Agriculture Reference
In-Depth Information
starvation is already highly resolved (Scheible et al.
2004
; Morcuende et al.
2007
;
Doerner
2008
). In contrast, sulfate is often poorly mobilised from the vacuole,
probably because soil sulfate uptake is sufficient for seed filling and vacuolar
sulfate export transporters, which are known to be responsive to sulfate starvation,
are not always induced (Davidian and Kopriva
2010
). When different regimes of
sulfate were applied to soil grown wheat, persistent sulfate deficiency led to yield
reductions and poor protein quality of the seeds. Application of sulfate, even at the
seed filling stage, was able to revert the phenotype with respect to improved starch
deposition and seed protein quality, especially of sulfur-rich seed proteins though it
could not compensate for previous losses in biomass accumulation (Z¨rb
et al.
2012
; Steinfurth et al.
2012
). Interestingly, the sulfur-rich defense compounds
of brassicaceae, the glucosinolates, are actively degraded in leaves during senes-
cence and might provide a source of reduced sulfur for export in contrast to
vacuolar sulfate (Yoshimoto and Saito
2012
; Watanabe et al.
2013
). However,
various sulfated compounds are not mobilised. Therefore sulfate and sulfurous
organic compounds are a substantial part of the leaf litter and contribute to the
fact that in humus rich soils the majority of nutrient sulfate is released by bacteria
from organic sources (Kertesz
1999
; Kertesz and Mirleau
2004
; Schmalenberger
et al.
2009
).
Senescence processes are clearly extremely complex, and the functions of
individual genes, not to mention the regulatory networks, are still not fully under-
stood. However, transcriptomic and metabolomic studies on developmental senes-
cence in
Arabidopsis thaliana
(Fig.
8.2
) are building a framework at the molecular
level for gluing together more detailed and targeted observations from
Arabidopsis
and other species including crops.
Inadequate Availability of Mineral Nutrients Leads
to Nutrient Depletion Induced Senescence
Crops in an agricultural environment are exposed to varying and often non-optimal
environmental conditions, which usually negatively affect carbohydrate synthesis
and nutrient uptake. Moreover, stresses typically occur in combinations: drought
stress is usually accompanied by high temperatures and high light (Long and Ort
2010
; Krasensky and Jonak
2012
), while unavailability of water in the root zone
hinders nutrient ion uptake and stomatal closure inhibits transpiration and
in planta
transport of nutrients. Deficiencies of mineral nutrients leads to nutrient depletion
induced senescence (NuDIS) in plants (Watanabe et al.
2010
). NuDIS is accompa-
nied by a mixture of transcriptomic and metabolomic responses depending on
which nutrient ion or combination of ions is depleted. These responses are to
some extent specific for particular nutrients,
e.g.
the induction of high affinity
transport systems (Hawkesford
2000
,
2003
; Gojon et al.
2009
; Davidian and
Kopriva
2010
; Stitt et al.
2002
; Xu et al.
2012
; Bouguyon et al.
2012
). Interestingly,
overlapping molecular
responses occur under various nutrient depletions
