Polyamines and Regulation of Ripening and Senescence - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 331

components or other anionic sites on membranes (Hong and Lee, 1996). PAs bind, and
stabilize membranes and proton-secreting systems of oat protoplasts (Galston and Kaur-
Sawhney, 1995). In senescing barley and oat leaves, external application of PAs stabilized
thylakoid membranes and prevented chlorophyll loss (Popovic et al., 1989; Besford et al.,
1993). Exogenous application of Spd to barley leaf discs inhibited RNase activity, loss of
chlorophyll, and protein degradation from thylakoid membranes, thus stabilizing them dur-
ing senescence (Legocka and Zajchert, 1999). Reduction in loss of chlorophyll and reduced
levels of malondialdehyde in oat leaves supplied with Spd, Spm, diaminopropane, and
guazatine (an inhibitor of PAs oxidase) suggested an anti-senescence role of PAs through
inhibition of lipid peroxidation (Borrell et al., 1997).
PAs conjugated to Ca 2 + -dependent transamidating enzymes, TGases, play an important
role in preventing senescence by stabilizing proteins. TGases catalyze the formation of
cross-linkages between glutaminyl- and lysyl-residues or between glutaminyl-residues and
PAs, thus forming mono- or bis-PA derivatives. Transglutaminase activity measured in
terms of formation of mono- and bis-derivatives of Put and bis-derivatives of Spd from
corolla of undetached flowers of Nicotianatabacum indicated an increase in mono-PAs and
a decrease in bis-derivatives during early senescence. Also, application of Spm to excised
flowers delayed senescence and cell death while increasing mono-Put (Serafini-Fracassini
et al., 2002).
15.11.4 Polyamines and gene expression
PAs can alter gene expression both at the transcriptional as well as the translational level.
PA depletion may stabilize mRNA due to lack of Spd for post-translational modification of
eIF5A, an important factor involved in mRNA turnover (Mehta et al., 1991; Veress et al.,
2000). In E. coli , PAs influence proteins synthesis at the level of translation through the PA
modulon (Yoshida et al., 2004; Igarashi and Kashiwagi, 2006). Expression of certain genes
including transcription factors constituting the PA modulon is altered by PAs at the level
of translation. Microarray analysis of a PA-requiring E. coli mutant showed that a large
number of genes involved in cellular processes such as central intermediary metabolism,
energy metabolism, iron, and zinc transport processes were upregulated by PAs. The PA
modulon influenced 58 of the 309 upregulated genes. Genes downregulated by PAs were
mainly associated with amino acid metabolism, biosynthesis of cofactors, prosthetic groups,
and carriers (Yoshida et al., 2004; Igarashi and Kashiwagi, 2006).
Genes associated with different stress signaling pathways such as transcription factors
involved in salt, cold, and dehydration responses were downregulated in plants overex-
pressing ADC. Additionally, AtGA20ox1 , AtGA3ox1 , and AtGA3ox3 genes involved in GA
biosynthesis and pathogen response were upregulated (Alcazar et al., 2005). Under chill-
ing stress, transgenic Arabidopsis thaliana overexpressing SpdSyn showed upregulation
of many genes including transcription factors involved in chilling tolerance. Other genes
upregulated in these transgenic plants encode protein kinases, calmodulin-related proteins,
cytochrome P450, and peroxidases. However, most of the downregulated genes did not
appear to be related to stress tolerance (Kasukabe et al., 2004).
NMR spectroscopy-based metabolite profiling revealed distinct metabolite trends in
high polyamine transgenic tomato fruits, overexpressing SAM decarboxylase compared to
control fruits. Gln, Asn, choline, citrate, fumarate, malate, and an unidentified compound