Cracking the Kranz enigma

30 September 2014 - By: Christine Raines

Cracking the Kranz enigma with systems biology

By Christine Raines

Cracking the Kranz enigma with systems biology J. Exp. Bot. (2014) 65 (13): 3327-3339 doi:10.1093/jxb/eru015, Jim P. Fouracre, Sayuri Ando, and Jane A. Langdale early/2014/02/06/jxb.eru015.abstract 

This paper presents a model to explain the development of Kranz anatomy in plants using the C4 pathway for fixation of atmospheric CO2, based on data from –omic based approaches. 

Kranz anatomy is the term given to the formation of bundle sheath cells that surround the veins in plants using the C4 photosynthetic pathway. The mesophyll cells are found between the between the veins and are functionally distinct, fixing atmospheric CO2 using PEP carboxylase. C4 species are amongst the most highly productive on the planet (eg Maize and sugar cane) and these plants are also more efficient in terms of both nitrogen and water use efficiencies. A major global effort of research in this area currently is to convert C3 species such as rice into a C4 plant. C4 species have evolved from the C3 ancestors on 65 separate occasions and for this reason it is believed that a relatively simple mechanism must underlie this process. However, despite much research the underlying processes have remained illusive.
The authors of this paper put forward a radial patterning model for Kranz development involving the shortroot (SHR) and scarecrow (SCR) genes, known to be part of a network specifying cell types in the roots of angiosperms. This new model proposes that SHR is expressed in bundle sheath cells, which leads to the movement of the SHR protein to adjacent cell layers, the interaction of SHR with SCR and other factors leads to cell specification. Four predictions, supported by experimental evidence, are proposed:

1. Expression of SHR should be similar in a range of C4 species. 
2. Ectopic expression of these genes in C3 species should alter the leaf anatomy. 
3. Genome wide phylogenetic studies of cis-regulatory elements should reveal C4 signatures 
4. Loss of function mutants should result in a change in the numbers of mesophyll and or increased numbers of bundle sheath cells. 

This paper together with approaches discussed in two further reviews in the Special Issue (Sage et al., From ProtoKranz to C4 Kranz: building the bridge to C4 photosynthesis and Lundgren et al., Deconstructing Kranz anatomy to understand C4 evolution) identify novel approaches to enable the manipulation of C3 crop species to use the C4 mechanism. 


Category: Plant Biology
Christine raines

Christine Raines

Christine Raines is a Professor of Plant and Molecular Physiology at the University of Essex as well as being Editor in Chief of the Journal of Experimental Botany. Christine is also a member of the SEB Plant Sections Committee and the SEB Council.