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et al.
2011
; Guo and Gan
2012
; Masclaux-Daubresse et al.
2005
; Watanabe
et al.
2013
), and numerous targeted investigations have greatly expanded our
understanding of the shift from anabolic to catabolic metabolism and senescence-
associated gene (SAG and SDG) expression changes.
Arabidopsis thaliana
is a long-day plant with a basal rosette and a life cycle of
about 2 months belonging to the brassicaceae family (Fig.
8.2a
). Older leaves
successively senesce during development in a highly ordered physiological process
with new appearing leaves acting as sinks. Upon inflorescence formation, a set of
upper rosette leaves as well as small leaves at the flower stem are generated. As
seeds ripen in siliques, the leaves further senesce and finally die off. The senescence
process at the level of genes or metabolites is the same in all leaves and appears to
follow a time shift according to the sequence of emergence (Breeze et al.
2011
;
Watanabe et al.
2013
). Even within a single leaf, a trajectory of senescence is
established along the basipetal axis with the (older) tip senescing earlier than the
base.
The characteristic shift from anabolic to catabolic processes is readily apparent
when analysing the metabolic composition of senescing plants or tissues over a time
course (Fig.
8.2c
; Watanabe et al.
2013
). Over the course of senescence, chloro-
phyll contents and chloroplast lipids such as mono- and di galactosyldiglycerides
(MGDGs and DGDGs) decrease, which point to the long-described senescence-
associated chloroplast degradation and decrease in photosynthetic capacity. Despite
this fact, the major sugars remain constant (sucrose) or increase (glucose and
fructose) in senescing leaves, and storage lipids such as triacylglycerides (TAGs)
increase. Protein contents are reduced due to proteolysis. Free branched-chain
amino acids, aromatic amino acids, and stress-related amino acids such as proline,
beta-alanine, and gamma-aminobutyric acid (GABA) accumulate. Conversely, the
major N-containing amino acids glutamine, arginine and their precursors glutamate
and aspartate decrease, probably due to export to sinks.
Furthermore, when looking at nutrient ions, nitrate and phosphate are efficiently
exported from the senescing leaf. In addition to inorganic phosphate, nucleic acids
are a likely source of phosphate for export, as indicated by the induction of RNases
(Himelblau and Amasino
2001
; Morcuende et al.
2007
; Lers et al.
2006
, Watanabe
et al.
2013
) and the concomitant induction of phosphate transporters (Chapin and
Jones
2009
). The regulatory response network of
Arabidopsis
to phosphate
Fig. 8.2 (continued) are presented as fold-change from stage 1 in log2 scale. No change to stage
1 is indicated by the
dashed line
, decrease by negative and increase by positive values. Typically
chlorophyll contents are reduced as well as chloroplast associated lipids as mono- and di
galactosyldiglycerides (MGDS, DGDS) and storage lipids as triacylglycerides (TAGs) accumu-
late. Protein degradation is accompanied by typical accumulation patterns of amino acids, most
importantly the phloem-transported amino acids (asp, glu, gln, and arg) are reduced, while
branched chain amino acids rather accumulate. Sulfur-rich glucosinolates are reduced and reduced
sulfur probably mobilized to the seeds and the nutrient ions phosphate and nitrate are mobilised to
the seeds, while sulfate is not mobilised. Sugars and anthocyanins as well as stress related amino
acids accumulate, these latter processes being linked to senescence induced reactive oxygen
species (ROS) accumulation. The slight increase of most metabolites at the end of the senescence
phase is due to a decrease of fresh weight due to water loss (wilting)
