2004, 2006). This suspicion was reinforced by the fact that exogenous cytokinin
application to vertical roots induced root bending towards the application site (Aloni
et al. 2004).
Ethylene also interacts with auxin to regulate root gravitropism. This relationship
has been firmly established from the discovery that the agravitropic Arabidopsis pin2
mutant was allelic with the ethylene insensitive mutants ein1 and agr1 (Müller et al.
1998). This interaction is complicated and multi-faceted, affecting not only auxin
efflux but the biosynthesis and signaling of both hormones. Furthermore, although
the agravitropic phenotype of pin2/agr1/ein1 is specific to the root, the overall
influence of the auxin/ethylene interaction is broad, not only affecting gravitropism,
but also regulating other important processes which respond to a mixture of external
and internal stimuli such as bending of the hypocotyl hook in etiolated seedlings,
root elongation and root hair development (Stepanova and Alonso 2005; Stepanova
et al. 2007). The regulation of auxin biosynthesis by ethylene is an aspect of the
relationship between the two hormones which has also been difficult to parse, with
ethylene inducing the expression of auxin biosynthetic genes, such as WEAK
ETHYLENE INSENSITIVE 2 (WEI2)/ANTHRANILATE SYNTHASE α1
(ASA1), WEI7/ASB1, WEI8/TRYPTOPHAN AMINOTRANSFERASE OF
ARABIDOPSIS 1 (TAA1)/TRANSPORT INHIBITOR RESPONSE2 (TIR2) and
its homolog TAR1 (Stepanova et al. 2005, 2008). Ethylene promotes auxin transport
in the root in both directions by upregulating the expression of several transcripts
encoding auxin transporters, including PIN1, PIN2, PIN4 and AUX1 (Ruzicka et al.
2007; Negi et al. 2008; Vandenbussche et al. 2010; Lewis et al., 2011; Muday et al.
2012).
Ethylene is synthesized from its precursor 1-aminocyclopropane-1-carboxylic
acid (ACC), which is, in turn, synthesized by ACETYL-COA SYNTHETASES
(ACS). Auxin induces the expression of ACS genes, increasing the production of
ethylene (Woeste et al. 1999; Tsuchisaka and Theologis 2004). Crosstalk between
auxin and ethylene has been most clearly demonstrated in a series of experiments
which showed that by removing specific auxin and ethylene responses by using aux1
and ein2 genotypes respectively, underlying cross-regulation by ethylene and auxin
could be observed. (Stepanova et al. 2007). A picture is emerging whereby auxin
mutants also lose sensitivity to ethylene, but ethylene mutants retain their sensitivity
to auxin. Such observations suggest that ethylene signaling acts through auxin and
not vice versa (Vandenbussche et al. 2012). This conclusion appears to be borne out
as the EIN3-dependent ethylene response in the root transition zone requires high
auxin activity (Stepanova et al. 2007), as accumulation of the protein is enhanced by
auxin. The auxin-mediated repression of two F-box proteins, EBF1 and EBF2 has
been implicated here as both proteins mediate EIN3 degradation (He et al. 2011).
The synergistic impact of ethylene and auxin on the asymmetric growth of the
gravitropic root also involves cytokinin. Street and colleagues (Street et al. 2016)
reported that the aforementioned cytokinin regulation of root cell elongation occurs
through ethylene-dependent and -independent mechanisms, both hormonal signals
converging on AUX1 as a regulatory hub (Street et al. 2016).
7.9 Interaction Between Auxin and Other Hormones
103
application to vertical roots induced root bending towards the application site (Aloni
et al. 2004).
Ethylene also interacts with auxin to regulate root gravitropism. This relationship
has been firmly established from the discovery that the agravitropic Arabidopsis pin2
mutant was allelic with the ethylene insensitive mutants ein1 and agr1 (Müller et al.
1998). This interaction is complicated and multi-faceted, affecting not only auxin
efflux but the biosynthesis and signaling of both hormones. Furthermore, although
the agravitropic phenotype of pin2/agr1/ein1 is specific to the root, the overall
influence of the auxin/ethylene interaction is broad, not only affecting gravitropism,
but also regulating other important processes which respond to a mixture of external
and internal stimuli such as bending of the hypocotyl hook in etiolated seedlings,
root elongation and root hair development (Stepanova and Alonso 2005; Stepanova
et al. 2007). The regulation of auxin biosynthesis by ethylene is an aspect of the
relationship between the two hormones which has also been difficult to parse, with
ethylene inducing the expression of auxin biosynthetic genes, such as WEAK
ETHYLENE INSENSITIVE 2 (WEI2)/ANTHRANILATE SYNTHASE α1
(ASA1), WEI7/ASB1, WEI8/TRYPTOPHAN AMINOTRANSFERASE OF
ARABIDOPSIS 1 (TAA1)/TRANSPORT INHIBITOR RESPONSE2 (TIR2) and
its homolog TAR1 (Stepanova et al. 2005, 2008). Ethylene promotes auxin transport
in the root in both directions by upregulating the expression of several transcripts
encoding auxin transporters, including PIN1, PIN2, PIN4 and AUX1 (Ruzicka et al.
2007; Negi et al. 2008; Vandenbussche et al. 2010; Lewis et al., 2011; Muday et al.
2012).
Ethylene is synthesized from its precursor 1-aminocyclopropane-1-carboxylic
acid (ACC), which is, in turn, synthesized by ACETYL-COA SYNTHETASES
(ACS). Auxin induces the expression of ACS genes, increasing the production of
ethylene (Woeste et al. 1999; Tsuchisaka and Theologis 2004). Crosstalk between
auxin and ethylene has been most clearly demonstrated in a series of experiments
which showed that by removing specific auxin and ethylene responses by using aux1
and ein2 genotypes respectively, underlying cross-regulation by ethylene and auxin
could be observed. (Stepanova et al. 2007). A picture is emerging whereby auxin
mutants also lose sensitivity to ethylene, but ethylene mutants retain their sensitivity
to auxin. Such observations suggest that ethylene signaling acts through auxin and
not vice versa (Vandenbussche et al. 2012). This conclusion appears to be borne out
as the EIN3-dependent ethylene response in the root transition zone requires high
auxin activity (Stepanova et al. 2007), as accumulation of the protein is enhanced by
auxin. The auxin-mediated repression of two F-box proteins, EBF1 and EBF2 has
been implicated here as both proteins mediate EIN3 degradation (He et al. 2011).
The synergistic impact of ethylene and auxin on the asymmetric growth of the
gravitropic root also involves cytokinin. Street and colleagues (Street et al. 2016)
reported that the aforementioned cytokinin regulation of root cell elongation occurs
through ethylene-dependent and -independent mechanisms, both hormonal signals
converging on AUX1 as a regulatory hub (Street et al. 2016).
7.9 Interaction Between Auxin and Other Hormones
103