{"id":3137,"date":"2011-01-21T11:11:14","date_gmt":"2011-01-21T09:11:14","guid":{"rendered":"http:\/\/localhost\/azgad\/wordpress\/?p=3137"},"modified":"2011-01-21T11:38:09","modified_gmt":"2011-01-21T09:38:09","slug":"%d7%9e%d7%99%d7%95%d7%97%d7%93-%d7%9c%d7%98%d7%95-%d7%91%d7%a9%d7%91%d7%98-unearthing-the-mechanisms-controlling-plant-size","status":"publish","type":"post","link":"https:\/\/azgad.com\/?p=3137","title":{"rendered":"\u05dc\u05e8\u05d2\u05dc \u05d8"\u05d5 \u05d1\u05e9\u05d1\u05d8: Unearthing the mechanisms controlling plant size"},"content":{"rendered":"

.
\nPlants have been cultivated and studied from the earliest days
\nof human civilization, yet much remains unknown about them.
\nA good example is the mechanism by which the size of plant
\ncells is determined. Keiko Sugimoto, leader of the Cell Function
\nResearch Unit in RIKEN\u2019s Plant Science Center, in working to
\nelucidate this mechanism has discovered a series of genes that
\ncontrol cell division or cell growth, attracting the attention of
\nresearchers and companies worldwide. \u201cOur research focuses
\non the cellular aspects of plants,\u201d says Sugimoto.
\n.
\nA garden of lilies<\/strong>
\n.
\nWhile a high-school student, Sugimoto noticed that a single lily that
\nhad blossomed in her garden the year before had become three lilies
\na year later, followed by ten the next year and as many as 100 the
\nyear after. \u201cBut what impressed me most was that all of those flowers
\nwere the same size, and had the same color and same shape every
\nyear,\u201d she says. \u201cAlthough I knew that this was a manifestation of
\nheredity, which I had learned at school, I was fascinated. I wanted
\nto understand the mystery of plants, and this led me into research.\u201d<\/p>\n

.
\n.
\n.
\n\"\"<\/a>.<\/p>\n

.
\n.
\n.
\nFigure 1.<\/strong> Mechanism underlying increases in plant size.<\/em>
\n.
\nAfter completing her master\u2019s course in Japan, Sugimoto gained
\nher PhD in plant science at the Australian National University. She
\nthen went to work at the John Innes Center in the UK\u2014a Mecca for
\nresearchers studying plant biology\u2014and in 2007 she set up the Cell
\nFunction Research Unit in RIKEN\u2019s Plant Science Center. \u201cHow is
\nplant cell size controlled? We are now working to solve this difficult
\nproblem.\u201d
\n.
\nThe aspect that had impressed Sugimoto most as a high-school student
\nwas that the sizes of flowers, leaves, seeds and other plant organs
\ndepend roughly on the species of plant. Each organ grows as its cells
\nself-divide and increase in number, and each cell expands. However,
\nplant organs do not continue to grow infinitely. \u201cFlowers and leaves
\nstop growing when they reach a certain size. Research has shown that
\nplant hormones such as auxin and cytokinin are involved in plant growth,
\nbut we still don\u2019t know how plant hormones control cell division and cell
\nexpansion to determine ultimate organ size,\u201d says Sugimoto. \u201cPlant size
\ncannot be understood without knowing what is happening in cells. We
\nare conducting research focusing on the cellular aspects of plants,
\nwhich is a unique approach.\u201d
\n.
\nDeterminants of plant growth<\/strong>
\n.
\n\u201cEvery time I cut radishes or carrots into long, thin strips for cooking, I cannot
\nhelp admiring the slices for a moment,\u201d says Sugimoto with a smile. \u201cIf you
\nlook closely at a slice, you can see a finely textured portion near the tip. This
\nis called the meristem. It is dividing tissue where the cells self-divide. Try taking
\na look next time you\u2019re preparing a meal.\u201d
\n.
\nIn plants, meristems (Fig. 1) are found only in the tips of roots and stems. Plant
\ngrowth is the result of cell division and proliferation at the meristem, which is
\nfollowed by cell expansion. \u201cThere are two key time points in plant growth. One
\nis a turning point when cell division switches to cell expansion. Once a cell begins
\nexpanding, it cannot return to the stage of proliferation by division. The other is the
\npoint when cell growth stops and cells no longer expand. Plants cannot
\ngrow normally unless these two points are strictly controlled.\u201d Sugimoto and
\nher colleagues have attracted global attention for their discovery of the
\ngenes that control these two growth points.
\n.
\nGenes control cell division and endoreduplication<\/strong>
\n.<\/p>\n

Photograph showing a cross-section of Arabidopsis roots.
\nPlant cells divide mainly in the meristems at the tips of roots
\nand stems. After dividing several times, cells begin to increase
\ntheir size due to endoreduplication. When the cell reaches a
\ncertain size, endoreduplication and cell growth cease. The
\nHPY2 gene controls the transition from cell division to
\nendoreduplication, whereas the GTL1 gene stops cell growth.
\nThe green fluorescence indicates the HPY2 expression.
\n.<\/p>\n

.
\n.<\/p>\n

\"\"<\/a>.
\n.
\nFigure 2<\/strong>. Endoreduplication and cell size in Arabidopsis<\/em>.
\n.
\n.<\/p>\n

A scanning electron microscopy image of the surface of an
\nArabidopsis leaf showing a trichome. Most cells have 2C nuclear
\nDNA content, but some cells have increased nuclear DNA contents
\nof 4C, 8C and 32C due to endoreduplication. The four insets show
\nthe correlation between cell size and the amount of nuclear DNA.
\nThe trichome has a nuclear DNA content of 32C<\/p>\n

.
\n.<\/p>\n

Sugimoto used a new technique to discover genes that control the
\nPoint at which cells stop dividing and begin increasing in size. \u201cMany
\nresearchers have tried to look for genes that control cell size, but most
\nof them were trying to find mutants with altered cell size. Mutants have
\nbeen identified merely based on the appearance of cells. We developed
\na method to accurately measure nuclear DNA content and isolate
\nmutants with altered DNA levels.\u201d
\n.
\nThe cells of Arabidopsis, a commonly used experimental material in
\nplant science, just like human cells, have two sets of chromosomes,
\none from the mother and the other from the father. The DNA of a cell
\nhaving two sets of chromosomes is denoted 2C. When a 2C cell
\ndivides, its DNA is first replicated to produce 4C, which is then equally
\ndistributed into the next two dividing cells, resulting in two 2C daughter
\ncells. In Arabidopsis, however, 2C and 4C cells are not the only cell
\ntypes to be found. Gametes (pollen, ovules) that have undergone
\nmeiosis, a special process of cell division that results in half the
\nnumber of chromosomes as found in somatic cells, are 1C cells, but
\nthere are also 8C, 16C and 32C cells (Fig. 2). \u201cIn plant cells, DNA
\nreplication is sometimes followed by doubling in DNA without mitosis,\u201d
\nsays Sugimoto. \u201cThis phenomenon is called endoreduplication,
\nwhich results in 8C, 16C and 32C cells. The nuclear DNA and
\ncell size are correlated; cells expand as their nuclear DNA
\nincreases.\u201d
\n.
\nTogether with Takashi Ishida, a postdoc in her lab, Sugimoto examined
\nthe nuclear DNA content of Arabidopsis cells and discovered a mutant
\nhaving fewer 2C and 4C cells and more 32C, 64C and 128C cells (Fig. 3).
\n\u201cUsually in plants, 2C and 4C cells in meristems continue to divide at a
\nconstant rate. We assume that in the mutant we discovered, these
\nmeristematic cells have undergone endoreduplication and switched into
\ncell expansion prematurely. A more detailed investigation revealed that
\nthis mutant had lost the function of the HPY2 gene. Hence, HPY2 plays
\na role in controlling the point of switching to endoreduplication, where
\ncells stop dividing and instead grow in size.\u201d
\n.
\n\"\"<\/a>.<\/p>\n

.
\n.
\nFigure 3<\/strong>. Regulation of cell division by the HPY2 gene.<\/em>
\n.
\nThis achievement was announced in August 2009, drawing attention
\nnot only from plant biologists, but also from researchers studying a wide
\nvariety of other organisms. \u201cThis is because HPY2 is involved in the
\nfunction of a small peptide known as SUMO, a small ubiquitin-like
\nmodifier. SUMO is found in a broad range of species, from humans to
\nplants and yeasts. It binds to other proteins to enhance or weaken their
\nfunctions, and to regulate the diverse functions of cells. The reason
\nwhy my result attracted so much attention is that researchers studying
\ndiverse ranges of organisms have been interested in SUMO.\u201d
\n.
\nSugimoto\u2019s group demonstrated that the protein produced by HPY2
\nmediates the binding of SUMO to other proteins, resulting in the regulation
\nof cell division. This was the first report of SUMO being associated with
\nthe regulation of cell division in multicellular organisms. \u201cI never thought
\nthat my studies on the mechanism of plant cell size control would lead
\nto SUMO. Research is fascinating because it can lead to unexpected
\nresults.\u201d
\n.
\nSugimoto has also discovered three other genes that control the switch
\ninto endoreduplication like HPY2. Her next task is to clarify the differences
\nin their functions.
\n.
\nA gene terminating cell growth<\/strong>
\n.
\nIn September 2009, following the discovery of HPY2, Sugimoto\u2019s group
\ndiscovered a gene involved in the second point\u2014when plant cells stop
\ngrowing in size. \u201cIt began with the discovery of a mutant having very large
\ntrichomes by Christian Breuer, a posdoc in my lab, who was searching
\nfor mutants with abnormal cell size.\u201d
\n.
\nTrichomes are hair-like outgrowths that cover the surfaces of Arabidopsis
\nleaves to protect them from insects, pathogens and even ultraviolet radiation
\n(Fig. 4). \u201cEach trichome comprises a single epidermal cell in Arabidopsis.
\nIt is large enough to be seen macroscopically.\u201d While even a normal-sized
\ntrichome is 500 times larger than an ordinary cell, the mutant discovered
\nby Breuer has trichomes that are more than twice this size.
\n.
\n.
\n\"\"<\/a>.<\/p>\n

em>Figure 4:<\/strong> Arabidopsis trichomes.<\/p>\n

(Upper) Nuclear DNA content of a cell population. The hpy2
\nmutant has lower ratios of 2C and 4C and higher ratios of 32C,
\n64C and 128C compared to the wild-type control. (Lower)
\nPhotographs of wild-type (left) and hpy2-mutant (right) plants
\nten days after germination. The mutant has very small roots
\nand leaves. The Blue staining indicates defective cell
\ndivision in the mutant.
\n.
\nThe mutant was found to have the GTL1 gene partially modified and
\nexpressed in excess. When the function of GTL1 was artificially
\nsuppressed, the mutant\u2019s trichomes became more than twice the size
\nof wild-type trichomes. Based on these experiments, Sugimoto\u2019s group
\nhypothesized that GTL1 functions to terminate cell growth. To test the
\nhypothesis, they examined when and where GTL1 is expressed. It was
\nfound not to be expressed in smaller trichomes in the early stage of
\ngrowth or trichomes that had stopped growing, but to be expressed
\nonly in trichomes that have just expanded to maximum size (Fig. 5).
\n.
\nPreviously, it had been thought that cell growth ceases when the supply
\nof cellulose and other components of the cell wall is stopped, or when
\nwater absorption in vacuoles ceases. However, the discovery of GTL1
\nshows that plants have an intrinsic mechanism for actively stopping
\ncell growth. The discovery is groundbreaking, overturning the traditional
\nconcept of plant growth.
\n.
\nTrichomes are cells undergoing endoreduplication, which is known to cease
\nat 32C. Sugimoto\u2019s group is conducting research on the hypothesis that
\nGTL1 may control endoreduplication. It is already known that the function
\nof the gene necessary for endoreduplication is activated in mutants
\nlacking the function of GTL1. \u201cGTL1 produces a protein known as a
\ntranscription factor, which binds to the DNA of a certain gene to promote
\nor suppress its transcription to RNA. In the future, I want to clarify how
\nGTL1 controls transcription and of which genes, and to discover the
\nmechanism of endoreduplication.\u201d
\n.
\nGiant prospects<\/strong>
\n.
\nSince the announcement of the discovery of GTL1, Sugimoto has received
\na flood of offers for joint research, including many inquiries from industry,
\nwho have great expectations for creating larger fruits and vegetables by
\nsuppressing the function of GTL1.<\/p>\n

Trichomes are hair-like outgrowth at the plant leaf surface, each
\ncomprising a single cell. These cells protect plants against insects
\nand pathogens, and trichomes in some species produce useful
\nsecondary metabolites such as aspirin. Sugimoto\u2019s group examined
\nArabidopsis mutants with abnormally expanded trichomes and
\ndiscovered GTL1, a gene that terminates cell growth. The photograph
\nshows a mutant having an increased number of trichome branches.
\nA normal trichome has three branches.
\n.<\/p>\n

.
\n\"\"<\/a>.
\n.
\n.
\nFigure 5.<\/strong> Trichome size and GTL1 expression.<\/em>.
\n.
\n.
\nSome cultivars are already available with increased yields thanks to artificial
\nduplication of nuclear DNA with chemical agents. However, this chemical
\ntreatment unavoidably duplicates the nuclear DNA in all cells constituting the
\nplant body, which in turn makes the plant unable to produce seeds. \u201cAdvanced
\nresearch on GTL1 may allow us to promote endoreduplication at desirable
\nportions of plants, such as fruits, flowers and leaves, or whenever needed,
\nto change their sizes without preventing seed production,\u201d says Sugimoto,
\nwho is keen to conduct joint research with industry.
\n.
\n\u201cNow is the most enjoyable time in my academic career,\u201d declares Sugimoto.
\nHowever, she is not satisfied with just discovering the genes that control
\nplant growth. Further extensive investigation of the functions of individual
\ngenes is needed. It is also necessary to identify the targets of HPY2 and
\nGTL1 to determine on which genes and proteins they act. She is also
\ninterested in the relationship between HPY2 and GTL1, and their
\nassociation with plant hormones. \u201cMuch remains to be done, and I have
\nnot found the answer to my question about lilies when I was a high school
\nstudent. In the plant kingdom, there are so many unanswered questions.
\nThis is why I am fascinated by plant research.\u201d<\/p>\n

Photograph (left) and fluorescence image (right) of wild-type and GTL1-lacking
\nmutants. GTL1 (labeled with green fluorescent protein) is expressed only
\nin trichomes that have just grown to maximum sizes, and not in younger
\nor older trichomes.<\/p>\n","protected":false},"excerpt":{"rendered":"

. Plants have been cultivated and studied from the earliest days of human civilization, yet much remains unknown about them. A good example is the mechanism by which the size of plant cells is determined. Keiko Sugimoto, leader of the Cell Function Research Unit in RIKEN\u2019s Plant Science Center, in working to elucidate this mechanism … <\/p>\n

\u05d4\u05de\u05e9\u05d9\u05db\u05d5 \u05d1\u05e7\u05e8\u05d9\u05d0\u05d4<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"footnotes":""},"categories":[10],"tags":[290,513,158],"class_list":["post-3137","post","type-post","status-publish","format-standard","hentry","category-10","tag-290","tag-513","tag-158","nodate","item-wrap"],"_links":{"self":[{"href":"https:\/\/azgad.com\/index.php?rest_route=\/wp\/v2\/posts\/3137","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/azgad.com\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/azgad.com\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/azgad.com\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/azgad.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=3137"}],"version-history":[{"count":16,"href":"https:\/\/azgad.com\/index.php?rest_route=\/wp\/v2\/posts\/3137\/revisions"}],"predecessor-version":[{"id":3153,"href":"https:\/\/azgad.com\/index.php?rest_route=\/wp\/v2\/posts\/3137\/revisions\/3153"}],"wp:attachment":[{"href":"https:\/\/azgad.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=3137"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/azgad.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=3137"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/azgad.com\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=3137"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}