurora kinase-dependent phosphorylation of polarity determinants plays an important role in animal neuron development 58, 59, 60, 61. Animal cells achieve ACD by translating polarizing cues into an asymmetric distribution of polarity regulators, for instance the partitioning defective (PAR) complex, an important regulator during neuron development [62]. Homologs of the PAR complex are not present in plant genomes, suggesting that plant cells use alternative mechanisms to establish cellular asymmetry [63]. Formative divisions are crucial for many important developmental steps and postembryonic organ growth, such as root branching to allow optimal water and nutrient acquisition and soil anchoring. One of the strategies plants use to increase the complexity of their root system is the continuous post-embryonic initiation of new lateral roots. Consequently, further research on plant Aurora kinases involved in formative divisions and polarity is therefore an important field of research, with potential impact on the modulation of root architecture in general, and lateral root formation in particular, for plant development, stress resistance, and yield.