Agriculture Reference
In-Depth Information
authors suggest that this mechanism is not only active during nutritional stress, but
also has a housekeeping role. In addition, autophagy was demonstrated to be
responsible for delivering endoplasmic reticulum (ER) to the vacuole during ER
stress (Liu et al. 2012 ).
Some differences in autophagy seem to depend on the plant species (Bassham
et al. 2006 ; Toyooka et al. 2006 ). These observations point to the possible existence
of lysosome/endosome-like organelles in plants such as tobacco (which also fuse
with the central vacuole) but not in most species such as barley or Arabidopsis .
Toyooka et al . ( 2006 ) observed two transport pathways in BY-2 tobacco cells:
direct delivery of autophagosomes to the central vacuole (in the absence of E-64
autophagy inhibitor) versus engulfment of autophagosomes by small vacuoles after
E-64 treatment. This second pathway may be a consequence of incomplete
autophagy inhibition, which could be solved with 3MA (3-methyl-adenine) but
further research on this possibility is required. It is possible that the time of
observation of E-64 treated plants has to be varied (to extract the same conclusions)
depending on the generation time of the plant. In other words, 24 h of E-64
treatment for Arabidopsis (a plant with a 3 month generation time) may have
proportionally stronger effect than such treatment for tobacco (a plants with
6 month generation time). Another reason for discrepancies may be the comparison
of protoplast or BY-2 cells obtained results with those derived from plant organs.
Genes for Plant Autophagy and Typical Phenotypes of atg
Mutants
Nomenclature of autophagy related genes (ATG, some were previously called
APG ) originates from yeast heterologous comparisons (Bassham et al. 2006 ), occa-
sionally leading to confusion between unique yeast and metazoan proteins with
plant ones. A common subset of over 30 proteins is required for all forms of
autophagy (Table 7.1 ). The significant subset of these proteins represent the com-
ponents of the so called “core autophagy” machinery required for both selective and
non-selective autophagy (Lynch-Day and Klionsky 2010 ). These proteins belong to
five large complexes responsible for the subsequent steps of autophagosome for-
mation, namely induction, nucleation and extension. In addition to this “core”
subset more proteins are required for certain pathways. Some proteins are present
only in one group of organisms and it is difficult to identify their orthologues
because of low sequence conservation. These proteins are involved in, so called,
selective autophagy and perform a quality control function by distinguishing
between cargoes ready for degradation and their functional counterparts (see
review: Schreiber and Peter 2013 ). Mutations in ATG proteins impair autophagy
in different ways. The effects depend to which complex the impaired protein
belongs. For example, atg5 or atg7 mutations cause the accumulation of
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