Response of Prolyl 4 Hydroxylases, Arabinogalactan Proteins and Homogalacturonans in Four Olive Cultivars under Long-Term Salinity Stress in Relation to Physiological and Morphological Changes

Abstract

Olive (Olea europeae L.) salinity stress induces responses at morphological, physiological and molecular levels, affecting plant productivity. Four olive cultivars with differential tolerance to salt were grown under saline conditions in long barrels for regular root growth to mimic field conditions. Arvanitolia and Lefkolia were previously reported as tolerant to salinity, and Koroneiki and Gaidourelia were characterized as sensitive, exhibiting a decrease in leaf length and leaf area index after 90 days of salinity. Prolyl 4-hydroxylases (P4Hs) hydroxylate cell wall glycoproteins such as arabinogalactan proteins (AGPs). The expression patterns of P4Hs and AGPs under saline conditions showed cultivar-dependent differences in leaves and roots. In the tolerant cultivars, no changes in OeP4H and OeAGP mRNAs were observed, while in the sensitive cultivars, the majority of OeP4Hs and OeAGPs were upregulated in leaves. Immunodetection showed that the AGP signal intensity and the cortical cell size, shape and intercellular spaces under saline conditions were similar to the control in Arvanitolia, while in Koroneiki, a weak AGP signal was associated with irregular cells and intercellular spaces, leading to aerenchyma formation after 45 days of NaCl treatment. Moreover, the acceleration of endodermal development and the formation of exodermal and cortical cells with thickened cell walls were observed, and an overall decrease in the abundance of cell wall homogalacturonans was detected in salt-treated roots. In conclusion, Arvanitolia and Lefkolia exhibited the highest adaptive capacity to salinity, indicating that their use as rootstocks might provide increased tolerance to irrigation with saline water.

Tolerance Mechanisms of Olive Tree (<em>Olea europaea</em>) Under Saline Conditions

“…The same authors noted that transcriptional analyses have indicated a repression of cell wall-related gene expression including AGPs and xyloglucan, as a result of salt stress. Similarly, Azariadis et al [224] reported that immunodetection revealed a weak AGP signal in the 'Koroneiki' cultivar, which was associated with irregular cells and intercellular spaces, resulting in the formation of aerenchyma after 45 days of salt treatment. Hydroxyproline-rich cell wall glycoproteins (HRGP), such as arabinogalactans proteins (AGP), are involved in salinity stress [232,233].…”

“…The results showed higher levels of MDA and HNE in the 'Arbequina', 'Cobrançosa', 'Pico-Limón' and 'Cornezuelo' cultivars compared with controls, while no significant difference was observed for the 'Changlot Real' and 'Picual' cultivars, renowned for their resistance to salinity. Similarly, Azariadis et al [224] found that MDA levels increased significantly in salt-sensitive varieties ('Koroneiki' and 'Arvanitoli'), but did not occur in tolerant varieties ('Lefkolia' and 'Gaidourelia'), when all were exposed to saline conditions. Meanwhile, in another study, MDA levels in two olive cultivars ('Canino' and 'Sirole') under different salt concentrations remained unchanged [225].…”

“…Nevertheless, the extent of its tolerance to saline stress is also conditioned by many other factors. Studies have revealed significant variation in the response of olive trees to stress depending on the cultivar, as some cultivars such as 'Leccino', 'Arvanitoli', 'Arbequina' displayed salt sensitivity, while others like 'Frantoio', 'Lefkolia', 'Gaidourelia', 'Manzanillo', and 'Hojiblanc'a showed more resilience [78,224]. The specific genetic characteristics of each cultivar define its salt tolerance, thereby affecting ion absorption/exclusion regulation, osmolyte accumulation, resistance to oxidative stress, and the activation of metabolic pathways [110,246,261].…”