Root apoplastic barriers block Na+ transport to shoots in rice (Oryza sativa L.)

Abstract

Rice is an important crop that is very sensitive to salinity. However, some varieties differ greatly in this feature, making investigations of salinity tolerance mechanisms possible. The cultivar Pokkali is salinity tolerant and is known to have more extensive hydrophobic barriers in its roots than does IR20, a more sensitive cultivar. These barriers located in the root endodermis and exodermis prevent the direct entry of external fluid into the stele. However, it is known that in the case of rice, these barriers are bypassed by most of the Na+ that enters the shoot. Exposing plants to a moderate stress of 100 mM NaCl resulted in deposition of additional hydrophobic aliphatic suberin in both cultivars. The present study demonstrated that Pokkali roots have a lower permeability to water (measured using a pressure chamber) than those of IR20. Conditioning plants with 100 mM NaCl effectively reduced Na+ accumulation in the shoot and improved survival of the plants when they were subsequently subjected to a lethal stress of 200 mM NaCl. The Na+ accumulated during the conditioning period was rapidly released when the plants were returned to the control medium. It has been suggested that the location of the bypass flow is around young lateral roots, the early development of which disrupts the continuity of the endodermal and exodermal Casparian bands. However, in the present study, the observed increase in lateral root densities during stress in both cultivars did not correlate with bypass flow. Overall the data suggest that in rice roots Na+ bypass flow is reduced by the deposition of apoplastic barriers, leading to improved plant survival under salt stress.

PDH45overexpressing transgenic tobacco and rice plants provide salinity stress tolerance via less sodium accumulation

“…Here, findings of less Na C accumulation in shoots in the tolerant rice varieties/ transgenic lines are similar with finding of Krishnamurthy et al 17 who reported that additional suberized hydrophobic barriers and reduced entry of Na C into shoots under salt stress conditions. Therefore, our findings in sustaining least Na C in shoot suggest that PDH45 gene may be involved similar salinity tolerance mechanism in defined tobacco (dicot) and rice (monocot) transgenic plants in response to salinity stress.…”

“…16 The salinity tolerant variety (Pokkali) revealed more extensive hydrophobic barriers in its roots as compared to the salinity susceptible variety (IR20). 17,20 The PDH45 overexpression enhances salinity tolerance without compromising yield as compared to WT (IR64). 11 The results of this study, revealed differential accumulation of Na C in salinity stress in 3 rice varieties and 2 transgenic lines analyzed.…”

“…16 Further, bypass flow closely paralleled the extent of suberin deposition over the course of exposure to salinity stress and is negatively correlated with suberin deposits. 17 CoroNa Green AM binds with sodium (Na C ) and is specific Na C indicator that shows increase in fluorescence intensity indicates increasing Na C accumulation. In this study, role of PDH45 helicase in salinity stress was investigated using CoroNa Green AM dye specific to Na C localization.…”

Sodium as nutrient and toxicant

“…Severe reductions in extracellular water potentials due to high salt loads can produce rapid dehydration and consequent damage of cells, and the similarities between salt and drought stresses are remarkable (Munns 2002). In the case of rice, most, if not all, salt-induced damage in the leaves might be due to osmotic stresses caused by Na + buildup in the leaf apoplast (the "Oertli hypothesis"; Flowers et al 1991;Krishnamurthy et al 2011). Less clear, however, are the mechanisms underlying the secondary, "ion-specific" aspects of salt stress (Munns et al 1995).…”

Cd-tolerance markers of Pfaffia glomerata (Spreng.) Pedersen plants: anatomical and physiological features

“…As seen under salt-stress, the development of apoplastic barriers (such as the endodermis) has been correlated with increasing resistance to the radial flow of water and solutes in roots, resulting in reduced ion uptakes (i.e. Na + ) into the shoots and in better survival under subsequent acute stress (Krishnamurthy et al, 2011).…”