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AOBPreview originally published online on January 31, 2009
Annals of Botany 2009 103(6):847-858; doi:10.1093/aob/mcp009
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© The Author 2009. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Novel thigmomorphogenetic responses in Carica papaya: touch decreases anthocyanin levels and stimulates petiole cork outgrowths

Brad W. Porter1,*, Yun J. Zhu2, David T. Webb3 and David A. Christopher1

1 Department of Molecular Biosciences and Bioengineering, University of Hawai'i at Manoa, 1955 East-West Rd, Agricultural Sciences Room 218, Honolulu, HI 96822, USA
2 Hawai'i Agriculture Research Center, 99–193 Aiea Heights Drive, Aiea, HI 96701, USA
3 Department of Botany, University of Hawai'i at Manoa, 3190 Maile Way, St John Room 412A, Honolulu, HI 96822, USA

* For correspondence. E-mail bporter{at}hawaii.edu

Received: 24 August 2008    Returned for revision: 5 November 2008    Accepted: 4 December 2008    Published electronically: 31 January 2009

Background and Aims: Because of its rapid growth rate, relative ease of transformation, sequenced genome and low gene number relative to Arabidopsis, the tropical fruit tree, Carica papaya, can serve as a complementary genetic model for complex traits. Here, new phenotypes and touch-regulated gene homologues have been identified that can be used to advance the understanding of thigmomorphogenesis, a multigenic response involving mechanoreception and morphological change.

Methods: Morphological alterations were quantified, and microscopy of tissue was conducted. Assays for hypocotyl anthocyanins, lignin and chlorophyll were performed, and predicted genes from C. papaya were compared with Arabidopsis touch-inducible (TCH) and Mechanosensitive channel of Small conductance-like genes (MscS-like or MSL). In addition, the expression of two papaya TCH1 homologues was characterized.

Key Results: On the abaxial side of petioles, treated plants were found to have novel, hypertrophic outgrowths associated with periderm and suberin. Touched plants also had higher lignin, dramatically less hypocotyl anthocyanins and chlorophyll, increased hypocotyl diameter, and decreased leaf width, stem length and root fresh weight. Papaya was found to have fewer MSL genes than Arabidopsis, and four touch-regulated genes in Arabidopsis had no counterparts in papaya. Water-spray treatment was found to enhance the expression of two papaya TCH1 homologues whereas induction following touch was only slightly correlated.

Conclusions: The novel petiole outgrowths caused by non-wounding, mechanical perturbation may be the result of hardening mechanisms, including added lignin, providing resistance against petiole movement. Inhibition of anthocyanin accumulation following touch, a new phenotypic association, may be caused by diversion of p-coumaroyl CoA away from chalcone synthase for lignin synthesis. The absence of MSL and touch-gene homologues indicates that papaya may have a smaller set of touch-regulated genes. The genes and novel touch-regulated phenotypes identified here will contribute to a more comprehensive view of thigmomorphogenesis in plants.

Key words: Thigmomorphogenesis, thigmomorphogenetic, mechanoreception, touch-regulated genes, periderm, cork, anthocyanins, lignin, Carica papaya


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