Environmental Engineering Reference
In-Depth Information
measure of the plant is to accumulate many metabolites which are usually called ―compatible
solutes‖ to increase the osmotic tolerance (Türkan and Demiral 2009). The compatible solutes
include sugars (fructose, sucrose and glucose), complex sugars (trehalose, raffinose and
fructans), quaternary amino acid derivatives, tertiary amines, sulfonium compounds and so on
(Munns and Tester 2008). They can not only counteract the ions but also balance the osmotic
potential of Na
+
and Cl
−
being sequestered into the vacuole (Wyn Jones et al. 1977). Among
the compatible solutes, proline (Pro) and glycine betaine (GB) are the most common and
efficient. Several decades ago, Flowers et al. reported that the Pro or GB is able to accumulate
at a high concentration as much as 0.1MPa in osmotic pressure (Flowers et al. 1977). With
the in-depth research, their vital functions in salt-stress are increasingly clearer.
Pro is able to act as molecular chaperone to stabilize the protein conformation, buffer
cytosolic pH and balance cell redox condition (Maggio et al. 2002). Under stresses the
accumulation of the Pro is duo to the increased biosynthesis and the decreased degradation. In
leaves of two rice cultivars with different salinity tolerance, the accumulative rate of proline
is quicker in the tolerant line (Demiral and Türkan 2004). Research on sorghum also
illustrated that the proline concentration is at a higher level subjected to salt stress (Yan et al.
2012). Now it is known that there are two pathways and three enzymes involved in the
synthesis of Pro (Verbruggen and Hermans 2008). For the three synthetic enzymes (P5CS,
P5CR and OAT), P5CS is the rate-limiting enzyme and is adjusted by feed-back and
transcriptional regulation (Savouré et al. 1995; Zhang CS 1995). In Arabidopsis the signaling
pathway for the induction of P5CS1 expression during salt stress depends on phospholipase C
(PLC) and ABA responsive (ABRE) element (Parre et al. 2007; Abrahám et al. 2003). The
development of molecular mechanism on Pro metabolism is accompanied by the progress of
the transgenic engineering. The first transgenic plant with accumulated Pro was acquired in
1995 (Kavi et al. 1995). In Arabidopsis overexpression of P5CS antisense or the insertional
mutant of this gene can reduce the salt tolerance of the transgenic plants (Nanjo et al. 1999;
Székely et al. 2008). Meanwhile, we may cut down the degradation of the Pro to maintain its
concentration, so the key enzyme PDH in degradation is also manipulated. However, in PDH
antisense lines higher stress tolerance emerged at times but not always (Nanjo et al. 1999;
Mani et al. 2002).
GB is synthesized and accumulated under a variety of abiotic stress and acts as an
osmolyte to protect PSII under salinity (Jagendorf and Takabe 2001). It can also protect the
integrity of membrane and the activity of enzymes against osmotic stress (Mäkelä et al.
2000). Generally high concentrations of salt are able to increase GB concentration in many
crop plants, such as sugar beet, spinach, barley, wheat, and sorghum (Weimberg et al. 1984;
Fallon and Phillips 1989; Yang et al. 2003). In view of its powerful features, transgenic
engineering was also employed to operate GB concentration. Transgenic Arabidopsis
expressing N-methyltransferase gene which is one of the three key enzymes in GB synthesis
possess improved seed yield compared to the control under salt stress (Waditee et al. 2005).
Similarly, overexpression of GOLS2, a GB synthase enhanced tolerance to high salinity in
Arabidopsis (Sun et al. 2013).
As a matter of fact, there are many more other compatible solutes. For example, in
Arabidopsis, overexpression of mannose-6-phosphate reductase was reported to increase
growth and photosynthesis in saline treatment (Sickler et al. 2007). Some plants which are
able to bear Na
+
at high level such as barley tend to adapt and maintain turgor in the face of
