Copyright 2007 Accademia dei Georgofili
The genetic control of resistance to stress in olive. I would like to dedicate this review to Claudio Vitagliano, the colleague who introduced me into the privileged academic world and as a friend encouraged me to operate with the mind in science and the eyes in agriculture. Plants can adapt to different abiotic stresses through either constitutive or adaptive mechanisms. The constitutive mechanisms are always active and have been acquired by the species during its evolution to survive in a permanently hostile environment while the adaptive ones are switched off in normal conditions and are rapidly activated when a stress occurs. This paper reviews the olive (Olea europaea L) adaptive response to water deficit, high salt concentration and low temperature, taking into consideration the literature on olive and general reviews as well. The olive plant undergoing water shortage reduces water loss by closing stomata and reducing the transpiration flux. Olive also reduces the photosynthesis; activates the SOD, CAT, APX, POD, and LOX enzymatic systems to protect itself from the ROS created by the excess of non utilised photosynthetic radiation; reduces the permeability of cellular membranes through modulation of aquaporins; increases the cytoplasmic osmotic potential through the production of compatible solutes, such as proline and mannitol, and increases the synthesis of abscisic acid, which in turn activates several ABA-dependent biosynthetic pathways. These biochemical mechanisms of response to water stress are well studied, and are often present in both adapted and non-adapted genotypes. These contrasting genotypes are supposed to differ in the rapidity of sensing and signalling, but little is known about these key processes even in model plants. Sensing is likely based on trans-membrane histidin-kinases, that should act as an osmotic sensor, while the signalling is likely based on systems such as MAPK (Mitogen-activated protein kinase), which is a rather general signalling system in eukaryotes. Fine biochemical and genetic analyses showed that in Arabidopsis approximately 70% of genes activated by water stress are also activated by salt stress, and this shows the large extent of overlap between the two processes. The differences arise from the management of large amounts of Na+ e Cl- in the circulating solution. Olive is a glycophytic species and, unlike halophytic species constitutively adapted to extreme salt concentration, is not capable to compartmentalize Na+ and Cl- ions in vacuoles. Olive avoids salt damage by extruding ions in the apoplast and by accompanying this process with the synthesis of compatible osmolytes, such as mannitol. This helps olive to tolerate NaCl concentrations up to 40-100 mM, depending on genotype. Olive is damaged by low temperatures in winter and in the intermediate seasons. Like other evergreen species, it tolerates better low temperatures in winter rather than in autumn or in spring, because in the last periods the temperatures drop suddenly and plants have not time to adapt progressively. The literature in olive is scarce and we can only guess that olive mimics other better studied plants, by reaching adaptation through the fatty acid desaturation of membrane phospholipids, the accumulation of compatible osmolytes, and the production of anti-freezing proteins (endochitinases, glucanases, osmotin-like proteins and others). The agriculture will have to cope more and more with planet warming and water scarcity. These phenomena are negatively impacting the olive crop that is traditionally consigned to marginal and arid soils. A renewed effort in breeding and selection is necessary to save the olive crop and its historical heritage. The recent advancements in molecular biology and genetics have improved the breeding programs in many crops. Olive must rapidly make up for the lost time.
