Agriculture Reference
In-Depth Information
downstream pathway proteins and speed up the plant life span (Minina et al.
2013
).
Some typical phenotypes of
Arabidopsis
autophagy mutants are shown in Table
7.1
.
Role of Autophagy in Plant Response to Nutrient Deficiency
Plants deficient in autophagy (
atg
mutants) do not show evident phenotypic differ-
ences from wild type during normal non-stress conditions (see Table
7.1
). How-
ever, some
atg
mutants show accelerated life cycle and early senescence when the
light intensity is low and during short day photoperiods (Thompson et al.
2005
).
This suggests that a basal or constitutive autophagy is important for normal plant
growth (Minina et al.
2013
). The senescence features, such as leaf yellowing in a
consequence of Rubisco breakdown and chlorophyll dismantling, are more evident
in
atg
mutants than in the wild type grown under nitrogen (N) or carbon
(C) deficiency (Doelling et al.
2002
; Hanaoka et al.
2002
; Thompson et al.
2005
;
Phillips et al.
2008
). Apparently, autophagy is important for recycling and deliver-
ing nutrients to the reproductive organs during senescence and as such it can be
considered as a nutrient redistribution process. In addition, autophagy plays an
important role in N management at the whole-plant level through the control of
remobilisation, under both limiting and ample nitrate conditions (Guiboileau
et al.
2012
). For example, heterologous expression of soybean ATG8c in
Arabidopsis
led to better performance of the transgenic lines under both starvation
and normal conditions (Xia et al.
2012
). Although the protein and soluble sugar
concentrations were similar in the wild type and transgenic line, the fresh weight of
the transgenic lines was significantly larger in both, N sufficient and N deficient
conditions. The transgenic plants survived the period of carbon-limiting conditions
induced by extended darkness much better than the wild type and promptly
recovered and resumed growth when moved back to normal conditions. Compar-
ison of the growth parameters in soil under a long-day photoperiod indicated that
the transgenic plants grew faster than the wild type plants, reached a larger size
before bolting, entered the reproductive stage slightly earlier, and produced more
flowers, siliques and seeds. The authors concluded that over-expression of soybean
ATG8 in
Arabidopsis
promoted the vegetative growth and facilitated the transition
into reproductive stage.
Similarly,
Arabidopsis
plants overproducing ATG8 protein fused on its N
terminus to green fluorescent protein (GFP) showed enhanced growth and leaf
size, accelerated flowering under nutrient-limiting and short-day growth, however
they were also slightly more sensitive to mild salt and osmotic stress (Slavikova
et al.
2008
). This study also suggested that ATG8 participates in cytokinin-
mediated root-shoot communication possibly by sequestration of proteins involved
in cytokinin transport or signaling.
It has been shown that ATG18a protein from
Arabidopsis
is required for the
formation of autophagosomes under nitrogen deficiency and sucrose starvation
(Xiong et al.
2005
). Interestingly,
the mutants with silenced expression of
