Changes in free polyamine ...

 

 

Changes in free polyamine concentration induced by salt stress in seedlings of different species

 

 

 

Pedro Javier Zapata1, Maria Serrano2, Maria Teresa Pretel2 and Maria Angeles Botella2 

 

(1)

Departamento Tecnología Agroalimentaria, Escuela Politécnica Superior de Orihuela, Universidad Miguel Hernández, Orihuela, Spain

 

 

 

 

(2)

División Fisiología Vegetal, Departamento Biología Aplicada, Escuela Politécnica Superior de Orihuela, Universidad Miguel Hernández, Ctra Beniel-Orihuela, Km 3.2, 03312 Orihuela, Alicante, Spain

 

 

 

 

Plant Growth Regulation

 

 Received: 4 July 2007  Accepted: 13 June 2008  Published online: 1 July 2008

 

Abstract  Growth rate, mineral composition and changes in polyamine concentration induced in response to salinity were studied in six crop species: spinach, lettuce, bean, pepper, beetroot and tomato. Salinity decreased growth rate, but sensitivity differed amongst the species: pepper being the most sensitive, followed by bean, tomato, lettuce and spinach, with beetroot being the most tolerant. The increase of Na+ and total cation with salinity in shoots was the highest in spinach and beetroot, the most tolerant species, while in pepper it was the lowest. Changes in putrescine (Put) concentration in shoots were related to salinity tolerance (increased in the most sensitive), while changes in spermidine (Spd; decreases) and spermine (Spm; increases) were similar with most species, except for pepper in which salinity strongly increased Put, Spd and Spm. Therefore, total polyamine concentration increased in pepper shoot, while it decreased in the other species. Thus, results show that Put accumulation was a consequence of salt stress in the most sensitive species, while salt tolerant species (beetroot) showed little change in polyamine concentration, and higher concentration in both Na+ and total cations. The role of polyamines or cation increased concentration after saline treatment in species with different salt tolerance is discussed.

 

Keywords  Polyamines - Putrescine - Salinity - Spermidine - Spermine - Mineral composition

 

 

 

 

 

 

 

 

 

Introduction

 

Salinity represents one of the most important environmental stresses, since it limits plant production. Therefore the study of salinity tolerance in crop plants is of special importance (Mansour and Salama 2004). Salinity affects plants through osmotic and ion effects (nutritional imbalance and/or toxic ion effects; Pitman and Lauchli 2002). Osmotic effects are due to a salt-induced decrease in soil water potential. Salinity results in a reduction of K+ and Ca2+ concentration and an increased concentration of Na+ and Cl−, which has ionic effects. The osmotic adjustment in plants exposed to saline stress can occur by accumulation of high concentrations of inorganic ions or low molecular weight organic solutes (Yeo 1998). Both mechanisms play important roles under saline conditions, but their relative contribution varies between species, varieties and even between different compartments within the same plant (Ashraf and Harris 2004). In plants exposed to saline stress, a range of nitrogenous compounds accumulate depending on species, the most frequent being amino acids, amides, proteins, ammonium quaternary compounds and polyamines. The proposed functions for these compounds under saline conditions are: osmotic adjustment, cellular macromolecule protection, nitrogen storage, cellular pH maintenance, decrease of cellular toxicity and elimination of free radicals. Accumulation of these compounds is usually correlated to plant salinity tolerance (Mansour 2000).

 

Polyamines are biologically active compounds involved in various physiological processes, and it has been suggested that changes in polyamine metabolism under stress conditions (such as salt, osmotic, drought and oxidative stresses) may be part of an integrated plant response to stress (Flores 1991). Since polyamine concentration increase significantly upon exposure to saline stress, it has been proposed that polyamines could be a protective mechanism of plants against saline stress, therefore conferring tolerance against stress in some cases (Katiyar and Dubey 1990a; Bouchereau et al. 1999; Reggiani et al. 1994; Iqbal and Ashraf 2005). However, in other plant species polyamine concentration decreases as a consequence of saline stress (Kakkar et al. 2000), and may even show increases and decreases in the same species, i.e. rice (Basu et al. 1988; Krishnamurthy and Bhagwat 1989; Lin and Kao 1995) as is the case in putrescine (Put) concentration under saline conditions. In Brassica campestris the total polyamine concentration increased at 25 and 50 mM NaCl, while at 100 mM NaCl Put decreased (Das et al. 1995). Recent studies demonstrated that exogenous Put, and to a lesser extent spermidine (Spd), application reduced the deleterious impact of NaCl on rice yield, at least in a salt sensitive genotype cv. I Kong Pao (Ndayiragije and Lutts 2007). Moreover, a role for Spd and spermine (Spm) in protecting plasma membrane under salinity stress, and thus enhancing salt tolerance, has been hypothesised (Gonzalez and Ramírez 1999). According to Zhao and Qin (2004) one of the mechanisms involved in attenuating salt injury, in barley seedlings, by exogenous polyamine application was to maintain tonoplast integrity and function.

 

It has been suggested that polyamines could contribute to osmotic adaptation and the problem of ion excess by maintaining an adequate cation–anion balance, and by stabilizing membranes at high external salinity (Krishnamurthy and Bhagwat 1989; Mansour and Al-Mutawa 1999). According to Kakkar and Rai (1997), the polyamine contribution to osmotic adjustment seems to be small compared to other nitrogenous compounds. However, as previously indicated, data do not always support a positive correlation between the increase in polyamine concentration and adaptation to saline stress. Therefore the physiological significance of polyamine changes in response to salt stress and their specific mode of action in higher plants remains tenuous. The aim of the present work was to study the effect of salinity (NaCl) on the growth and mineral nutrition of a range of different species, selected for their different sensitivities to salinity, and to determine if changes in polyamine concentration can be related to salinity tolerance, or if these changes are a general response of plants to saline stress