Transcriptome changes in reciprocal grafts involving watermelon and bottle gourd reveal molecular mechanisms involved in increase of the fruit size, rind toughness and soluble solids

Authors: GARCIA-LOZANO, MARLENY - West Virginia State University; DUTTA, SUDIP KUMAR - West Virginia State University; NATARAJAN, PURUSHOTHAMAN - West Virginia State University; TOMASON, YAN - West Virginia State University; LOPEZ, CARLOS - West Virginia State University; KATAM, RAMESH - West Virginia State University; Levi, Amnon; NIMMAKAYALA, PADMA - West Virginia State University; REDDY, UMESH - West Virginia State University

Submitted to: Plant Molecular Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/4/2019
Publication Date: 12/16/2019
Citation: Garcia-Lozano, M., Dutta, S., Natarajan, P., Tomason, Y.R., Lopez, C., Katam, R., Levi, A., Nimmakayala, P., Reddy, U. 2019. Transcriptome changes in reciprocal grafts involving watermelon and bottle gourd reveal molecular mechanisms involved in increase of the fruit size, rind toughness and soluble solids. Plant Molecular Biology. https://doi.org/10.1007/s11103-019-00942-7.
DOI: https://doi.org/10.1007/s11103-019-00942-7

Interpretive Summary: The watermelon crop in the USA is highly susceptible to soil-borne diseases. Grafting watermelon on a pumpkin or a bottle gourd rootstock is a common practice in Asia and throughout the world and is considered a highly effective procedure to control soil-borne diseases of watermelon. However, grafting watermelon on pumpkin or bottle gourd rootstocks can negatively affect watermelon fruit quality, like fruit size, rind toughens, flesh texture and flavor. There is no knowledge about the mechanisms and molecules that are being produced in the pumpkin or bottle gourd rootstocks and migrate to watermelon scion, and consequently affect the quality of the grafted watermelon fruit. In this study, ARS scientists collaborated with researchers at West Virginia State University on elucidating the molecular mechanisms that affect watermelon plants grafted on pumpkin. The researchers identified small molecules named “microRNA” which migrate from the bottle gourd rootstock onto the watermelon scion. These microRNA molecules are known to play a role in regulating molecular activity, including the translation of genes to proteins in plant cells. The findings in this study are useful for ARS and University researchers and for plant breeders aimed to develop rootstocks that confer resistance to soil-borne diseases and at the same time produce high quality watermelon fruits.

Technical Abstract: Grafting is a sustainable alternative for watermelon breeding to control soil-borne pathogens and to increase tolerance to abiotic stresses. However, several reports have shown that grafting can negatively affect watermelon fruit quality. Field studies examined the effect of grafting on watermelon fruit quality. However, the regulation of this process at the molecular level has not been studied. The objective of this study was to elucidate molecular mechanisms in tissues of hetero-grafted watermelon and bottle gourd plants. Grafting on bottle gourd rootstock increased the size and rind thickness of watermelon fruits. On the other hand, grafting on watermelon rootstock produced bottle gourd fruits with high soluble solid content and thin rinds. Genes related to ripening, softening, cell wall strengthening, stress response, and disease resistance were differentially expressed in watermelon fruits grafted on bottle gourd while genes associated with sugar metabolism were differentially expressed in bottle gourd fruits grafted on watermelon. RNA-seq technology and validation using PlaMoM (a database for plant mobile macromolecules) revealed more than 400 mobile transcripts across the hetero-grafted sets. Some of these mobile transcripts contained a transfer RNA-like structure. This transcriptome study provides a valuable information on molecular mechanisms underlying grafted tissues of watermelon.