Agriculture Reference
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(Oono et al. 2011 ). The over-production of citrate in transgenic tobacco (L ´ pez-
Bucio et al. 2000 ) as well as mitochondrial citrate synthase in A. thaliana (Koyama
et al. 2000 ) enhanced P i uptake. Key enzymes which have been studied in
A. thaliana are citrate synthase, malic enzyme and aconitase, which exhibited
variation in protein abundance between ecotypes during P deficiency (Chevalier
and Rossignol 2011). In other species, the activity of aconitase correlated with
organic acid secretion (Neumann and R ¨ mheld 1999 ) and in alfalfa, the
overexpression of malate dehydrogenase (MDH) resulted in increased P accumu-
lation (Tesfaye et al. 2001 ). Another approach was the expression of phytase genes
of alfalfa or of a fungal origin in Arabidopsis and tobacco plants, resulting in
improved acquisition of organic P sources (George et al. 2005 ). However, the
length of P starvation influences the synthesis and degradations of proteins which
are potentially involved in enhancing the plants adaptation to P deficiency. For
instance, genes encoding for isocitrate dehydrogenase were suppressed in rice roots
only after a certain time of exposure to P starvation, resulting in a suppression of
citrate degradation (Oono et al. 2011 ).
The replacement of phospholipids by galactolipids or sulpholipids is a well-
known adaptation process in plants during P deficiency (Andersson et al. 2003 ;
Hammond et al. 2003 ; Byrne et al. 2011 ), even if phospholipid degradation is
differently mediated in different species (Calder´n-V´zquez et al. 2011 ). For
instance, in potato, an array study identified novel roles for the main storage protein
in potato tubers, the patatin-like proteins, which also have lipase activity and are
potentially involved breakdown of phospholipids for P i recycling (Hammond
et al. 2011 ). Numerous studies in model plants or crops reported the induction of
genes related to an altered lipid metabolism, for example UDP-sulfoquinovose
synthase 1 (SQD1) or glycerophosphoryl diester phosphodiesterase (GDPD) or
lipid transfer proteins (Hammond et al. 2011 ; Oono et al. 2011 ; Morcuende
et al. 2007 ; Calder ´ n-V ´ zquez et al. 2008 ; Wasaki et al. 2003 ). Glyceropho-
sphodiester phosphodiesterases (GPX-PDE) catalyse the hydrolysis of phospho-
lipids to glycerol-3-phosphate and the corresponding alcohol. Recently, GPX-PDE
genes were identified which were highly expressed in cluster roots of white lupine
under P i -deficiency (Cheng et al. 2011 ; Uhde-Stone et al. 2003 ).
To date, the knowledge about functional consequences of replacing phospho-
lipids in membranes is very limited (Veneklaas et al. 2012 ).
Post-translational Modifications
Post-translational modifications are important factors in P signalling and metabolic
pathways involved in PUE (Alexova and Millar 2013 ; Plaxton and Tran 2011 ),
highlighted when comparing proteome with transcriptome studies in P-starved
maize (Calder´n-V´zquez et al. 2008 ; Li et al. 2008a , b ) and Arabidopsis
(Morcuende et al. 2007 ). A striking example underpinning the importance of
post-translational modifications within adaptation to low P exposure is the potential
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