7.3 Could the Under-expression of ACC Synthase Genes
Improve Nitrogen Use Efficiency?
Because a multigene family in Arabidopsis encodes ACS aminotransferases,
acs mutants of this specie, mutated at multiple loci, has been engineered to understand the function and regulatory roles of these proteins (Tsuchisaka et al. 2009).
Analyses of pentuple, hexuple, and octuple acs mutants demonstrated that in normal
conditions ethylene acts as growth repressor in dark- or light-grown plants since
pentuple and hexuple mutants are bushier and display significant greater height with
the progressive decrease in ethylene production. Indeed, ethylene is known to downregulate photosynthetic genes (Van Zhong et al. 2003). These acs mutants also
exhibited a delayed flowering time, a less response to gravity, and enhanced
susceptibility to the necrotrophic pathogen Botrytis cinerea. Among the mutants,
growth of the octuple acs mutant is delayed during the initial stage of development
and becomes taller and less bushy after 50 days of growth. Moreover, transcription
analyses of acs mutants confirmed that there exists a relationship between expression
of light signaling genes and ethylene biosynthesis. This study clearly highlights
the importance of spatial and temporal combination between ACS isoforms in
multiple ethylene-mediated physiological processes during growth and development. The multiple-locus acs mutants appear to be a valuable tool for deciphering
how ethylene biosynthesis interacts with nitrogen metabolism in response to different levels of nitrate availability and how decrease in ethylene production affects the
levels of Asp, Met, AdoMet, and PAs. In this respect, the crosstalk between nitrogen
nutrition and the ethylene plant hormone in Col-0 and 20 natural accessions of
Arabidopsis seedlings have been recently investigated (De Gernier et al. 2016).
Comparison of the behavior of the 20 accessions behavior revealed that changes in
root biomass and ethylene production were negatively correlated at 1 mM but
positively correlated at 10 mM nitrate. Greater ethylene release and root biomass
production under nitrogen limitation were mainly due to higher transcription levels
in the roots of ACS6 and of ACO2 and ACO4 genes, respectively. Taken together
these studies indicate that ethylene modulates plant morphology and biomass allocation probably in relation to nitrate availability.
This conclusion is in line with recent findings demonstrating that endogenous
glucose signals increase growth by promoting auxin signaling and by antagonizing
ethylene signaling through the glucose sensor HXK1 (Moore et al. 2003). Indeed,
glucose enhances the degradation of EIN3 and EIL1 (EIN3-Like1) components of
ethylene signaling through a proteasome-dependent mechanism controlled by
HXK1 (Yanagisawa et al. 2003; Yoo et al. 2008). During primary root growth in
Arabidopsis, EIN3 is known to exert a positive feedback loop on the expression of
anthranilate synthase and TAA1/TAR2 genes involved in auxin biosynthesis
whereas IAA enhances EIN3 stability by repressing EBF1/2(EIN3 Binding F-Box
protein1)-mediated degradation of EIN3 (He et al. 2011). Although HXK1 and TOR
kinase signaling systems in response to glucose seem to be mostly uncoupled
more studies are required to understand possible interactions between both sensory
systems (Sheen 2014).
280
E. Le Deunff
Improve Nitrogen Use Efficiency?
Because a multigene family in Arabidopsis encodes ACS aminotransferases,
acs mutants of this specie, mutated at multiple loci, has been engineered to understand the function and regulatory roles of these proteins (Tsuchisaka et al. 2009).
Analyses of pentuple, hexuple, and octuple acs mutants demonstrated that in normal
conditions ethylene acts as growth repressor in dark- or light-grown plants since
pentuple and hexuple mutants are bushier and display significant greater height with
the progressive decrease in ethylene production. Indeed, ethylene is known to downregulate photosynthetic genes (Van Zhong et al. 2003). These acs mutants also
exhibited a delayed flowering time, a less response to gravity, and enhanced
susceptibility to the necrotrophic pathogen Botrytis cinerea. Among the mutants,
growth of the octuple acs mutant is delayed during the initial stage of development
and becomes taller and less bushy after 50 days of growth. Moreover, transcription
analyses of acs mutants confirmed that there exists a relationship between expression
of light signaling genes and ethylene biosynthesis. This study clearly highlights
the importance of spatial and temporal combination between ACS isoforms in
multiple ethylene-mediated physiological processes during growth and development. The multiple-locus acs mutants appear to be a valuable tool for deciphering
how ethylene biosynthesis interacts with nitrogen metabolism in response to different levels of nitrate availability and how decrease in ethylene production affects the
levels of Asp, Met, AdoMet, and PAs. In this respect, the crosstalk between nitrogen
nutrition and the ethylene plant hormone in Col-0 and 20 natural accessions of
Arabidopsis seedlings have been recently investigated (De Gernier et al. 2016).
Comparison of the behavior of the 20 accessions behavior revealed that changes in
root biomass and ethylene production were negatively correlated at 1 mM but
positively correlated at 10 mM nitrate. Greater ethylene release and root biomass
production under nitrogen limitation were mainly due to higher transcription levels
in the roots of ACS6 and of ACO2 and ACO4 genes, respectively. Taken together
these studies indicate that ethylene modulates plant morphology and biomass allocation probably in relation to nitrate availability.
This conclusion is in line with recent findings demonstrating that endogenous
glucose signals increase growth by promoting auxin signaling and by antagonizing
ethylene signaling through the glucose sensor HXK1 (Moore et al. 2003). Indeed,
glucose enhances the degradation of EIN3 and EIL1 (EIN3-Like1) components of
ethylene signaling through a proteasome-dependent mechanism controlled by
HXK1 (Yanagisawa et al. 2003; Yoo et al. 2008). During primary root growth in
Arabidopsis, EIN3 is known to exert a positive feedback loop on the expression of
anthranilate synthase and TAA1/TAR2 genes involved in auxin biosynthesis
whereas IAA enhances EIN3 stability by repressing EBF1/2(EIN3 Binding F-Box
protein1)-mediated degradation of EIN3 (He et al. 2011). Although HXK1 and TOR
kinase signaling systems in response to glucose seem to be mostly uncoupled
more studies are required to understand possible interactions between both sensory
systems (Sheen 2014).
280
E. Le Deunff