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Role of Proline and Glycine Betaine in Overcoming Abiotic Stresses
Murat Dikilitas1, Eray Simsek1, and Aryadeep Roychoudhury2
1Department of Plant Protection, Faculty of Agriculture, Harran University, S. Urfa, Turkey
2Department of Biotechnology, St. Xavier's College (Autonomous), Kolkata, West Bengal, India
1.1 Introduction
Plants are continuously under exposures of many kinds of abiotic and biotic stresses starting from vegetative to reproductive stages (Roychoudhury et al., 2011; Hajihosseinlo et al., 2015; Aksakal et al., 2017; Aamer et al., 2018; Cheng et al., 2018; Dawood and ElâAwadi 2018; Duhan et al., 2018; Choudhary et al., 2019; Ghaffari et al., 2019; Sahitya et al., 2018; Paul and Roychoudhury 2019). Salinity, waterlogging, chilling or cold stress, drought, heat, light, heavy metal stress, pesticide wastes, nutrient deficiency, UVâB damages, and pathogen stress and their combinations might lead to more devastating consequences on crop plants (Roychoudhury et al. 2011). Even if crop plants may exhibit tolerance to stress arising from either biotic or abiotic stress agents or to both, they may not be productive as desired in terms of crop production and quality. Although plants are able to accumulate osmolytes to defend themselves against stress, the synthesis of osmolytes could be reduced under severe stress conditions. In general, increases of osmolytes have been regarded as the reflection of stress tolerance. There are quite a few osmolytes commonly measured in cells such as proline, polyamines, glycine betaine (GB), sugar and sugar products, glycerol, sorbitol, mannitol, etc., that have significant roles in protecting cells from cellâdamaging stress factors (Chen and Jiang 2010; Rabbani and Choi 2018). Therefore, increasing amino acid or osmolyte contents under stress conditions would be a proper approach to tackle at least one of the stress factors. However, if one of the stress agents is biotic, the mechanisms for tolerance would be more complex due to the adaptation of attacking pathogens to these stressful conditions. It is possible that the pathogens may use the compounds having low molecular weights such as sugars, polyamines, or lowâmolecularâweight amino acids as substrates. These âready to takeâin substratesâ could even increase the virulence of the pathogens and result in more devastating effects on crop production. Therefore, modulation of abiotic stress tolerance under the combined stress conditions may not be satisfactory for crop plants. Applications of proline, sugar, and amino acid might increase the pathogen dissemination via secretion of secondary metabolites under abiotic or combined stress occurrences. For example, Dunn et al. (1998) reported that NaCl stress led to increased production of arginine levels and decreased phenylalanine ammonia lyase (PAL) activity in citrus plants, thus causing increased susceptibility to nematode attack caused by Tylenchulus semipenetrans. The authors also stated that the reduction in PAL activity due to an increase in arginine levels increased higher infection rates. It was concluded that the increased level of arginine following salinity stress led to reduction in host defense responses against the attacking mites. In another study, Mathur et al. (2013) stated that the elevated CO2 (550 ppm) led to reduction in disease severity of Alternaria blight and downy mildew caused by Alternaria brassicae and Hyaloperonospora brassicae, respectively, in Brassica juncea cv. Pusa Tarak (mustard plants). They concluded that elevated CO2 resulted in the accumulation of higher amounts of epicuticular wax with the increase of total phenolics and PAL activity levels, which might have enabled the mustard plants to resist the infection caused by those pathogens. A reduction in pore size and stomatal density with reduced stomatal conductance might have played significant roles in decreasing the disease index of downy mildew caused by H. brassicae, which is a stomataâinvading pathogen. However, the same authors also stated that the pathogenicity of Albugo candida, a causal agent for white rust infection, increased in the same plant. The authors suggested that three times higher sugar concentration levels than plants grown in ambient CO2 might have played significant roles and led to higher incidence and severity of the white rust disease. Therefore, we have to consider the biotic stress cases while we aim to develop and improve the level of tolerance or resistance of crop plants under stress.
In this chapter, we mainly focus on the modulation of abiotic stress issues through proline and GB application, but we would discuss on the biotic stress involvement as well.
