Agriculture Reference
In-Depth Information
the generation of glucosinolate hydrolysis products, which serve as plant defence
molecules. The system, however, seems to work both ways: it deters usual patho-
gens but it may also attract some specialised herbivores (Renwick
2001
). Many of
them use glucosinolates as a food or use glucosinolates-containing plants for
oviposit (Halkier and Gershenzon
2006
).
Phytosulfokines (PSK) are another sulfur-containing metabolite of importance
to plant growth and fitness, which promote various stages of plant growth such as
somatic embryogenesis, adventitious bud and root formation and pollen germina-
tion (Chen et al.
2000
; Kobayashi et al.
1999
; Yamakawa et al.
1998
). PSK-
ʱ
is the
first sulfated peptide found in plants. Its unique signal strongly promotes cell
proliferation in plant cells at low concentrations. PSK-
is universally distributed
in the plant kingdom (Yang et al.
2000
) and is synthesised from an ~80 amino acid
precursor peptide which has a secretion signal at its N-terminus (Yang et al.
1999
).
In
Arabidopsis
five genes encoding the precursor peptide have been identified
(Yang et al.
2001
). The expression of PSK genes in
Arabidopsis
is not limited to
tissues characterised by active cell division and differentiation. It has been detected
in most plant organs including mature leaves, stems, roots, and calluses, which
indicate that it is not a simple mitogen or differentiation initiator. PSK precursor
over-expression causes no apparent changes in plant growth or development under
normal growth conditions (Matsubayashi and Sakagami
2006
). It acts by binding to
the specific binding sites which have been identified on the surface of suspension-
cultured cells and in plasma membrane-enriched fractions of various plant species
(Matsubayashi and Sakagami
1999
). Based on the internal sequence of the
PSK-binding protein, the component of the functional PSK receptor, LRR-RLK,
was identified (Matsubayashi et al.
2002
) and
LRR-RLK Arabidopsis
knock-out
mutants are used to study the
in vivo
function of PSK (Matsubayashi and Sakagami
2006
). The same receptor is used by another sulfated peptide called PSY1 which is
also involved in the promotion of cell proliferation and expansion at nanomolar
concentrations (Amano et al.
2007
). Recently discovered RGF family peptides
seem to be involved in root development (Matsuzaki et al.
2010
; Meng
et al.
2012
), however, the exact role of these proteins and its sulfation remains
unclear.
ʱ
Regulation of Sulfate Assimilation
Understanding the tight regulation of the sulfate assimilation pathway is extremely
important for two reasons: the essential role of sulfur for plant growth and quality,
and the potential cytotoxicity of sulfite and sulfide, which are intermediates in
sulfate assimilation. Various stages of the assimilatory pathway are regulated in
both positive and negative feedback mechanisms in a demand-driven manner
(Fig.
3.5
).
