The Lamia, Part 8, Force of Nature

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A large spectrum of mutants is now available Menda et al. Functional analyses of candidate genes are routinely conducted, since tomato is highly suitable to stable transformation via Agrobacterium tumefaciens , and susceptible to transient gene expression via agroinjection Orzaez et al.

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Additionally, the repression of target genes can be obtained by virus-induced gene silencing VIGS Orzaez et al. Finally, the release in of the full tomato genome sequence represented an extraordinary breakthrough for developmental studies in fleshy fruits Tomato Genome Consortium, The development of plant organs requires mechanisms of differentiation by which tissue identity is imposed on cells as well as mechanisms regulating growth that dictate final organ size.

The determination of organ size thus relies on the fine regulation of the number and size of cells that is determined by the cell division and cell expansion processes, respectively. These processes respond to spatial and temporal controls that come under the influence of internal genotypic and external environmental cues. As a general feature of angiosperms, cell expansion is frequently ascribed to nuclear endopolyploidization in several plant organs, according to the process of endoreduplication Chevalier et al.

This review addresses the current knowledge on fruit development in tomato, in particular describing those genes that are involved in the developmental processes governing fruit growth.

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Fruits typically develop from pre-existing organs, such as carpels inside flowers. Obviously, the number of cells formed in the ovary before anthesis is critical for the final size of fruit, and such a positive correlation is frequently observed Tanksley, From flower initiation to the double fertilization occurring in ovules Gillaspy et al.

In order to keep the ovary in a temporally protected and dormant state, abscisic acid ABA and ethylene work to stop growth within the ovary shortly before anthesis when the ovary has reached its mature size Vriezen et al.

Growth resumes only after successful pollination and fertilization of ovules triggers fruit set—the first phase of fruit development—through the action of ovary-synthesized auxins and GAs Ruan et al. Fruit growth is the longest phase of fruit development, as it ranges from 5 to 8 weeks depending on the genotype.

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  • Growth proceeds first by a period of intense mitotic activity according to a spatially and temporally organized pattern of cell division. Active cell division within the pericarp is usually restricted to an initial period of 1—2 weeks after fruit set Fig. Remarkably, cell division occurs within discrete cell layers with well-defined planes of division fulfilling specific purposes. For instance, the two subepidermal layers of the pericarp undergo several rounds of periclinal divisions, thus leading to an increase in the number of pericarp cell layers, whereas the two epidermal cell layers undergo anticlinal divisions in response to the resulting increase in fruit volume Cheniclet et al.

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    These various types of cell divisions are differently regulated, because cell divisions promoting cell layer formation occur only within 5—8 days post-anthesis dpa , whereas randomly oriented cell divisions occur for longer periods up to 10—18 dpa Cheniclet et al. How this spatio-temporal pattern of development is related to gene expression, metabolic profiles, and cellular characteristics remains poorly understood Lemaire-Chamley et al. Fruit development in tomato. The fruit originates from the development of the ovary.

    Following fertilization concomitant with anthesis and fruit set, fruit growth starts with a period of very active cell division inside the ovary that lasts for 8—10 days post-anthesis dpa. Cell enlargement starts concomitantly with cell division as early as 1 or 2 dpa and contributes to the increase in fruit size. It has been reported that a second phase of growth related to cell expansion occurs separately after the cell division phase Gillaspy et al.

    In fact, cell expansion starts within very few days after fruit set, concomitantly with cell division Cheniclet et al. This cell enlargement mostly occurs through dramatic increases in the vacuolar compartment and cell vacuolation index.

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    Remarkably, this spectacular cell hypertrophy is closely correlated to an increase in nuclear DNA levels due to endopolyploidization. Endopolyploidy is the occurrence of different ploidy levels within an organism. As a consequence, hypertrophic nuclei arise from successive cycles of DNA replication without segregation of sister chromatids, thereby resulting in the production of polytene chromosomes Bourdon et al.

    The physiological and developmental importance of endoreduplication is still a matter of debate. However, the frequent observation that cell size and ploidy levels are highly and positively correlated in many different plant species, organs, and cell types Chevalier et al. This theory, which was formulated as early as the beginning of the 20th century, states that there is a causal relationship between nuclear and cytoplasmic growth to maintain a constant ratio of nucleus to cell volume Sugimoto-Shirasu and Roberts, ; Chevalier et al.

    In the course of tomato fruit development, endoreduplication produces high levels of nuclear DNA ploidy within the mesocarp and the locular jelly-like tissue Bergervoet et al. Ploidy levels as high as C where C is the haploid DNA content have been observed to occur in a large array of tomato genotypes. This extraordinary extent of endoreduplication occurring in tomato fruit is unmatched by any other plant species and makes tomato an excellent model to study the role of endoreduplication as a determinant of fruit size.

    Cheniclet et al. The endoreduplication-associated cell expansion in fruit is characterized by profound cellular modifications. It is evident that the formation of polytene chromosomes impacts on nuclear, nucleolar, and chromatin organization within hypertrophied nuclei delimited by a highly invaginated nuclear envelope Bourdon et al.

    These profound nuclear grooves are filled with numerous mitochondria, whose number increases according to nuclear DNA content. Apparently, endoreduplication triggers efficient communication between the nucleus and the cytoplasm, despite the increase in nuclear volume. In addition, Bourdon et al. As a morphogenetic factor, endoreduplication thus contributes to maintain homeostasis of cytoplasmic components through the establishment of a highly structured cellular system, where multiple physiological functions are integrated to support cell growth during fruit development.

    Therefore, modifying fruit growth by targeting genes involved in mitosis and endoreduplication has been considered for tomato Fig.

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    Genes involved in tomato fruit growth. Mapping of QTLs, cloning of the associated genes, in planta functional analyses, and characterization of mutants have identified genes that impact fruit growth both positively and negatively. Theses genes are reported according to the respective stages of fruit development in which they are involved: A ovary development; B fruit set; C phase of cell division; and D phase of cell expansion. Progression of the cell cycle is ensured by the activity of core regulators consisting of conserved heterodimeric protein complexes.

    These complexes are formed by a catalytic subunit referred to as cyclin-dependent kinase CDK and a regulatory cyclin CYC subunit.

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    Specific CDK—CYC complexes operate at the boundaries between the various phases of the cell cycle in order to phosphorylate target proteins. These post-translational modifications which can be either inhibitory or activating, are essential for the progression through cell cycle boundaries De Veylder et al. To allow a subtle regulation of cell cycle progression, the kinase activities of the CDK—CYC complexes are regulated in several ways: i CDK activity is finely tuned by phosphorylation and dephosphorylation of the CDK on conserved residues by specific kinases and phosphatases; ii the proteolytic destruction of the cyclin subunit by the ubiquitin—proteasome system UPS is sufficient to abolish activity of the complex, as CDK alone does not display kinase activity without its cyclin partner; and iii the CDK—CYC complexes are inactivated by the specific binding of CDK inhibitors Churchmann et al.

    The endoreduplication cycle endocyle consists of successive rounds of DNA synthesis S phases in the absence of mitosis, and thus represents a partial and modified cell cycle. Consequently, cell cycle and endocycle progression involves control of CDK—CYC activity levels by common regulatory mechanisms on the molecular level, such as those mentioned above De Veylder et al.

    Central to this regulation is the maintenance of a certain threshold of CDK—CYC activity to allow the commitment to mitosis. When mitotic CDK complexes do not form or their activity is suppressed, this threshold is not exceeded and the level of CDK—CYC activity is insufficient to drive cells into mitosis; endoreduplication can then take place. In tomato, several functional analyses have attempted to modulate the CDK—CYC complex activity during fruit development, either through direct targeting of CDK gene expression or through post-translational regulation of the complex Fig.

    Czerednik et al. The amiCDKA plants produced smaller fruits than those of the wild type, and which had a thinner pericarp due to an overall decreased number of cell layers within the exocarp displaying the smallest cells. In contrast, the mesocarp displayed normal, enlarged cells without any significant difference in ploidy levels. CDKA1-overexpressing fruits were visually indistinguishable from those of the wild type, with similar fruit weights and diameters.

    However, they exhibited a larger placental area, a thicker pericarp, and a reduced number of seeds. Across the pericarp, the number of constituting cell layers was significantly increased, as well as the number of cells per mm 2 , which was also observed in the placenta.

    Overall, the observations showed that mean cell size was smaller, and was accompanied by a decrease in endoreduplication levels in cells from the mesocarp, placenta, and jelly-like tissues. These data provided another example of the relationship between endoreduplication and final cell size, suggesting a pivotal role for CDKA1 in the regulation of mitotic activities during fruit development.

    Additionally, CDKA1 overexpression affected the production of seeds in developing fruits that indirectly affected pericarp cell expansion, which is normally driven by seed-produced phytohormones. In general, the observed data are difficult to interpret, most probably because they are associated with the specific tissular and developmental pattern of expression of the TPRP promoter, which was used to drive gene misexpression.

    The TPRP promoter is a fruit-specific promoter whose expression starts early in the ovary and reaches a maximum during the cell expansion phase of fruit development Fernandez et al. In the absence of measured CDK activity, we speculate that the phenotypes observed by down-regulating CDKA genes, or up-regulating CDKB genes are caused by the lack of endocycle-specific CDKA—CYC complex activity, which is necessary to permit the youngest cells of the outermost layers of pericarp namely the exocarp to enter into the phase of endoreduplication-driven cell expansion.

    In a similar manner, overexpression of CDKA;1 induced mitosis across the pericarp and altered the endoreduplication index Czerednik et al. The inhibitory effect of phosphorylating CDKA on the Tyr15 residue through the action of the WEE1 kinase represents a good example of a post-translational regulatory mechanism impairing the cell cycle, and potentially triggering the endocycle Fig. Gonzalez et al. Smaller fruits were produced displaying a thinner pericarp composed of smaller cells. The WEE1 phosphorylation activity on its CDK targets appears to be an important mode of regulation for the promotion of endoreduplication during fruit development, and contributes to cell size determination, which ultimately influences final fruit size Chevalier et al.

    The completion of mitosis and progression from mitosis back into interphase requires the loss of CDK—CYC complex activity, which occurs through proteolytic destruction of the cyclin moiety by the UPS. In plants, the commitment towards endoreduplication involves the selective destruction of mitotic cyclins, thereby preventing the formation of a proper mitotic CDK—CYC complex, and thus impairing the associated kinase activity.