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Open Access Highly Accessed Research article

Gene expression in developing watermelon fruit

W Patrick Wechter1, Amnon Levi1*, Karen R Harris1, Angela R Davis2, Zhangjun Fei3, Nurit Katzir4, James J Giovannoni3, Ayelet Salman-Minkov5, Alvaro Hernandez6, Jyothi Thimmapuram6, Yaakov Tadmor4, Vitaly Portnoy4 and Tova Trebitsh5

Author Affiliations

1 USDA, ARS, U.S. Vegetable Lab, 2700 Savannah Highway, Charleston, SC, USA

2 USDA, ARS, South Central Agricultural Research Laboratory, P.O. Box 159 Hwy 3 West, Lane, OK, USA

3 USDA, ARS, Robert Holly Center and Boyce Thompson Institute for Plant Research, Tower Road, Ithaca, NY, USA

4 Agricultural Research Organization, P.O. Box 1021, Ramat Yishay 30095, Israel

5 Department of Life Sciences, Ben-Gurion University of the Negev, Beer-Sheva, Israel

6 University of Illinois at Urbana-Champaign, 1201 W. Gregory Drive, Urbana, IL 61801, USA

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BMC Genomics 2008, 9:275  doi:10.1186/1471-2164-9-275

Published: 5 June 2008

Abstract

Background

Cultivated watermelon form large fruits that are highly variable in size, shape, color, and content, yet have extremely narrow genetic diversity. Whereas a plethora of genes involved in cell wall metabolism, ethylene biosynthesis, fruit softening, and secondary metabolism during fruit development and ripening have been identified in other plant species, little is known of the genes involved in these processes in watermelon. A microarray and quantitative Real-Time PCR-based study was conducted in watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai var. lanatus] in order to elucidate the flow of events associated with fruit development and ripening in this species. RNA from three different maturation stages of watermelon fruits, as well as leaf, were collected from field grown plants during three consecutive years, and analyzed for gene expression using high-density photolithography microarrays and quantitative PCR.

Results

High-density photolithography arrays, composed of probes of 832 EST-unigenes from a subtracted, fruit development, cDNA library of watermelon were utilized to examine gene expression at three distinct time-points in watermelon fruit development. Analysis was performed with field-grown fruits over three consecutive growing seasons. Microarray analysis identified three hundred and thirty-five unique ESTs that are differentially regulated by at least two-fold in watermelon fruits during the early, ripening, or mature stage when compared to leaf. Of the 335 ESTs identified, 211 share significant homology with known gene products and 96 had no significant matches with any database accession. Of the modulated watermelon ESTs related to annotated genes, a significant number were found to be associated with or involved in the vascular system, carotenoid biosynthesis, transcriptional regulation, pathogen and stress response, and ethylene biosynthesis. Ethylene bioassays, performed with a closely related watermelon genotype with a similar phenotype, i.e. seeded, bright red flesh, dark green rind, etc., determined that ethylene levels were highest during the green fruit stage followed by a decrease during the white and pink fruit stages. Additionally, quantitative Real-Time PCR was used to validate modulation of 127 ESTs that were differentially expressed in developing and ripening fruits based on array analysis.

Conclusion

This study identified numerous ESTs with putative involvement in the watermelon fruit developmental and ripening process, in particular the involvement of the vascular system and ethylene. The production of ethylene during fruit development in watermelon gives further support to the role of ethylene in fruit development in non-climacteric fruits.