regulated by multiple kinases. Although it has previously been proposed that specific
serine residues are modified by specific kinases either to control auxin flux or PIN
localization (Michniewicz et al. 2007; Zourelidou et al. 2014), recent studies using
phospho-specific antibodies show that the same sites are targeted by kinases which
regulate each process (for example D6PK and PINOID), meaning the regulation is
likely to be complex, dynamic and context-specific (Weller et al. 2017).
Other regulatory mechanisms are likely also to be at work. For example, it has
been known for decades that the localized synthesis and directed transport of
flavonols, plant specific phenolic compounds, modulate auxin transport and
gravitropism (Buer and Muday 2004; Buer et al. 2007). Experiments with the
synthetic polar auxin transport inhibitor (and functional flavonol analog) N-1naphthylphthalamic acid (NPA) and transparent testa mutants with altered flavonol
levels revealed altered gravitropic responses (Taylor and Grotewold 2005; Teale and
Palme 2018). Hence application of nanomolar levels of the flavonol quercetin to the
pin2 mutants with strong defects in gravitropic response was sufficient to restore
wild type-like auxin distribution patterns and partially restore the gravitropic
response (Santelia et al. 2008). The question of whether NPA and flavonols can be
considered to be functionally equivalent with respect to the gravity response is fairly
complex and considered elsewhere (Teale and Palme 2018). However, observations
may suggest that flavonoids possibly exert their function by directly binding to PIN
proteins and regulating their auxin transport capacity and subcellular localization
(Buer and Muday 2004; Kuhn et al. 2017).
Other proteins possibly involved in polar auxin transport and gravitropic response
are ATP-binding cassette (ABC) transporter family proteins (Nagashima et al. 2008)
with the anion channel blocker 5-nitro-2-(3-phenylpropylamino)-benzoic acid
inhibited ABC-dependent transport activity in a heterologous assay and the root
gravitropic response (Cho et al. 2014). Mechanistically it has been suggested that
members of this family may directly form complexes with PIN proteins on the
protein level, but solid evidence is yet lacking to back up this hypothesis.
7.9 Interaction Between Auxin and Other Hormones
Several lines of evidence suggest auxin works in concert with other plant hormones
to regulate root gravitropism (Philosoph-Hadas et al. 2005). For example, gibberellic
acid (GA) shows asymmetric action during gravitropic responses (Löfke et al. 2013).
GA signaling at the lower side of the root stabilizes the auxin transporter
PIN-FORMED2 (PIN2) at the plasma membrane through a specific GA effect on
protein trafficking lytic vacuoles, hence suggesting an interplay between asymmetric
auxin and gibberellin activities in the modulation of auxin fluxes for root gravitropic
responses.
Besides GA, it was shown that cytokinin synthesized in root cap cells redistributes towards the lower side of the gravistimulated root within minutes, suggesting
that cytokinin is acting as early during root gravitropism as well as auxin (Aloni et al.
102
7 Gravitropism in Higher Plants: Molecular Aspects
serine residues are modified by specific kinases either to control auxin flux or PIN
localization (Michniewicz et al. 2007; Zourelidou et al. 2014), recent studies using
phospho-specific antibodies show that the same sites are targeted by kinases which
regulate each process (for example D6PK and PINOID), meaning the regulation is
likely to be complex, dynamic and context-specific (Weller et al. 2017).
Other regulatory mechanisms are likely also to be at work. For example, it has
been known for decades that the localized synthesis and directed transport of
flavonols, plant specific phenolic compounds, modulate auxin transport and
gravitropism (Buer and Muday 2004; Buer et al. 2007). Experiments with the
synthetic polar auxin transport inhibitor (and functional flavonol analog) N-1naphthylphthalamic acid (NPA) and transparent testa mutants with altered flavonol
levels revealed altered gravitropic responses (Taylor and Grotewold 2005; Teale and
Palme 2018). Hence application of nanomolar levels of the flavonol quercetin to the
pin2 mutants with strong defects in gravitropic response was sufficient to restore
wild type-like auxin distribution patterns and partially restore the gravitropic
response (Santelia et al. 2008). The question of whether NPA and flavonols can be
considered to be functionally equivalent with respect to the gravity response is fairly
complex and considered elsewhere (Teale and Palme 2018). However, observations
may suggest that flavonoids possibly exert their function by directly binding to PIN
proteins and regulating their auxin transport capacity and subcellular localization
(Buer and Muday 2004; Kuhn et al. 2017).
Other proteins possibly involved in polar auxin transport and gravitropic response
are ATP-binding cassette (ABC) transporter family proteins (Nagashima et al. 2008)
with the anion channel blocker 5-nitro-2-(3-phenylpropylamino)-benzoic acid
inhibited ABC-dependent transport activity in a heterologous assay and the root
gravitropic response (Cho et al. 2014). Mechanistically it has been suggested that
members of this family may directly form complexes with PIN proteins on the
protein level, but solid evidence is yet lacking to back up this hypothesis.
7.9 Interaction Between Auxin and Other Hormones
Several lines of evidence suggest auxin works in concert with other plant hormones
to regulate root gravitropism (Philosoph-Hadas et al. 2005). For example, gibberellic
acid (GA) shows asymmetric action during gravitropic responses (Löfke et al. 2013).
GA signaling at the lower side of the root stabilizes the auxin transporter
PIN-FORMED2 (PIN2) at the plasma membrane through a specific GA effect on
protein trafficking lytic vacuoles, hence suggesting an interplay between asymmetric
auxin and gibberellin activities in the modulation of auxin fluxes for root gravitropic
responses.
Besides GA, it was shown that cytokinin synthesized in root cap cells redistributes towards the lower side of the gravistimulated root within minutes, suggesting
that cytokinin is acting as early during root gravitropism as well as auxin (Aloni et al.
102
7 Gravitropism in Higher Plants: Molecular Aspects