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Addresses*Department of Cell and Developmental Biology, John Innes Centre,Colney, Norwich NR4 7UH, UK; e-mail: keith.roberts@bbsrc.ac.uk†Laboratory of Plant Molecular Biology, Rockefeller University,1230 York Avenue, New York, NY10021, USA; e-mail: chua@mail.rockefeller.edu

Current Opinion in Plant Biology 2001, 4: 475–4771369-5266/01/$ — see front matter© 2001 Elsevier Science Ltd. All rights reserved.

AbbreviationMAP microtubule-associated protein

It’s Cell Biology’s turn again, but this year there is adifference. The subject area has received a shot in thearm, which perfuses all of the contributions in this issue, inthe form of the complete Arabidopsis genome sequenceand the increasing availability of gene sequence informa-tion from other plant species. This has had two effects.First, it has facilitated faster routes from genes to func-tions, helped by reverse genetics and the various -omicstechnologies. Second, it has changed the intellectuallandscape, from one in which it was easy to worry aboutvast hinterlands of unknown genes, which may or may notimpact on your own chosen gene of interest, to one thatis more comprehensible, with a known boundary, lessintimidating and with defined groups of genes. It’s a morecomforting framework within which to pursue any focussedproblem in cell biology, and the opportunities to exploitecotype variation and evolutionary relationships make itall the more conducive.

The articles we have commissioned in this issue all reflect,in one way or another, the pervasive impact of genomeavailability. Yet the topics still reflect the classical concernsof the cell biologist: the production of cells; their growth,shaping, differentiation and death; and the intracellularmachineries that underlie these activities. Our choice is ofcourse selective and personal, but we believe it reflects thecurrent energy and excitement that this shot in the armhas produced.

Post-genomic cell biologyWe open with a review by Assaad (pp. 478–487) that takesan overall look at the Arabidopsis genome, looking at thedifferences and similarities in gene families in humans andplants. Given the walled and static existence of plant cells,and the capacity of plants to deal with unpredictableenvironmental changes, it is not too surprising that there isconsiderable novelty in the groups of plant genes that areconcerned with even generalised cellular functions, such astranscription, cytoskeletal dynamics and vesicle trafficking.A walled existence is of course a key difference between

plant and animal cells, and there may well be as many as1000 genes involved directly and indirectly in assemblingthe complex extracellular matrix that allows plant cells togrow and that determines their shape. Dhugga (pp. 488–493)explores some of the families of genes involved and high-lights, in particular, the large family of glycosyltransferasesinvolved in the production of the major carbohydratepolymers of the wall. This review highlights the power ofArabidopsis mutants and the contribution they have made toour molecular understanding of wall biosynthesis. Thistheme is picked up in almost all the subsequent reviews. Inparticular, the article by Verbsky and Richards (pp. 494–500)looks at the impact that mutants, derived both from for-ward genetic screens and reverse genetics, have had on ourunderstanding of chromatin-based control in plants.Chromatin remodelling, either by core histone modificationor through ATP-dependent chromatin remodelling com-plexes, has a major influence on gene expression, genesilencing, and plant development.

The birth of cellsPlant growth and development begins in meristems withthe proliferation of cells, and how the cell cycle is regu-lated is a key area of interest in cell biology. The core cellcycle machinery is reasonably well conserved amongplants and other eukaryotic organisms (although theArabidopsis genome does throw up some oddities, forexample, no Cdc25 gene is found). Potuschak and Doerner(pp. 501–506) describe recent work that uses increasinglysophisticated biochemical assays and genetic approachesto tackle the function of the key cell cycle regulators inplant meristems, with an emphasis on the cyclin gene fam-ily. An area of cell proliferation in which plants show offtheir differences from other eukaryotes to advantage is incytokinesis. The phragmoplast is a conspicuous andplant-specific cytoskeletal apparatus that, along with thepreprophase band of microtubules, provides the machineryfor ensuring that a new cell wall is placed appropriatelybetween the two daughter cells as division is completed.Nishihama and Machida (pp. 507–512) describe thedynamic growth of the phragmoplast; the microtubulerearrangements that occur during the expansion of thegrowing cell plate; the importance of microtubule-associatedproteins (MAPs), both structural and motor proteins; andthe crucial role that phosphorylation plays in the regulationof MAP activity. The role of the phragmoplast highlights amajor problem in plant cell biology, targeted secretion.How secretion sites on the plasma membrane are identifiedand marked, and how specific classes of vesicles are targetedto these regions, remains obscure but the problem crops upin many of the reviews in this issue.

Cell biologyNow underpinned by the genomeEditorial overview Keith Roberts* and Nam-Hai Chua†

The growth of cellsThe phragmoplast is one of several plant-specific microtubulearrays that include a preprophase band and the corticalarray. Microtubules exert their function through severalfamilies of MAPs that, until recently, have been poorlycharacterized. A combination of genome information andmolecular genetics has begun to identify key plant MAPs,some of which turn out to be plant-specific. Our under-standing of the relationship between these MAPs and thecell-division- and growth-related dynamics of thecytoskeleton will need to build on these foundations withmore detailed biochemistry and cell biology. Azimzadehand colleagues (pp. 513–519) review the recent literature inthis area. Even a cursory look at plant anatomy makes itclear that, shortly after birth, plant cells usually acquire alongitudinal axis and often show clear signs of polarityalong that axis. This is particularly clear in cells leaving theroot apical meristem, in trichomes and in pollen tubes. Inall of these cases, the interconnections among the cell wall,the plasma membrane and the cytoskeleton are crucial.Grebe and colleagues (pp. 520–526) review current work onthe generation of axes and asymmetries within developingplant cells, with an emphasis on the role of polarisedsecretion and ion channels. There is increasing evidence tosupport the idea that ROP-GTPase-dependent signallingnetworks underpin the control of polar growth. Again, thefield has benefited from the availability of Arabidopsismutants but also, crucially, from the availability of greenfluorescent protein (GFP)-tagged probes for studyingcytoskeletal dynamics. For new cells to grow, the wall mustloosen and allow the incorporation of new wall material thatis targeted specifically to the expanding wall. The molecularmachinery that allows the wall to remodel and incorporatenew material, while still retaining its mechanical strength,remains much of a mystery. Many candidates have beensuggested to play a crucial role in this process, none more sothan members of the expansin gene family. Lee and col-leagues (pp. 527–532) review the members of this genefamily, which in Arabidopsis number over 30, and the recentwork on the alpha and beta sub families and their possiblerole in expanding cells. The intriguing presence of hor-mone-responsive elements in the promoters of expansins,and in general the correlation between the presence ofexpansins and expansion growth, is reviewed. This is anarea in which protein structure work would be of consider-able value and might help to illuminate the mysteriousmode of action of this intriguing family of proteins.

The final size of a plant is a product of cell number andcell size. Controls on the cell cycle and on cell expansionare therefore key to understanding overall organ-sizeregulation. Mizukami (pp. 533–539) reviews the differ-ences between plants and animals with respect to sizecontrol. In particular, the post-meristematic processes ofendoreduplication and polyploidy are highlighted as plant-specific control mechanisms. The molecular basis forbalancing cytosolic volume and nuclear DNA content is farfrom clear, but the controls on the production of cells

through the maintenance of meristematic competence arebeginning to be elucidated. Attention is drawn, especially,to the possible function of AINTEGUMENTA.

Cell shaping, differentiation and deathThe cell wall is crucial not only for providing mechanicalrigidity to the plant and anchoring the component cellstogether but also in shaping the individual cells within theplant. Leaf hairs, or trichomes, provide a conspicuousexample of the shaping of a specific cell type. The localmodification of the cell wall, the production and localiseddeposition of new wall material, and the involvement ofthe cytoskeletal machinery, are all intimately involved incell shaping. The local cell–cell interactions that result incell specification are beginning to be better understood,but how these signals mobilise the cellular processes thatresult in shaping are far from clear. Martin and colleagues(pp. 540–549) review the area, contrasting diffuse growthwith tip growth, highlighting the importance of cytoskeletaldynamics, and putting an increased emphasis on the role ofturgor. One exquisitely shaped cell, the hair cell (ortrichloblast) of the root epidermis, is a model for under-standing the detailed process of cell-shape determination,and Dolan (pp. 550–554) reviews the recent work in thisarea. The genes involved in hair-cell patterning and cellspecification are beginning to be well understood inArabidopsis. In addition, a wide range of developmentalmutants is facilitating our understanding of the roles ofion channels, wall synthesis and deposition, and thecytoskeleton in the controlled tip-growth processesrequired to make a root hair.

An excellent example of the complete developmentalpathway of cell proliferation, growth, size control, specifi-cation, shaping and differentiation is provided by thestomatal complex. von Groll and Altmann (pp. 555–560)review recent advances in our knowledge of the cellularevents involved in the formation of stomata. Again,molecular genetics has been crucial, and they discussthe family of genes that is emerging that controls thepatterning and subsequent development of the stomatal complex.

Where more appropriate to finish than with cell death?Although plant research has lagged behind in manyrespects in this field, there has recently been rapidprogress in understanding some of the regulatory net-works that underlie the hypersensitive response and otherforms of cell suicide in plants. This progress has againbeen facilitated by the use of Arabidopsis mutants. TheArabidopsis sequence has emphasised both similarities anddifferences between the core death machinery in plantsand animals. For example, there appear to be no Bcl2 fam-ily members in plants, neither do there appear to be p53genes or a conventional caspase cascade. The functionscarried out by these latter components in animals maywell be performed in plants by groups of cysteine proteases.It is increasingly clear that the mitochondrion may

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mediate some forms of programmed cell death in plants,as in animals. Beers and McDowell (pp. 561–567) reviewthis rapidly moving field and emphasise possible linksbetween signalling in defence and in development withrespect to cell death.

These are exciting times for cell biology, and when weassemble at the same time, same place next year we aresure that the genomics shot in the arm will have pervadedand enhanced yet more areas of cell biology. We look for-ward to the corresponding issue then!

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