Chemistry Reference
In-Depth Information
2.8 Senescence
Plant senescence is a phenomenon that resembles age of the plant that closely
connects with cell death. It is developmentally well defined that optimizes the
growth and reproductive capacity of plants by recycling of resources from
senescing leaves into young leaves or seeds. After well documented the impor-
tance of role of SA in photosynthesis and flowering, it is not unanticipated that this
plant hormone is also involved in regulation of senescence. This process is
characterized by yellowing of leaves due to chlorophyll degradation (Vogelmann
et al.
2012
) and increased ROS levels (Rivas-San and Plasencia
2011
). It is
believed that these events are due to SA accumulation. In Arabidopsis senescent
leaves, SA levels increase 4-fold at the mid-senescent stage. Consistent with this
observation, Arabidopsis plants affected in SA biosynthesis, such as the transgenic
NahG and the mutant pad4, or with a disrupted SA signalling pathway, such as
npr1, exhibit altered senescence patterns that include delayed yellowing and
reduced necrosis compared with wild-type plants (Morris et al.
2000
). Moreover,
senescence is escorted by important changes in gene expression, and SA paly
pivotal role in successful execution of this process. Transcripts of several SAGs,
such as SAG12, are considerably reduced or undetectable in SA-deficient Ara-
bidopsis plants (Morris et al.
2000
). In addition this, SA activates the expression of
the Arabidopsis senescence-related genes aVPE, vVPE, WRKY6, WRKY53, and
SEN1 that encode two vacuolar processing enzymes, two transcription factors, and
a protease, respectively (Robatzek and Somssich
2001
; Miao et al.
2004
; Schenk
et al.
2005
). The involvement of the SA signalling pathway in senescence was
confirmed through a detailed microarray analysis in Arabidopsis senescent leaves
(Buchanan-Wollaston et al.
2005
). Almost 20 % of the up-regulated genes during
senescence show at least 2-fold reduced expression in SA-deficient NahG trans-
genic plants. Most of the senescence enhanced genes that are dependent on the SA
pathway encode kinases, transferases, and hydrolases, but their function in
senescence progression remains to be elucidated. Although a great deal of effort
has been put into identifying the signalling factors required for senescence regu-
lation, further research must determine whether SA is involved in different stages
of senescence, and the interconnecting networks with other phytohormones that
promote (ABA, JA, an ET) or delay (CKs and GAs) senescence.
3 Effect of SA on Yield
The credibility on any exogenously sourced plant hormones evaluate in terms of
biological yield. SA is known to be a natural signal molecule has been shown to
play an important role in regulating various physiological processes in plants
including yield. Yildirim and Dursan (
2009
) revealed that foliar application of SA
showed positive effect on early yield and total yield and also proposed that highest
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