Priming applied to commercial seed lots is widely used by seed technologists to enhance seed vigour in terms of germination potential and increased stress tolerance. Priming can be also valuable to seed bank operators who need improved protocols of ex situ conservation of germplasm collections (crop and native species). Depending on plant species, seed morphology and physiology, different priming treatments can be applied, all of them triggering the so-called ‘pre-germinative metabolism’. This physiological process takes place during early seed imbibition and includes the seed repair response (activation of DNA repair pathways and antioxidant mechanisms), essential to preserve genome integrity, ensuring proper germination and seedling development. The review provides an overview of priming technology, describing the range of physical–chemical and biological treatments currently available. Optimised priming protocols can be designed using the ‘hydrotime concept’ analysis which provides the theoretical bases for assessing the relationship between water potential and germination rate. Despite the efforts so far reported to further improve seed priming, novel ideas and cutting-edge investigations need to be brought into this technological sector of agri-seed industry. Multidisciplinary translational research combining digital, bioinformatic and molecular tools will significantly contribute to expand the range of priming applications to other relevant commercial sectors, e.g. the native seed market.

Seed priming: state of the art and new perspectives.

Chemical Priming of Plants Against Multiple Abiotic Stresses: Mission Possible?

Seed priming to alleviate salinity stress in germinating seeds

The physiological relationship between abiotic stress in plants and polyamines was reported more than 40 years ago. Ever since there has been a debate as to whether increased polyamines protect plants against abiotic stress (e.g., due to their ability to deal with oxidative radicals) or cause damage to them (perhaps due to hydrogen peroxide produced by their catabolism). The observation that cellular polyamines are typically elevated in plants under both short-term as well as long-term abiotic stress conditions is consistent with the possibility of their dual effects, i.e., being protectors from as well as perpetrators of stress damage to the cells. The observed increase in tolerance of plants to abiotic stress when their cellular contents are elevated by either exogenous treatment with polyamines or through genetic engineering with genes encoding polyamine biosynthetic enzymes is indicative of a protective role for them. However, through their catabolic production of hydrogen peroxide and acrolein, both strong oxidizers, they can potentially be the cause of cellular harm during stress. In fact, somewhat enigmatic but strong positive relationship between abiotic stress and foliar polyamines has been proposed as a potential biochemical marker of persistent environmental stress in forest trees in which phenotypic symptoms of stress are not yet visible. Such markers may help forewarn forest managers to undertake amelioration strategies before the appearance of visual symptoms of stress and damage at which stage it is often too late for implementing strategies for stress remediation and reversal of damage. This review provides a comprehensive and critical evaluation of the published literature on interactions between abiotic stress and polyamines in plants, and examines the experimental strategies used to understand the functional significance of this relationship with the aim of improving plant productivity, especially under conditions of abiotic stress.

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Polyamines and abiotic stress in plants: a complex relationship1

Experience and memory of environmental stimuli that indicate future stress can prepare (prime) organismic stress responses even in species lacking a nervous system The process through which such organisms prepare their phenotype for an improved response to future stress has been termed 'priming' However, other terms are also used for this phenomenon, especially when considering priming in different types of organisms and when referring to different stressors Here we propose a conceptual framework for priming of stress responses in bacteria, fungi and plants which allows comparison of priming with other terms, eg adaptation, acclimation, induction, acquired resistance and cross protection We address spatial and temporal aspects of priming and highlight current knowledge about the mechanisms necessary for information storage which range from epigenetic marks to the accumulation of (dormant) signalling molecules Furthermore, we outline possible patterns of primed stress responses Finally, we link the ability of organisms to become primed for stress responses (their 'primability') with evolutionary ecology aspects and discuss which properties of an organism and its environment may favour the evolution of priming of stress responses

Priming and memory of stress responses in organisms lacking a nervous system.

Plants are exposed to any number of potentially adverse environmental conditions such as water deficit, high salinity, extreme temperature, submergence, etc. These abiotic stresses adversely affect the plant growth and productivity. Nowadays various strategies are employed to generate plants that can withstand these stresses. In recent years, seed priming has been developed as an indispensable method to produce tolerant plants against various stresses. Seed priming is the induction of a particular physiological state in plants by the treatment of natural and synthetic compounds to the seeds before germination. In plant defense, priming is defined as a physiological process by which a plant prepares to respond to imminent abiotic stress more quickly or aggressively. Moreover, plants raised from primed seeds showed sturdy and quick cellular defense response against abiotic stresses. Priming for enhanced resistance to abiotic stress obviously is operating via various pathways involved in different metabolic processes. The seedlings emerging from primed seeds showed early and uniform germination. Moreover, the overall growth of plants is enhanced due to the seed-priming treatments. The main objective of this review is to provide an overview of various crops in which seed priming is practiced and about various seed-priming methods and its effects.

Seed priming for abiotic stress tolerance: an overview

The effects of β-amino butyric acid (BABA) on abiotic stress tolerance potential of three Vigna radiata varieties were studied. The reduction in the growth of seedlings subjected to NaCl/polyethylene glycol (PEG) stress is alleviated by BABA seed priming, which also enhanced photosynthetic pigment content and photosynthetic and mitochondrial activities, and also modified the chlorophyll a fluorescence-related parameters. Moreover, BABA seed priming reduced malondialdehyde content in the seedlings and enhanced the accumulation of proline, total protein, total carbohydrate, nitrate reductase activity, and activities of antioxidant enzymes like guaiacol peroxidase and superoxide dismutase. Most of these positive features of BABA priming were predominantly exhibited when the plants were encountered with stress (NaCl/PEG). The BABA content in the BABA-treated green gram seeds and seedlings was also detected and quantified with high-performance thin layer chromatography (HPTLC), and it revealed that the priming effect of BABA initiated in seeds and further gets carried over to the seedlings. It was concluded that BABA seed priming improved the drought and salinity stress tolerance potential of all the three green gram varieties, and it was evident in the NaCl-tolerant variety Pusa Vishal as compared to Pusa Ratna (abiotic stress sensitive) and Pusa 9531(drought tolerant). Dual mode in cost effectiveness of BABA priming is evident from: (1) the positive features of priming are being exhibited more during the exposure of plants to stress, and (2) priming of seedlings can be carried out by BABA application to seeds at very low concentration and volume.

Seed priming with BABA (β-amino butyric acid): a cost-effective method of abiotic stress tolerance in Vigna radiata (L.) Wilczek

Seed Priming with Beta-Amino Butyric Acid Improves Abiotic Stress Tolerance in Rice Seedlings

The investigation was carried out to study the effect of halopriming on NaCl and polyethylene glycol-6000 (PEG-6000) induced stress tolerance potential of three Vigna radiata (L.) Wilczek varieties, with varied abiotic stress tolerance potential. Halopriming is a seed priming technique in which the seeds were soaked in various salt solutions (in this study NaCl was used). The results of the study indicated that the application of stresses (both NaCl and PEG) induced retardation of growth attributes (measured in terms of shoot length, fresh weight, dry weight) and decrease in physiological attributes like total chlorophyll content, metabolites, photosynthetic and mitochondrial activity of the seedlings in all three V. radiata (L.) varieties. However, halopriming of the seeds could reduce the extent of decrease in these biological attributes. NaCl and PEG stress also caused increase in MDA content (a product of membrane lipid peroxidation) in all the varieties studied and this increase was significantly minimized under halopriming. From the present investigation it was evident that among the green gram varieties studied, Pusa Vishal, a NaCl tolerant variety showed enhanced tolerance to NaCl and PEG induced stress, when the seeds were subjected to halopriming followed by Pusa Ratna (stress sensitive variety). Pusa 9531 (drought tolerant variety) also showed positive halopriming effects but it was less significant when compared to other two varieties. It could be concluded that halopriming improved the drought and salinity stress tolerance potential of all varieties and it was significantly higher in the Pusa Vishal as compared to Pusa 9531 and Pusa Ratna.

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Halopriming of seeds imparts tolerance to NaCl and PEG induced stress in Vigna radiata (L.) Wilczek varieties

Abiotic stress is one of the major factors limiting plant growth and yield globally. Though substantial progress has been made in breeding and genetic manipulation of plants to enhance abiotic stress tolerance, the task remains as a challenge even today. Investigations on the priming activity of various chemicals in plants for enhancing abiotic stress tolerance have been undertaken over the past few years. Priming with γ-amino butyric acid (GABA) and β-amino butyric acid (BABA) gains greater attention, because priming with these non-protein amino acids equips the plants to resist abiotic stresses effectively without suffering costly energy investments in operating defence mechanisms. It is well documented that the protective effect of non-protein amino acids like BABA and GABA on plants is due to a potentiation of natural defence mechanisms against abiotic stresses but at the same time not activating the complete defence arsenal before the stress exposure. The exact mode of action of priming with GABA/BABA in plants is still a puzzle, though their importance as signaling molecules during stress is undoubtful. The better understanding of molecular, physiological, and ecological aspects of GABA/BABA priming might lead to the emergence of this technique as a successful strategy for enhancing the abiotic stress(es) tolerance potential of plants in the field, without compromising much on productivity.

K. C. Jisha

Papers

Seed priming for abiotic stress tolerance: an overview

Seed priming with BABA (β-amino butyric acid): a cost-effective method of abiotic stress tolerance in Vigna radiata (L.) Wilczek

Seed Priming with Beta-Amino Butyric Acid Improves Abiotic Stress Tolerance in Rice Seedlings

Halopriming of seeds imparts tolerance to NaCl and PEG induced stress in Vigna radiata (L.) Wilczek varieties

GABA/BABA priming: a means for enhancing abiotic stress tolerance potential of plants with less energy investments on defence cache