acids present in cytosol or circulating in the apoplast. In this regard, plant GLRs
could act as an internal and external N sensor system and play a major role in the
regulation of amino acids biosynthesis or their compartmentalization in different
organelles in relation to the extracellular flows of amino acids.
5 Relationship Between Ethylene and N Metabolism:
The Central Role of Aminotransferases
A close examination of Fig. 3 indicates that no less than six PLP-dependent aminotransferases, namely AAT/PAT, OASTL, CgS, CbL, ACS, and an unknown AAT,
are required for ethylene biosynthesis and methionine salvage pathways (Fig. 3).
Therefore, among the different plant amino acid pathways (Fig. 1), the methionine
biosynthetic pathway needs the highest involvement of aminotransferases for its
biosynthesis and is highly dependent on the availability in PLP cofactor (vitamin
B 6 ). This ascertainment is exemplified in recent studies of pyridoxine synthase1
enzyme involved in PLP biosynthesis and encoded by two paralogous genes, namely
PDX1.1 and PDX1.3 in Arabidopsis (Chen and Xiong 2009a, b; Boycheva et al.
2015). Disruption of either of these genes in single mutants results in vitamin B 6
deficiency and a differential reduction of ethylene production and auxin levels while
disruption of both genes in double mutants is lethal for seedlings. At the phenotypic
level, these mutants displayed impairment in root growth, reduction of meristem
size, and altered root cell division and elongation (Chen and Xiong 2009a, b; Titiz
et al. 2006). However, the single pdx1.3 mutant is more impaired in root development than the pdx1.1 mutant. These phenotypic differences are mainly explained by
the presence of distinct regulatory elements in the upstream region of both genes that
lead to a deficit in ethylene production and/or signaling. Thus, PDX1.1 expression is
repressed by sucrose and its promoter possesses a sugar response element whereas
PDX1.3 promoter contains ethylene and auxin response elements. Several evidences
indicate that some components of the ethylene signaling pathway act upstream of the
effect of auxin biosynthesis and transport (Růžička et al. 2007; Boycheva et al.
2015). Indeed, the root phenotype of the pdx1.3 mutant is partially restored by ACC
treatment but not by auxin treatment (Chen and Xiong 2009a, b; Boycheva et al.
2015). These results perfectly demonstrate that PLP availability plays a central role
in ethylene biosynthesis and probably the methionine pathway as well as in phytohormone homeostasis. However, as previously mentioned it is not known how de
novo PLP synthesis in cytosol or PLP salvage pathway modulate PLP pool in the
plastid and which plastidial carriers are involved in the transport of different forms of
vitamin B 6 (Gerdes et al. 2012).
From Aspartate to Ethylene: Central Role of N, C, and S Shuttles by. . .
271
could act as an internal and external N sensor system and play a major role in the
regulation of amino acids biosynthesis or their compartmentalization in different
organelles in relation to the extracellular flows of amino acids.
5 Relationship Between Ethylene and N Metabolism:
The Central Role of Aminotransferases
A close examination of Fig. 3 indicates that no less than six PLP-dependent aminotransferases, namely AAT/PAT, OASTL, CgS, CbL, ACS, and an unknown AAT,
are required for ethylene biosynthesis and methionine salvage pathways (Fig. 3).
Therefore, among the different plant amino acid pathways (Fig. 1), the methionine
biosynthetic pathway needs the highest involvement of aminotransferases for its
biosynthesis and is highly dependent on the availability in PLP cofactor (vitamin
B 6 ). This ascertainment is exemplified in recent studies of pyridoxine synthase1
enzyme involved in PLP biosynthesis and encoded by two paralogous genes, namely
PDX1.1 and PDX1.3 in Arabidopsis (Chen and Xiong 2009a, b; Boycheva et al.
2015). Disruption of either of these genes in single mutants results in vitamin B 6
deficiency and a differential reduction of ethylene production and auxin levels while
disruption of both genes in double mutants is lethal for seedlings. At the phenotypic
level, these mutants displayed impairment in root growth, reduction of meristem
size, and altered root cell division and elongation (Chen and Xiong 2009a, b; Titiz
et al. 2006). However, the single pdx1.3 mutant is more impaired in root development than the pdx1.1 mutant. These phenotypic differences are mainly explained by
the presence of distinct regulatory elements in the upstream region of both genes that
lead to a deficit in ethylene production and/or signaling. Thus, PDX1.1 expression is
repressed by sucrose and its promoter possesses a sugar response element whereas
PDX1.3 promoter contains ethylene and auxin response elements. Several evidences
indicate that some components of the ethylene signaling pathway act upstream of the
effect of auxin biosynthesis and transport (Růžička et al. 2007; Boycheva et al.
2015). Indeed, the root phenotype of the pdx1.3 mutant is partially restored by ACC
treatment but not by auxin treatment (Chen and Xiong 2009a, b; Boycheva et al.
2015). These results perfectly demonstrate that PLP availability plays a central role
in ethylene biosynthesis and probably the methionine pathway as well as in phytohormone homeostasis. However, as previously mentioned it is not known how de
novo PLP synthesis in cytosol or PLP salvage pathway modulate PLP pool in the
plastid and which plastidial carriers are involved in the transport of different forms of
vitamin B 6 (Gerdes et al. 2012).
From Aspartate to Ethylene: Central Role of N, C, and S Shuttles by. . .
271