Agriculture Reference
In-Depth Information
critical control points for crop quality (Hoefgen and Galili 2002 ; Hoefgen
et al. 2004 ). Chloroplasts contain approximately 80 % of total leaf nitrogen, mostly
in the form of Rubisco, and so not surprisingly, the major nitrogen resources for
export to the developing seeds are derived from chloroplast degradation in
senescing leaves (Feller et al. 2008 ; Kato und Sakamoto 2010 ). Leaf senescence
is further characterised by the formation of senescence-associated vacuoles,
involvement of the autophagy machinery (see chapter of Collados-Rodriguez
et al. in this topic), and activation of proteolytic and other degradative enzymes
(Ishida et al. 2008 ; Wada et al. 2009 ; Izumi et al. 2010 ;H¨rtensteiner 2012 ;
Yamada et al. 2001 ; Martinez et al. 2008 ; Watanabe et al. 2010 ). As a result of
these degradation processes, cellular integrity is compromised and reactive oxygen
species (ROS) accumulate, which may regulate senescence associated gene (SAG)
expression and additionally induce stress mitigation responses to maintain cellular
function as long as possible (Navabpour et al. 2003 ; Guo and Crawford 2005 ).
For breeders focused on increasing crop yields, prolongation of leaf integrity has
long been a trait of interest. Some cultivars with extremely late onset of senescence,
so called “stay-green” cultivars, do indeed show higher starch filling and yields
(Richards 2000 ). However, as robust senescence is required for maximal
remobilisation of nutrients, seed quality might be impaired in late-senescing vari-
eties (Bogard et al. 2011 ; Gong et al. 2005 ; Hirel et al. 2007 ; Masclaux-Daubresse
and Chardon 2011 ; Gregersen et al. 2008 ). Further, the effects of late onset
senescence have been found to vary depending on the species, genotype, and
environmental conditions. In wet and cool conditions wheat yield was negatively
correlated with the stay-green characteristic (Naruoka et al. 2012 ), while on the
other hand, the stay-green trait is often correlated with increased drought tolerance
or accentuated by low nitrogen supply (Yan et al. 2004 ; Robson et al. 2004 ; Jordan
et al. 2012 ). Thus, the relation of gross physiological characteristics such as stay-
green to final biomass and seed yield is still not well understood. We are still far
from being able to provide general rules, particularly as the stay-green phenotype is
caused by different mutations, some of which do not affect senescence parameters
only chlorophyll content (for summary see: Gregersen et al. 2013 ).
The case of stay-green cultivars highlights the need to consistently describe plant
development and senescence processes at the systems level. This kind of integrative
knowledge would greatly facilitate breeding schemes as it may identify those
processes most relevant for crop breeding.
Transcriptomic and Metabolomic Consequences of Plant
Maturation and Senescence
Arabidopsis thaliana provides a tractable and sufficiently valid model for systems
level analysis of senescence processes. Several detailed transcriptomics and
metabolomic studies (Guo et al. 2004 ; Buchanon-Wollaston et al. 2005 ; Breeze
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