Page 2 Plant-Bacteria Interactions Edited by Iqbal Ahmad, John ...

198j 10 Principles, Applications and Future Aspects of Cold-Adapted PGPR
Table 10.1 Percent abundance of membrane fatty acids at different temperatures.
components to remain fluid, a number of changes must occur in the pattern of fatty
acids. Unsaturation of fatty acid chains is the most common change that occurs
when the temperature is reduced; this increases the fluidity of the membrane
because unsaturated fatty acid groups create more disturbance to the membrane
than saturated chains. This process is achieved by desaturases situated in the membrane
itself and thus is able to react quickly. In cyanobacteria, four desaturase genes
(desA–desD) have been reported; moreover, desA, desB and desD have been demonstrated
to be cold inducible in Synechocystis [17].
There are, however, a number of other alterations that can occur after a decline in
temperature [18]. The average fatty acid chain length may be shortened, which would
have the effect of increasing the fluidity of the cell membrane because there are
fewer carbon–carbon interactions between the neighboring chains [19]. A psychrophilic
organism, for example Micrococcus cryophilus, which contains high proportions
of unsaturated fatty acids under all growth conditions, responds to a decrease
in temperature, from 20 to 0 C, by a reduction in the average chain length of the
fatty acids [20]. All these changes, as summarized in Table 10.1, result in the
membrane maintaining its fluidity by producing lipids with a lower gel-to-liquid
crystalline transition temperature and by incorporating proportionally more low
melting point fatty acids into membrane lipids.
10.2.2
Carbon Metabolism and Electron Flow
Percent abundance of fatty acids
Fatty acids (IUPAC name) No. of carbon 20 C 18 C 15 C 10 C 5 C
Didecanoic C12 (0) 1.94 2.97 1.18 1.0 0.83
Tridecanoic C13 (0)* 2.08 2.24 2.17 1.46 0.84
n-Tetradecanoic C14 (0) 2.53 3.78 2.57 1.8 0.97
Pentadecanoic C15 (0) 1.50 1.69 1.66 2.41 2.50
cis,cis-9,12-Hexadecanoic C16 (2) 0.65 1.35 1.18 0.67 1.63
cis-9-Hexadecanoic C16 (1) 33.8 33.3 34.7 40.5 49.8
n-Hexadecanoic C16 (0) 27.5 25.4 24.9 20.2 13.2
cis,cis-9,12-Octadecadienoic C18 (2) 0.92 1.08 0.99 2.00 1.00
cis-9-Octadecanoic C18 (1) 21.2 22.5 23.9 26.6 25.9
n-Octadecanoic C18 (0) 7.89 4.30 4.9 3.06 3.31
* Source: Russell [21].
Sardesai and Babu [22] reported that carbon metabolism and electron flow are also
affected by low temperature. Cold stress induces a change from respiratory metabolism
to anaerobic lactate formation in a psychrophillic Rhizobium strain. Analysis of
glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase of the
pentose phosphate pathway showed an upward regulation of an alternative pathway