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Plants are sessile in nature and cannot move away from adverse environmental conditions. Therefore, plants need to respond in other ways to protect themselves from environmental stress. In nature, different stresses such as cold, drought or heat may demand diverse responses, leading to distinct adaptations in plants (Mittler, 2002). The visible adaptive response depends on the severity of stress and can include leaf growth arrest, lesion formation, onset of leaf senescence and delayed or early flowering. In addition, plants can gauge mildly stressful conditions and respond by preparing for survival against future stressful conditions of the same kind in a process called hardening or priming (Tuteja and Gill, 2013). Exogenous application of osmoprotectants or priming agents prior to an expected stress can successfully be used to induce enhanced resistance to multiple abiotic stresses in plants and this strategy is being successfully used in agriculture to improve stress resistance of crop plants (Savvides et al., 2016). For example, natural or chemical compounds including putrescine, spermine, vitamins, hormones and oligosaccharides can be exogenously applied prior to the expected stress event (Aranega-Bou et al., 2014; Ebeed et al., 2017). Hence, plants have evolved effective mechanisms to cope with a variety of stresses and stress severities and these mechanisms can be manipulated to increase crop productivity.

1.3.1 Priming-induced abiotic stress tolerance in plants

Priming is an emerging field in the management of crops and produce against damaging effects of environmental stress (Savvides et al., 2016). The commercial storage of fruits at low temperature increases shelf life, but storage between 0-5 ˚C can cause chilling injuries or cold stress, especially in fleshy fruits. Recently, a study has shown that priming of peach fruits right after harvest by controlled, delayed cooling for 48-hours at 20 ˚C resulted in reduced chilling injuries as compared to fruits directly exposed to cold temperature (Tanou et al., 2017). In the field, soil salinity is a major agricultural problem because of decreased irrigation water quality and mineral weathering causing a slow increase in soil salinity (Flowers, 2004). Recent studies have shown that priming of Triticum aestivum L. plants

with 1 mmol ABA caused higher antioxidant activity and resistance to salt stress by protecting the photosynthetic electron transport chain (Wang et al., 2017). Also, pre-treatment of tomato plants with the proton pump inhibitor omeprazole increased ROS levels and resulted in improved shoot and root mass and plant nutritional status during salt stress (Van Oosten et al., 2017). Moreover, drought is a common abiotic stress to plants and priming has been confirmed by various studies to enhance the tolerance to water deficit conditions: the priming of Medicago sativa plants with the antioxidant melatonin enhanced the osmoprotection and antioxidant activity and conferred enhanced tolerance to a prolonged drought period (Antoniou et al., 2017). In addition, seed priming with salicylic acid, jasmonic acid or paclobutrazol significantly increased drought resistance in rice plants (Samota et al., 2017). Increased stress resistance as a result of priming resulted in marked improvements in shoot biomass compared to the non-primed plants. For example, maize seeds which were primed with silicon produced plants with significantly increased leaf size and fresh weight and after being exposed to alkaline stress as compared to plants developed from non-primed seeds (Abdel et al., 2016). Moreover, Arabidopsis

plants pre-treated with 50 mM NaCl displayed enhanced tolerance to desiccation, resulting in plants with greener leaves and a larger rosette size than non-primed control plants (Sani et al., 2013). Thus, priming has proven to be an effective approach in the improvement of crops under stressed conditions because it not only improves survival, but also yield.

Priming is believed to increase stress tolerance because the priming induces an endogenous stress responses that allows the plants to handle future stress with greater tolerance (Gamir et al., 2014). It is suggested that in primed plants the protective effect is caused by increased ROS signalling, which activates several signalling cascades, hormones, small peptides and antioxidants (Borges et al., 2014; Colcombet and Hirt, 2008; Mittler et al., 2011). Indeed, exogenous application of H2O2 increased drought and salt stress resistance in plants by modulating multiple processes including photosynthetic activity, ROS scavenging and turgor (Hossain et al., 2015). An effect of the priming-induced increased ROS levels may be enhanced antioxidant activity, resulting in restricted overproduction of ROS as a result of subsequent stress (Afzal et al., 2011; Hussain et al., 2016; Rejeb et al., 2014). Therefore, priming induces increased stress resistance, without apparent negative effects on plant growth, primarily by controlling ROS overproduction in response to various subsequent stresses (Conrath, 2011).

1.3.2 Abiotic stress-induced programmed cell death as an adaptive response in plants

Under certain stressed conditions like UV or ozone, cells that are no longer needed commit suicide, which is mediated by a highly coordinated process known as PCD (Tuzhikov et al., 2008). During this process only specific cells are destroyed so that neighbouring cells can survive the adverse effect of the

environmental stress (Wang and Bayles, 2013). The overproduction of ROS mediates PCD in many cell types (Petrov et al., 2015). In plants different stress factors like, heat shock, water deficit and salinity can cause the initiation of PCD (Gaussand et al., 2011; Zuppini et al., 2010). UV-B radiation that reaches the earth’s surface can also cause the activation of PCD and this is visible by the appearance of lesions or chlorotic areas (Nawkar et al., 2013). UV-B exposure results in reduced photosynthetic capacity and cessation of leaf growth because of delayed cell division and cell expansion (Hectors et al., 2010; Lo et al., 2005; Milchunas et al., 2004). Leaf growth resumes once the leaf’s injury is repaired, but in the case of severe stress, it can lead to premature leaf senescence (Suchar and Robberecht, 2015). Ozone is a major photochemical oxidant and can also cause lesion formation in leaves. For example, ozone caused leaf lesions in ozone-sensitive Arabidopsis accession Wassilewskija, while ozone-tolerant accession Columbia was not affected by the same stress levels (Tamaoki et al., 2003), indicating that ozone-stress resistance is a genetically controlled trait.

Mild abiotic stress such as drought, salinity or heat rapidly reduces plant growth and development. The primary cause of stunted plant growth is stomatal closure which is useful in terms of reducing the transpiration rate, but the closed stomata also lead to a reduction in photosynthesis (Kaya et al., 2006; Manickavelu et al., 2006; Hancock et al., 2001; Sahoo et al., 2017). The reduced energy production, leading to delayed cell elongation and cell expansion restrict the plant growth (Munns and Termaat, 1986). Tolerance to mild osmotic stress can be achieved by accumulation of osmoprotectants such as proline which helps in adjusting the osmotic pressure as well as scavenging of various ROS radicals and this may cause only minor effects on plant growth (Nanjo et al., 1999; Saradhi et al., 1995). Nevertheless, more pronounced osmotic stress can adversely affect plant growth (Maggio et al., 2001; Fahad et al., 2017).

One of the most conspicuous consequences of abiotic stress is the onset of senescence in adult leaves (Munns et al., 1995; Petronia et al., 2011). Leaf senescence is a natural process that is a result of ageing or initiation of reproduction. It is the final stage of leaf development, which is usually marked by the yellowing and withering of leaves and ultimately leads to the death of a leaf (Buchanan-Wollaston, 1997; Diaz et al., 2006; Guo and Gan, 2014). Senescence is an essential process that allows the remobilisation of the nutrients within the old senescent leaf so these can used to support young growing tissues (Himelblau and Amasino, 2001). However, senescence as a result of abiotic stress can be considered unwanted as it may result in decreased plant growth and yield (Sharabi-Schwager et al., 2009). However, as stress may result in the shut-down of photosynthesis and growth, nutrients made available through the senescence of the older leaves may allow the plant to survive and reproduce. The apparent positive effect of abiotic stress-induced senescence of the older leaves on the survival of the whole plant can also be seen in a number of transgenic plant lines in which stress resistance was affected by the transgene. For example, in Arabidopsis, the autophagy-related mutants atg5, atg7 and nbr1 are

more sensitive to heat and drought stress as compared to wild type plants (Zhou et al., 2013). However, careful observation of plant phenotypes shows that the old leaves have undergone senescence and the young leaves remained green and viable. Similarly, drought stress on ascorbate and GSH-deficient mutants (vtc-2 and pad-2) also displayed advanced yellowing in old leaves but young leaves remained green (Koffler et al., 2014).

The biosynthesis of ABA is upregulated during environmental stress and as part of the senescence program (Khan et al., 2013). Pyrabactin resistance1-like (PYL) belongs to the family of ABA receptors that function in ABA and drought-stress signalling. Arabidopsis lines overexpressing PYL9 displayed a greater tolerance to drought stress and ABA-induced leaf senescence of old leaves in an ethylene- independent manner (Zhao et al., 2016). This example clearly indicates that ABA-induced senescence of the old leaves during water deficit is important for the survival of young tissues. Thus, premature leaf senescence as a result of stress affects productivity but it is also an important strategy adopted by plants to assure the survival of young leaves. Therefore, the stress response depends on leaf age and I propose that ROS is a factor linking the two. Consistent with this, ARABIDOPSIS A-FIFTEEN (AAF) modulates redox homeostasis in Arabidopsis and transgenic lines overexpressing AAF (oxAAF) displayed early senescence and stress sensitivity in an age-dependent manner (Chen et al., 2012). The physiological parameters used to monitor the progression of senescence in the third rosette leaves at 49 days after germination displayed accelerated senescence as compared to 28 days old leaves. Moreover, accelerated senescence was found during ozone exposure due to overproduction of ROS in Arabidopsis

(Miller et al., 1999). In addition to this, research has shown that expression of ROS responsive genes are upregulated in fully expanded Arabidopsis rosette leaves, prior to the initiation of senescence (Breeze et al., 2011). This indicates that once leaves reach maturity, an increase in ROS production leads to the progressive onset of senescence and this is consistent with ROS playing a central role in the initiation of senescence in old leaves (Sedigheh et al., 2011; Schippers et al., 2008).

As described above, ROS, ABA and ethylene also function in plant survival during environmental stress. Here, I propose that (1) the dual role of ROS, ABA and ethylene in either leaf death or survival is ultimately determined by leaf age and (2) that stress-induced leaf senescence of old leaves benefits the survival of the whole plant. One purpose of early senescence in stressed plants is to complete the life cycle quickly thereby allowing early initiation of flower development and seed production (Buchanan-Wollaston, 1997; Lamb, 2012; Kazan and Lyons, 2016). However, whether plants can survive and complete reproduction depends on the severity of stress (Xu et al., 2010). Under severe stress, suppression of various biochemical and physiological processes eventually leads to the death of the whole plant (Munns et al., 1995; Pandey et al., 2017).

Altogether, plants’ response to stress can have at least three different outcomes: mild stress may have limited impact on plant growth and development, but it can result in a priming effect, hardening the

plant for future stress of a similar kind. Secondly, intermediate stress will induce leaf senescence in the older leaves, allowing those leaves to continue to function as a nutrient and energy source and allowing the growing parts of the plant to survive. Finally, severe stress will lead to the death of the plant. These