Background |
Quinoa have been considered as an important alternative crop in Asian and African due to its high nutrition content and more importantly, the tolerance to abiotic stress factors such as drought and salinity (Jacobsen et al., 2003). It has been presented that there are several of mechanism related to drought rsistant in quinoa, including drought escape, tolerance and avoidance(Jensen et al., 2000). Early maturation is the most important strategy for quinoa to escape drought condition. On the other hand, qunioa tolerate the drought through growth plasticity, tissue elasticity,and low osmotic potential which may be an important part of quinoa's drought resistance. Quinoa also can avoid the negative effects of drought by growing deep and dense root system along with the reduction of leaf area, leaf dropping, special vesicular gland, small and thick-walled cells, keeping the turgor and stomatal closure which only occurs when leaf water potential below -1.2MPa. This insensitive stomatal response is one of the special drought tolerant characteristics for quinoa (Jensen et al.,1996).
In the study of salt resistance mechanism, it has been demonstrated that quinoa control and adjust leaf water potential by accumulating salt ions in its tissue under saline condition. This behaviour keeps cells turgor and limits the transpiration and therefore avoids the physiology damage from drought and potential death. However, the ability of resistance to salinity has varied significant differently among cultivars and different saline levels. It has been found that an electrical conductivity higher than 15 mScm-1 can cause the yield reduction of quinoa (Jacobsen et al.,2000) . But under the mild saline conditions (10-20mscm-1), quinoa actually had better performance on leaf area, biomass production, seed yield and harvest index. This indicates that quinoa is a facultative halophyte (Christiansen et al.,1999;Jacobsen et al.,1999). Different from response to the drought, quinoa under saline condition has showed sensitive stomatal response, together with other characteristic of sensitivity such as inflorescence size and plant height, which can be the parameters to test the different saline tolerance among cultivars.
Although several of the mechanisms of resistance to drought and salinity have been explained, a large part of it still remains to be explored from the aspects of perception and chemical and hydraulic signalling pathway. In our investigation, a great attention will be paid on the stomatal behaviour and osmotic adjustment in plants under drought and salinity and more importantly, the signalling pathway behind. It has been documented that the first step of plant molecular response to the abiotic stress such as water deficit is through perception by specific receptors. Followed by the activation, it induces the changes of osmotic potential in cell and triggers dehydration-indued gene experssion (Posas et al.,1996; Ura et al., 1999). The signal transduction cascade after the perception of osmotic stress involves protein phophorylation and dephosphorylation. This process can be mediated by several protein kinases and two of the most abundant ones are Ca2+ dependent(CDPK)and mitogen activated(MAPK). In the drought responses from cell to organ, different signalling pathway occurs : ABA(abscisic acid) dependent or ABA independent pathway(Chaves et al., 2003).
In ABA-mediated responses, ABA is synthesised in the shoot and root in response to stresses such as drought (Taylor et al. 2000). ABA acts as a chemical signal transported in xylem from roots to other parts of the plants and finally induces the stomata closure in order to protect plants from extensive water loss (Schulze 1986b; Davies and Zhang 1991). In this long distance transportation, xylem sap and leaf tissue pH as factors have involved in modulation of ABA concentration at the guard cells of leaves. A increase in pH has been found in well-watered plants while stomatal closure (Pekic et al. 1995). Other study found that species differences in maximum stomatal conductance may rely on the maximum xylem conductance and endogenous ABA concentrations were also found related to the xylem conductance (Aasamaa et al. 2001, 2002). However, Holbokk et al.(2002) has found that ABA-deficient tomato mutants had stomatal closure in response to soil drying in the absence of leaf water deficits. It suggested that ABA effects on stomatal closure may be mediated by a chemical signal originated in the roots other than ABA itself.
Along with ABA accumulation, sustained root growth under moderated levels of water stress has been found (Hsiao and Xu, 2000). The investigation of maize has suggested that endogenous ABA accumulation under drought restricts ethylene production and further prevents the ethylene-induced growth inhibition. It suggests that role of ABA in the control of shoot and root growth under drought is not a direct one but rather results from limitation of ethylene production (Sharp, 2002). Others also found the maintance of root growth was independent of ABA content which all suggested the probability of existence of ABA- independent mechanism. Several genes induced by cell dehydration in ABA-deficient and ABA-insensitive mutants also suggests the same results(Shinozaki and Yamaguchi-Shinozaki 1997; Luan 1998 ). Others genetic work suggests the sharing of ABA-dependent and ABA-independent pathway may occur downstream of the first stress recognition and signalling events, while others suggest the interaction of ABA and ethylene signalling cascades determine the resistance to drought (Shinozaki and Yamaguchi-Shinozaki 2000; Fujimoto et al. 2000). The further investigation will be carried out through our experiments in order to further elucidate and clarify the mechanism. In the recent study of quinoa, ABA increases in xylem during soil drying compared to the control followed by a rapid closure of stomata and leaf area. However the increase of ABA occurred before the decrease of root water potential suggests that ABA played a minor role in mechanism of drought tolerance of quinoa(Jacobsen et al., 2009). But the real mechanism behind still remains unclear. Several questions have been raised: Do the different cultivars of quinoa have the same result? Is ABA-independent pathway the explanation or the cross-talk between ABA-dependent and ABA- independent does exist? Does osmotic adjustment play the big role here? Is ethylene more involved over than ABA?
In another recent study of quinoa, the increase of proline content has been found in quinoa leaves together with glucose and total soluble sugar content. This finding indicated that osmotic adjustment might be the signalling of quinoa response to drought (González et al., 2009). Further clarification in this area is required together with the investigation of other important osmotic compounds such as glycine betaine, dyhydrins, mannitol and sorbitol etc.
In addition, there is a realistic meaning to investigate the effect of salinity and drought together due to the two factors is often combined occurring in nature. Salinity increases sucrose and glucose content in salt-stressed cotyledons of quinoa while the source-sink relations are changed in order to supply soluble sugars and proline for the osmotic adjustment. Meanwhile the activities of sucrose phosphate synthase and solube acid invertase are both activated in salt-stressed cotyledons (Rosa et al., 2009). Recent microarray data from Arabidopsis and rice have shown that a large number of drought-inducible genes can also be induced by ABA and high salinity. This found has indicated the potential cross-talk or common regulatory system may exist between mechanism of resistance of drought and salinity (Shinozaki and Yamaguchi-Shinozaki, 2007). |
