Agriculture Reference
In-Depth Information
(Kanki et al.
2009
). In plants, much less work on mitophagy has been performed.
Electron microscopy pictures of mitochondria being fused to acidic compartments
(presumably autophagosomes) suggest that mitophagy also takes place in plant
cells (Toyooka et al.
2006
). However, autophagosomes colocalise preferentially
with protein aggregates rather than with mitochondria. The autophagy postulated
by Toyooka et al. (
2006
) seems to be independent of starvation. Although the role
of mitochondria in oxidative stress-induced autophagy and the process of
mitophagy in plants have been recently reviewed (Minibayeva et al.
2012
), further
studies on causes of mitophagy in higher plants are desirable.
Literature related to plant pexophagy (selective degradation of peroxisomes) is
limited. It has been shown that the process of pexophagy contributes to virulence of
fungal pathogen,
Colletotrichum orbiculare
, since the
atg26
mutant of this patho-
gen, specifically affected in peroxisomes failed to penetrate the host cells (Asakura
et al.
2009
).
Autophagic degradation of chloroplastic material (chlorophagy) via Rubisco-
containing bodies (RCBs) is specifically linked to carbon deficiency (caused by
darkness) but not to nitrogen deficiency. This process supports energy availability
at night for normal growth by providing amino acids (Izumi et al.
2013
). On the
other hand, autophagy also contributes to leaf starch degradation at night (Wang
et al.
2013
). This process takes place in vacuoles and is independent of the classic
chloroplast pathway of starch degradation. No proof for degradation of entire
chloroplasts is available. On the other hand, vacuolar localisation of the chloro-
plasts in protoplasts from the mesophyll leaves of the senescing wheat leaves seem
to support such possibility (Wittenbach et al.
1982
). Plastid degradation has mostly
been studied in the conditions of starvation or in leaves undergoing senescence. It
has been suggested that remobilisation of Rubisco in RCBs represents the first step
of chloroplasts degradation i.e. chloroplast downsizing due to RCB formation and
subsequent deposition into the vacuole (Wada et al.
2009
). Rubisco is the most
abundant plant protein and catalyses carboxylation and oxidation competing reac-
tions of photosynthesis. It is a substantial nitrogen source stored in chloroplasts that
the plant may use in case of N deficiency. Plants rely on chloroplasts to survive;
therefore they undergo senescence only when the day becomes short and the
respiratory period surpasses the photosynthetic period. During stress and senes-
cence a compromise is made between maintaining functional chloroplasts and
reusing the nutrients accumulated in the form of Rubisco (Wada et al.
2009
). The
extraplastidic degradation processes, namely chlorophagy and RCB rely on
autophagy, demonstrated as some autophagy-deficient mutants (
atg5
) do not
show RBCs (Ishida et al.
2008
). Moreover, it was recently shown that degradation
of stromal proteins takes place within chloroplasts largely via the autophagy-
independent process. However, autophagy is required for the complete degradation
of these proteins (Lee et al.
2013
).
The existence in plants of the ribophagy-like mechanism, targeting ribosomes
for recycling under normal growth conditions and similar to ribophagy observed in
yeast under nutrients deficiency, has been suggested (Hillwig et al.
2011
). The
