Agriculture Reference
In-Depth Information
each other, systems biology approaches will be necessary to unravel these
relationships. Complicating this analysis, the biochemistry of a plant under nutrient
deprivation typically shows pleiotropic effects, which are difficult to directly
connect to the initial nutrient stress (Nikiforova et al.
2004
,
2005
). Metabolites
related to a distinct nutrient are often involved in diverse biochemical processes
different from the immediate assimilatory pathway. For example, sulfate starvation
results in reduced
S
-adenosylmethionine (SAM) levels in plants. As SAM is one of
the main methyl group donors for biochemical reactions, pleiotropic effects of
sulfate starvation are inevitable (Nikiforova et al.
2005
; Morcuende et al.
2007
).
Therefore, it is necessary to differentiate between early response processes, such
as the induction of high affinity transporters or assimilatory genes, later processes
triggered by already downstream effects (Fig.
8.3b
) and general processes such
as the NuDIS induced induction of SAGs (Figs.
8.2b
,
8.3b
, and
8.5b
).
Arabidopsis thaliana
, due to its amenability to systems biology approaches, has
been widely used to investigate the effects of single nutrient starvations on the
transcriptome (Wang et al.
2003
; Amtmann and Armengaud
2009
; Watanabe
et al.
2010
,
2013
) and the metabolome (Nikiforova et al.
2005
; Morcuende
et al.
2007
). Taken together, these studies reveal that responses to distinct nutrient
stresses overlap to a certain extent (Figs.
8.5a
and
8.6
), even when focusing on the
subfraction of the SAGs (Fig.
8.5b
). Some trends are obvious and corroborate
previous findings. During senescence protein and chlorophyll levels are reduced
(Fig.
8.2c
). Anthocyanin accumulation, a typical feature of stressed plants, is shared
across N, P, and S starvation due to the induction of the senescence-associated
transcription factor PAP1 (MYB75; Fig.
8.5a, b
; Tohge et al.
2005
; Watanabe
et al.
2010
,
2012
). Also, one of the most responsive genes to sulfate starvation, low
sulfur induced (LSU; Hubberten et al.
2012a
), is induced under all three nutrient
conditions compared here. LSU has been postulated to be involved in autophagy
(Zientara-Rytter et al.
2011
), a degradatory process, which mobilises nutrients from
cellular macromolecules, even organelles (Fig.
8.3b
) during senescence and nutri-
ent deficiency rescue programmes (Wu et al.
2012
). See also the chapter of
Collados-Rodriguez et al. in this topic.
The transcriptome responses to N, P, and S starvation overlap substantially
(Fig.
8.5
; Watanabe et al.
2012
). Among the 1,240 genes induced at least twofold
under phosphate starvation, 12 % are shared with sulfate and 43 % with nitrate
starvation, and from 1,602 sulfate starvation induced genes, 39 % are shared with
nitrate and 9 % with phosphate starvation. When assigning such expression profiles
to functional categories (Fig.
8.6
) using MAPMAN annotations (Thimm
et al.
2004
), complex patterns of up- and down-regulation within individual cate-
gories and even within gene families are observed which are not easy to interpret. It
becomes obvious that certain categories are over-represented,
e.g.
genes of photo-
synthesis and carbohydrate biosynthesis are down-regulated and that this is a
common trend for all nutrient starvations and also senescence (Fig.
8.6
). Further-
more, the strength of the response mirrors the severity of symptoms (Figs.
8.5a
and
8.6
) as nitrogen starvation is usually stronger than phosphate, followed by sulfate
starvation (Wulff-Zottele et al.
2010
). The molecular basis and the physiological
