the lie of kkplan

 
the lie of kkplan

Expression of cyanobacterial ictB in higher plants
"enhanced" photosynthesis and growth
Judy Lieman-Hurwitz^ Leonid Asipov Shimon Rachmilevitch\ Yehouda Marcus^ and
Aaron Kaplan^
'Department of Plant and Environmental Sciences, The Hebrew University of Jerusalem,
91904 Jerusalem, Israel
^Department of Plant Sciences, Tel Aviv University, 69978 Tel Aviv, Israel
Summary. Under many environmental conditions plant photosynthesis and growth are
limited by the availability of CO2 at the site of ribulose 1, 5-bisphosphate carboxylase/oxygenase (RubisCO). We expressed ictB, a gene involved in HCO3" accumulation
in Synechococcus sp. PCC7942, in higher plants. The transgenic Arabidopsis thaliana
and Nicotiana tabacum plants exhibited significantly faster photosynthetic rates than the
wild types under limiting, but not under saturating CO2 concentrations. Similar results
were obtained m Arabidopsis plants bearing ictB from Anabaena sp. PCC7120. Growth
of transgenic A. thaliana plants maintained under low humidity was considerably faster
than that of the wild type. There was no difference in the amount of RubisCO or the activity of the enzyme activated in vitro in the wild types and the transgenic plants. In contrast, the in vivo RubisCO activity, without prior activation of the enzyme, in plants
grown under low humidity was considerably higher in /c/j5-expressing plants than in their
wild types. The CO2 compensation point in the transgenic plants was lower than in the
wild types suggesting a higher CO2 concentration in close proximity to RubisCO. This
may explain the higher activation level of RubisCO and enhanced photosynthesis and
growth in the transgenic plants. These data indicated a potential use of ictB for the stimulation of crop yield.
Key words. Growth, IctB, Inorganic carbon. Photosynthesis
1. Introduction
Plants that belong to the physiological C4 or the Crassulacean acid metabolism groups
possess biochemical CO2 concentrating mechanisms (CCM, Cushman and Bohnert 2000;
Hatch 1992) whereas in many photosynthetic microorganisms a biophysical CCM is
functional (Kaplan and Reinhold 1999). CCM mechanisms enable these organisms to
raise the concentration of CO2 in close proximity to RubisCO and hence overcome, at
least partly, the low affinity of the enzyme for CO2. In contrast, plants that belong to the
C3 group, including most crop plants, do not possess this ability. Therefore, under many
environmental conditions plant photosynthesis is rate-limited by the concentration of CO2
at the carboxylation site and/or by the activity of RubisCO. Attempts are being made to
raise the apparent photosynthetic affinity of C3 plants for CO2 by various biotechnologiPlant Responses to Air Pollution and Global Change
Edited by K. Omasa, L Nouchi, and L J. De Kok ( Springer-Verlag Tokyo 2005 )
134 J. Lieman-Hurwitz et al.
cal approaches. These include expression of genes involved in C4 metabolism within C3
plants (Matsuoka et al. 2001; Surridge 2002); a search for a RubisCO that exhibits an
elevated specificity for CO2 among natural photosynthetic populations (Tabita 1999); site
directed modifications of the enzyme (Spreitzer and Salvucci 2002); and expression of a
cyanobacterial gene encoding fructose-l, 6/sedoheptulose-l, 7-bisp hospate phosphatase
thereby raising the level of intermediates of the Calvin cycle (Miyagav^a et al. 2001).
2. Results and discussion
High-C02-requiring mutants of the cyanobacterium Synechococcus sp. strain PCC 7942
(hereafter Synechococcus) were raised by transformation with an inactivation library.
One of them, IL-2, was severely impaired in the ability to accumulate inorganic carbon
internally, implicating IctB in this activity (Bonfil et al., 1998).
Analysis of the IctB sequence, well conserved among cyanobacteria, suggested that it
is a hydrophobic protein presumably with 10 trans-membrane domains. Interestingly, a
hydrophilic region (Fig. 1) showed very high conservation in homologous genes from
various cyanobacteria. The exact role of IctB is not known since it was not possible to directly inactivate it or its homologue slrl515 from Synechocystis sp. strain PCC 6803.
IctB LVAVLGLEPUWRVLSIFVGREDSSMiFRINVWUW^   
Anabaena 7120 LIAVIJVEPVRFRVLSIFAI»QPSSNNraRNVWDA    VF
Nostoc punc. UAWFVEEVRIi?VFSIFAIH3DSSWFRRNVVCAVF   
TriGhodesmiim IIJWVlXEPUOM;VFAGRQPSSWFTMJVVWS^
Slrl515 GGALIAVEPIRLRA^6IFAGREDSSW^FK[^Ml«E    )G^^
TlTermosynechcxxcjcus MCTIVSVPPU?ERAASIFVARC3DSSWFRDJVVf1    AM3
Prodhlorocxxxxis 9313 VIAATQIEPIimtriSLIAGRSDSSNNFRINVWES    SL
SynechcxxxDCUs WH MAriai)PIRIRVLSLVftGRGDSSMJFimJVWI^   
• ilfjilf^ It ^ ^ ^ It 4r4r4rA4t4r4r •• •
IctB aGQroPWLGIGPGNTAFNLVYPLYQQ-ARPrALS
Anabaena 7120 EMIRDRPIIGIGP(M«F1«VYPLY3^-PRYSALS
Nostoc punc. EKIRDRPIPGIGPGHNSFT«VyPLYQH-PRXTALS   
Trichodesmion Dtm?i:M>ILGIGPCM>VFT«aYPLYQ|^-PRYSA    LS
31x1515 AMIRARPIIGIGPGNEAFH}IYPYYMR-PRFTALS   
TheiiriDsyneGhDCOocus (JO«ARPWIJGIGPC3^VAFN3IYPLYCVNVRFIM    ^
ProGhlorocoocus 9313 EMIQARPWII5IGP(a^AAFT^RIYPIJ\X^PRFl    JALS
Synedhoooccus WH EMVQP?PWLGIGP(1JAAFNSIYPLYQQ-PKFDAL    S
Fig. 1. Alignment of a hydrophilic region (positions 301-372) in IctB from Synechococcus PCC
7942 to homologous genes from Anabaena sp. strain PCC 7120, Nostoc punctiforme, Trichodesmium erythraeum, Slrl515 from Synechocystis PCC 6803, Thermosynechococcus elongates BP-1, Prochlorococcus marinus MIT9313 and Synechococcus WH 8102.