C4 Photosynthesis – Testing Candidate Maize Genes in Rice

It would be no surprise to regular readers of the Legume Laboratory that we hold significant interest in the possibility of engineering C4 photosynthesis into C3 crops. Projects such as the C4 Rice Project are dedicating their efforts to this goal and its promise of more efficient crop growth and improved food security in the face of a changing climate.

Scientific Reports recently published an article funded through the C4 Rice Project which was looking to take the next step in procuring this biological engineering feat.

Maize is an interesting plant as it contains the two forms of photosynthesis, utilising C4 photosynthesis in foliar leaves while husk leaves use C3 photosynthesis. Previous studies have compared the transcriptomes of the two types of maize leaves to identify differences in gene expression. Given both leaves contain the same genome, hidden within the 283 differences in gene expression should be those differences which convert a leaf from using C3 photosynthesis to using C4 photosynthesis.

Working from this base, the researchers selected 60 of these genes to analyse further. The choice to look at 60 genes instead of the 283 differently regulated genes identified was born of the lack of high throughput technology needed to test gene functions efficiently enough for all to be analysed. The 60 genes tested were selected as they were predominantly transcription factors or leucine rich repeat receptor like kinases. A maize ubiquitin promoter was ligated upstream of the each individual gene resulting in 60 gene constructs that were individually transformed into rice plants.

In order to identify whether the genes made any change to transgenic rice plants, the researchers compared the leaf anatomy of the transformed leaves to the wild-type leaves. Regression analysis of vein number and leaf width in rice plants found a linear relationship between the two (a wider leaf will have a greater number of veins proportional to the change in width when compared to a narrower leaf). Given the difference in cells surrounding veins in C4 photosynthetic plants compared to C3 plants, there is a difference in the space between veins in the two plant types. The hope was to identify changes in the ratio of vein number to leaf width in transformed plants, an indication of a possible change in the method of photosynthesis.

Results

The constitutive expression of 47 of the 60 genes made no observable difference to the ratio of veins to width of leaf normally seen in rice plants and the plants exhibited similar phenotypes to the wild type plants.

Three genes resulted in the transformed plantlets being unable to regenerate. The authors explain that rice tissue culture requires an excess of cytokinin to auxin in order for shoot growth to be promoted. Research into the three genes involved (ZmIDD16, ZmbHLH106 and ZmHCA2) revealed homologous genes in Arabidopsis that show evidence of effects on auxin biosynthesis. As a result, it was hypothesised that each of the three genes have resulted in an increase in auxin biosynthesis, throwing out the ratio of cytokinin to auxin needed for regeneration to occur, although further research was noted as necessary to confirm this hypothesis.

A different set of three genes disrupted shoot and root development, again believed to be due to alterations in the cytokinin/auxin ratio. The YUCCA gene seemingly caused the biosynthesis of excessive amounts of auxin, resulting in characteristic curled leaflets and hairy roots. A gene encoding a SMALL AUXIN UPREGULATED RNA (SAUR) disrupted the formation of lateral roots, with homologous genes demonstrating a disruption in normal auxin transport, reducing auxin-related root elongation and retarding initiation of lateral roots. The third gene (ZmSACL3) affected cytokinin, inhibiting its synthesis and resulting in similar phenotypes to that caused by the YUCCA gene due to the altered auxin/cytokinin ratio.

The overexpression of transcription factor R2R3 MYB caused intermediate veins to lose sclerenchyma cell connections to the abaxial epidermis. The loss of strengthening cells resulted in curled leaves. A different and novel gene resulted in the opposite effect on sclerenchyma formation, causing an increase in the proliferation of this cell type and resultant thicker walls than in wild type plants. Further, a couple of larger than usual bundle sheath cells around each vascular bundle formed. The result of the overexpression of this unnamed gene were infertile plants with short shoots and roots.

Figre 4 from C4 OX study

Figure 4 from article showing the effects of over expression of an as-yet unidentified gene. Figure b shows the reduced root growth while figure c shows the reduced shoot formation. Panel g shows normal sclerenchyma in wild type rice plants while panels h and l show the increased cell wall thickness described above.

ZmWRKY12, a gene which inhibited callus formation and therefore had to be inserted behind an inducible promoter to allow phenotypic analysis, caused dwarfing and reduced root growth in transgenic plants. The vein number to leaf width ratio was unaffected by the transgene but the normally lobed mesophyll cells in rice plants were lost in the transgenic lines. The orthologous gene in Arabidopsis is known to be involved in the formation of secondary cell walls, but these researchers have discovered that it also seems to play a role in cell lobing, a phenomenon for which the genetic basis was previously unknown.

Figure 5 from C4 OX study

Figure 5 from article showing the effects of ZmWRKY12 expressed in rice plants. The most interesting phenotype change was the change in mesophyll cell lobing from wild-type (panels d, f, and h) to the lack of lobing in the transgenic lines (panels e, g, and i).

Finally, three closely related Zinc finger nuclease genes showed normal vein spacing but caused spindly plants with drooping stems and leaves. It was theorised that the cause of the phenotype was due to alterations in the production gibberllic acid in the plants.

Conclusion

This paper is a great example of the incremental advancement of knowledge that builds upon earlier increments and is built upon by later increments. Those increments fill gaps in knowledge, or rule out particular cause/effect relationships, which help to direct and propel research towards its particular end (although, of course, research and enquiry need not always have a particular end to be directed at).

In the case of this research, the experiment resulted in some interesting and novel results, even though it didn’t yield any specific advance towards understanding the difference in gene regulation between the two forms of photosynthesis. That being said, we know that constitutive expression of any of 60 genes isn’t enough to trigger C4 photosynthesis. That leaves the remaining 223 genes to test, or perhaps a temporally regulated expression of one or more of the candidate genes, or some other combination of gene expression not yet contemplated.

The possible causes of C4 photosynthesis are numerous. This study also highlighted the deficiency in technology related to the efficient transformation, validation and phenotyping of transgenic plants. At present we lack the ability to quickly test over-expression of individual genes, and this is a relatively simple analysis to perform. The possibility that differential expression of a number of the genes being required to establish C4 photosynthesis requires significant advances in both wet lab and computation biology if we are to quickly make significant leaps in this type of research.

Great work by a great group of committed scientists.

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