EDITORIAL

Negotiating on identity—glimpses on biological borders

Peter Nick

# Springer-Verlag 2012

All organisms need a border delineating an internal spacethat is buffered against the fluctuating challenges of theouter world. These borders, at the same time, have to rec-oncile stability with dynamics and therefore are anythingelse but simple surfaces. Evolutionary processes very oftenare targeted to changes of biological borders, for instance,when organisms intrude (or are devoured) and subsequentlymaintain a partial border integrity. It is mostly the extent towhich this border integrity is maintained or modulated thatsets the frame for interactions that can range from symbio-genesis to biotrophic pathogenicity. The selective perme-ability of borders will ensure the maintenance of aninternal homeostasis, and specific adherence to cell bordersoften defines the specificity of organismal interactions. Inother words, biological borders are the sites where theidentity of organisms is negotiated by interaction with theexternal world including other organisms. Several reviewsand original papers in the current issue highlight differentaspects of this fascinating biological phenomenon for exam-ples from different domains of life.

Microtubule-based flagellar beat represents, along withamoeboid movements, a central mechanism for eukaryoticmotility and is located at the external border of many proto-zoan, animal, and generative plant cells. The evolutionaryorigin of eukaryotic flagellae seems to be linked with origi-nally ectoparasitic spirochaete-like bacteria that subsequentlywere domesticated in consequence of an endosymbiotic event.This might be one explanation for the fact that flagellae, inmany aspects, behave as closed regulatory systems posing

interesting questions with respect to the maintenance of theirhomeostasis. The work by Williamson et al. (2012) in thecurrent issue investigates how the transport of flagellar com-ponents to the growing tip of the flagellum is balanced withthe exit of turnover products. This transport involves direc-tional microtubule motors (kinesin-II for transport to the tipand dyneins for the retrograde transport) that form two com-plexes, termed A and B. Using conditional temperature-sensitive mutants of the model Chlamydomonas reinhardtii,they can show that the B complex confers dynein import,whereas the A complex is responsible for dynein exit fromthe flagellum. The initiation of flagellar differentiation islinked with a widely distributed, functionally diverse groupof proteins, called centrins, reviewed by Zhang and He (2012)in the current issue. These calcium-binding proteins are organ-ised in distinct domains that differ in structure and confer quitediverse functions to the different members of the centrinfamily. In addition to a participation of microtubule nucle-ation, they have also been shown a long time ago to participatein flagellar function (Melkonian 1979). These centrin func-tions are surprisingly divergent and can be not only structural,as shown for one of the Chlamydomonas centrins associatedwith the inner dynein arms, but also regulatory, as centrin 1from Paramecium caudatum that controls the calcium chan-nels that are responsible in ciliary reversal.

Although the border usually is located at the outer surfaceof an organism, this does not necessarily need to be the case.The endodermis of vascular plants represents an interestingexception of the rule that the border is outside. Whereas theouter layers of the root allow free diffusion of the externalmedium by mobility through the apoplast, the endodermissorts and selects nutrients and ions and thus represents thetrue border, where a fluctuating exterior is separated from abuffered interior. The core element of this border function isa cell-wall modification, where a water-impermeable

Handling Editor: Peter Nick

P. Nick (*)University of Karlsruhe,Karlsruhe, Germanye-mail: peter.nick@bio.uka.de

ProtoplasmaDOI 10.1007/s00709-012-0415-5

suberin layer blocks apoplastic flow forcing the mediumthrough the filter of the plasma membrane. The anatomicalfeatures of this Casparian strip had been studied in greatdetail since its discovery in 1865, and the commitment foran endodermal cell fate by diffusible transcription factorscould be elucidated a decade ago. However, it has remainedobscure until recently, how the equatorial domains of theendodermal cell are committed for transport of the suberinring. The review by Alassimone et al. (2012) in the currentissue surveys the discovery of the novel CASP proteins thatrepresent key components of this polarisation process andseem to guide the localisation of specific transporters to thecentral or distal faces of the endodermal plasma membraneshowing interesting mechanistic analogies of this “innerskin of plants” with polarised epithelia characteristic foranimals. However, partitioning of apoplastic and symplastictransport by Casparian strip-like structures seems to existalso outside the root, although the endodermis represents thebest known example. In gymnosperm leaves, similar struc-tures seem to control symplastic transport and phloem load-ing (Liesche et al. 2011).

The complexity of biological borders is illustrated by athird model, reviewed by Bak et al. (2012) in the currentissue. They consider cases of viral transmission, where thevirus does not undergo a specific propagation cycle in thevector organism, but is just attached to the exterior. Forinstance, the cauliflower mosaic virus binds to specificstructures of the exterior mouthparts of the transmittingaphid and thus can cross the plant boundary during thefeeding process. This seemingly trivial mechanism revealsupon closer scrutiny a surprising degree of complexity,where the viral particle forms a transmission complex with

specific helper proteins that mediate the binding to a specificprotein receptor in the specific acrostyle domains of theaphid stylet. Upon entering the host cell, viral particles enterthe nucleus and are decapsidated and propagated. Theysubsequently reprogramme the host cell to emit specificvolatiles and initiate chlorosis, both signals that will attractfurther aphids to close the life cycle.

Conflict of interest The author declares that there is no conflict ofinterest.

References

Alassimone J, Roppolo D, Geldner N, Vermeer JEM (2012) Theendodermis—development and differentiation of the plants innerskin. Protoplasma. doi:10.1007/s00709-011-0302-5

Bak A, Irons SL, Martinière A, Blanc S, Drucker M (2012) Host cellprocesses to accomplish mechanical and non-circulative virustransmission. Protoplasma. doi:10.1007/s00709-011-0328-8

Liesche J, Martens HJ, Schulz A (2011) Symplasmic transport andphloem loading in gymnosperm leaves. Protoplasma 248:181–190

Melkonian M (1979) Ultrastructural-study of the flagellate Tetraselmiscordiformis Stein (Chlorophyceae) with emphasis on the flagellarapparatus. Protoplasma 98:139–151

Williamson SM, Silva DA, Richey E, Qin H (2012) Probing the role ofIFT particle complex A and B in flagellar entry and exit of IFT-dynein in Chlamydomonas. Protoplasma. doi:10.1007/s00709-011-0311-4

Zhang Y, He C (2012) Centrins in unicellular organisms: functionaldiversity and specialization. Protoplasma. doi:10.1007/s00709-011-0305-2

P. Nick