The role of small RNAs in C4 photosynthesis
The C4 cycle represents a series of biochemical and anatomical modifications that are targeted to overcome the effects of photorespiration caused by the oxygenase capability of Ribulose Bisphosphate Carboxylase/Oxygenase (RuBisCO). The cycle has evolved independently in over 60 lineages, which suggests that recruitment of genes into the C4 cycle is a relatively easy process. However, the mechanisms by which the anatomy and cell-specificity of the components of the C4 cycle is achieved is poorly understood. Preliminary work in maize indicated several components of the C4 cycle may be targeted by microRNAs (miRNAs). To explore this, a library of sRNA sequences from mature leaf tissue of the model C4 species Cleome gynandra L. was generated and then searched against a list of expressed sequence tag sequences for candidate genes of the C4 cycle. To complement this, transgenic C. gynandra containing the viral p19 protein, which is capable of suppressing miRNA activity, were produced. A limited subset of the candidate C4 genes showed a high level of sRNA read alignment. In C. gynandra plants expressing p19 photosynthesis was compromised and transcripts of several genes (most notably RbcS and RCA) were upregulated. These data were complemented by examining the effect of illumination on developing C. gynandra cotyledons, and attempts to generate a hybrid between C. gynandra and the C3 C. hassleriana Chodat. RbcS also showed elevated abundance in etiolated cotyledons, although this rapidly declined after illumination. The remainder of the C4 genes profiled accumulated in etiolated tissue, but were upregulated within 6 hours of illumination. Therefore, this study has illustrated that miRNA activity may play a role in maintaining the C4 photosynthetic cycle at optimum efficiency, although it has not been possible to identify at which point(s) this regulation is applied. Secondly, RbcS appears to be subject to multiple regulatory mechanisms in C. gynandra, and it is possible that miRNAs have a role in negatively regulating expression of RbcS.
C4 photosynthesis, Small RNAs, Cleome, p19, Etiolation, Hybridisation
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