Agriculture Reference
In-Depth Information
Introduction
Response to Nutrient Deficiency in Plants
Plants convert CO
2
, light and water into biomass, however they also require
essential nutrients to complete their lifecycle. The sessile nature of plants makes
them vulnerable to the environmental fluctuations that cause nutrient limitation
(among other stresses). In plants, response to nutrient deficit is complex and
depends on the kind of limiting nutrient (macro- or micronutrient) and the extent
of the shortage. Nutrient deficits result in phenotypic adaptation, such as root length
and branching, shoot biomass reduction and faster transfer from vegetative stage of
growth into the generative one (Amtmann and Armengaud
2009
; Gruber
et al.
2013
). Plant responses to nutrient deficit may also be monitored at various
molecular levels, such as gene expression, posttranslational modification of various
proteins, qualitative and quantitative changes of proteins and metabolites.
Researchers from many laboratories have revealed that transcriptome, metabolome
and proteome profiles are reprogrammed in plants which encounter nutrient deficit
(for example, Hoefgen and Nikiforova
2008
; Howarth et al.
2008
; Liang
et al.
2013
). Limitation of the particular nutrient (and/or an increased internal
requirement for that nutrient) is sensed by plants and this information is transferred
to the appropriate effectors, which modify the pathway responsible for assimilation
and metabolism of the nutrient. The sensing and regulatory mechanisms are not
completely characterised in most nutrients. In addition, the need to coordinate
assimilation and metabolism of various nutrients makes this regulation even more
complicated. The growing body of evidence suggests that protein degradation
processes not only adjust plant metabolism to long lasting starvation periods, but
they are also important in early responses to short-term nutrient deficit.
Ubiquitin and Protein Turnover Processes
Ubiquitin (Ub) is a 76 amino acid polypeptide that folds up into a compact globular
structure and is heat-stable. It mediates selective proteolysis after enzymatic con-
jugation to the target proteins that can be either monoubiquitinated, when a single
Ub unit is attached through its C-terminal glycine (G-76) to one lysine (K) residue
of the substrate, or multi-monoubiquitinated, when single Ub units are attached
through G-76 to several K residues of the substrate or polyubiquitinated when
multiple Ub units are attached to the single K residue (Schreiber and Peter
2013
;
Vierstra
2012
). Ub is evolutionary conserved and contains seven highly conserved
lysines that can be used for poly(Ub) chain formation in vivo. Some of these K
residues are traditionally associated with particular degradation systems, for exam-
ple, K-48-linked (poly)Ub appears to be linked rather to proteasomal degradation
while K-63-linked (poly)Ub appears to be more linked to autophagic degradation.
In addition, the role of unconventional ubiquitination in targeting proteins for
degradation indicates the complexity of the process (Xu et al.
2009
).
