Insights into salt tolerance mechanisms from comparative analyses of salt stress responses in taxonomically related species adapted to different habitats

Autores UPV
Año
CONGRESO Insights into salt tolerance mechanisms from comparative analyses of salt stress responses in taxonomically related species adapted to different habitats

Abstract

All plants activate a series of basic, conserved mechanisms as a response to high soil salinity, including the control of ion transport, the synthesis of specific osmolytes, or the activation of antioxidant systems. It is generally assumed that salt tolerance depends on these processes, but the ecological relevance and the relative contribution to tolerance of different responses, in a given species, remain largely unknown. We have addressed this question by comparing the responses to salt stress ¿ specifically regarding accumulation of ions and osmolytes ¿ in three Plantago (P. crassifolia, P. coronopus and P. major) and three Juncus (J. maritimus, J. acutus and J. articulatus) species, which differ in their degree of salt tolerance according to their distribution in nature. In the Plantago taxa, salt-induced growth inhibition roughly corresponded to their relative degree of salt tolerance (P. crassifolia ≥ P. coronopus > P. major) ¿ although P. major, considered as a glycophyte since it is never found in saline environments, is actually quite resistant to salt. As for all Plantago species studied to date, sorbitol is the major osmolyte, responsible for osmotic adjustment, in P. crassifolia, P. coronopus and P. major; yet sorbitol contents are already high in the absence of salt, and their relative increase with increasing external NaCl concentrations ¿ even though it was higher in P. crassifolia ¿ did not explain the different tolerance of the three species. Salt tolerance appears to depend instead, to a large extent, on the constitutive capacity to store toxic ions for osmotic adjustment, even at low soil salinities. All three species, when grown in the presence of salt, showed a concentration-dependent accumulation of Na+ and Cl- in the leaves, but ion levels in non-treated controls were much higher in the tolerant P. crassifolia (Na+ and Cl-) and P. coronopus (Na+) than in the more sensitive P. major. It is also important to note the large increase in proline (Pro) contents at very high salt concentrations (600 and 800 mM NaCl, salinity levels that the plants will normally not encounter in nature) in the tolerant taxa, but not in P. major. These data suggest that the Plantago halophytes possess built-in mechanisms to rapidly adapt to possible increases of salinity in their natural habitats, by activating Pro biosynthesis. Studies in Juncus confirmed that salt tolerance in monocots partly depends on the inhibition of ion transport to the aerial part of the plants: in the three selected taxa, Na+ and Cl- accumulated to the same extent in the roots of salt treated plants, in parallel to increasing external NaCl concentrations; however, ion contents were lower in the culms and correlated with the relative salt sensitivity of the species: the lowest levels were measured in the most tolerant species, J. maritimus, followed by J. acutus, also a halophyte, whereas the highest were determined in the glycophyte J. articulatus. Maintenance of cellular osmotic balance is mostly based on the accumulation of soluble sugars, although with qualitative differences among taxa: sucrose appears to be the predominant osmolyte in the halophytes and glucose and fructose in J. articulatus. Yet, neither the relative salt-induced increase in sugar contents, nor the absolute concentrations reached in each species, can explain the observed differences in salt tolerance. Pro contents, on the contrary, increased significantly ¿ almost 50-fold in the presence of 400 mM NaCl, as compared to the non-treated controls ¿ only in the salt-tolerant J. maritimus and J. acutus, but not in J. articulatus. This result supports a functional role of Pro in salt tolerance mechanisms in Juncus, probably not only contributing to osmotic adjustment, but also because of its additional 'osmoprotectant' activities.