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AOBPreview originally published online on October 17, 2007
Annals of Botany 2008 101(1):89-102; doi:10.1093/aob/mcm258
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© The Author 2007. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Changes in Photosynthetic Rates and Gene Expression of Leaves during a Source–Sink Perturbation in Sugarcane

A. J. McCormick1,2, M. D. Cramer3,4 and D. A. Watt1,2,*

1 South African Sugarcane Research Institute (SASRI), Crop Biology Resource Centre, Private Bag X02, Mt Edgecombe, 4300, South Africa
2 University of KwaZulu-Natal, School of Biological and Conservation Sciences, Howard College Campus, Durban, 4041, South Africa
3 University of Cape Town, Botany Department, Private Bag X1, Rondebosch, 7701, South Africa
4 School of Plant Biology, Faculty of Natural and Agricultural Sciences, The University of Western Australia, 35 Stirling Highway, WA 6009, Australia

* For correspondence. E-mail alistair.mccormick{at}sugar.org.za

Received: 29 May 2007    Returned for revision: 3 August 2007    Accepted: 4 September 2007    Published electronically: 17 October 2007

Background and Aims: In crops other than sugarcane there is good evidence that the size and activity of carbon sinks influence source activity via sugar-related regulation of the enzymes of photosynthesis, an effect that is partly mediated through coarse regulation of gene expression.

Methods: In the current study, leaf shading treatments were used to perturb the source–sink balance in 12-month-old Saccharum spp. hybrid ‘N19’ (N19) by restricting source activity to a single mature leaf. Changes in leaf photosynthetic gas exchange variables and leaf and culm sugar concentrations were subsequently measured over a 14 d period. In addition, the changes in leaf gene response to the source–sink perturbation were measured by reverse northern hybridization analysis of an array of 128 expressed sequence tags (ESTs) related to photosynthetic and carbohydrate metabolism.

Key Results: Sucrose concentrations in immature culm tissue declined significantly over the duration of the shading treatment, while a 57 and 88% increase in the assimilation rate (A) and electron transport rate (ETR), respectively, was observed in the source leaf. Several genes (27) in the leaf displayed a >2-fold change in expression level, including the upregulation of several genes associated with C4 photosynthesis, mitochondrial metabolism and sugar transport. Changes in gene expression levels of several genes, including Rubisco (EC 4·1·1·39) and hexokinase (HXK; EC 2·7·1·1), correlated with changes in photosynthesis and tissue sugar concentrations that occurred subsequent to the source–sink perturbation.

Conclusions: These results are consistent with the notion that sink demand may limit source activity through a kinase-mediated sugar signalling mechanism that correlates to a decrease in source hexose concentrations, which, in turn, correlate with increased expression of genes involved in photosynthesis and metabolite transport. The signal feedback system reporting sink sufficiency and regulating source activity may be a potentially valuable target for future genetic manipulation to increase sugarcane sucrose yield.

Key words: C4 metabolism, array, hexose, photosynthesis, sucrose, Saccharum spp., source–sink, gene expression


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