Jos T. Puthur

Seed priming for abiotic stress tolerance: an overview

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.

Vesicular arbuscular mycorrhizal fungi improves establishment of micropropagated Leucaena leucocephala plantlets

Investigations were carried out to achieve cent per cent transplantation success of micropropagated Leucaena leucocephala (a fast growing multipurpose leguminous tree species) plantlets using two vesicular arbuscular mycorrhizal fungi, Glomus fasciculatum and Glomus macrocarpum. Plantlets were obtained by rooting the shoots [obtained through; hypocotyl callus in presence of 10-5M BAP + 10-6M NAA; and axillary bud sprouting from cotyledonary and other nodes in presence of 10-5M BAP, on Gamborgs B5 medium], on half strength B5 medium supplemented with 5×10-6M IBA. Subsequent to the nodulation of their roots with Rhizobium (strain PRGL 001)in soilrite, these plantlets were tranferred to sterilized garden soil by laying inoculum of either Glomus fasciculatum or Glomus macrocarpum around their roots. Only 20% of the plantlets survived in soils lacking VAM fungus. In contrast, cent per cent of the plantlets of Leucaena leucocephala established very well and showed good growth in VAM inoculated soil. Roots of the later plantlets showed presence of both external and internal hyphae with well formed arbuscules and vesicles confirming the establishment of good mycorrhizal association. These studies convincingly demonstrate that the mycorrhizal association help in successful establishment of tissue culture raised plantlets of Leucaena leucocephala in the field conditions by alleviating the transplantation shock.

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

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.

Surviving metabolic arrest: photosynthesis during desiccation and rehydration in resurrection plants

Photosynthesis is the key process that is affected by dehydration in plants. Desiccation‐tolerant resurrection plants can survive conditions of very low relative water content. During desiccation, photosynthesis is not operational, but is recovered within a short period after rehydration. While homoiochlorophyllous resurrection plants retain their photosynthetic apparatus during desiccation, poikilochlorophyllous resurrection species dismantle chloroplasts and degrade chlorophyll but resynthesize them again during rehydration. Dismantling the chloroplasts avoids the photooxidative stress in poikilochlorophyllous resurrection plants, whereas it is minimized in homoiochlorophyllous plants through the synthesis of antioxidant enzymes and protective proteins or metabolites. Although the cellular protection mechanisms in both of these species vary, these mechanisms protect cells from desiccation‐induced damage and restore photosynthesis upon rehydration. Several of the proteins synthesized during dehydration are localized in chloroplasts and are believed to play major roles in the protection of photosynthetic structures and in recovery in resurrection species. This review focuses on the strategies of resurrection plants in terms of how they protect their photosynthetic apparatus from oxidative stress during desiccation without membrane damage and with full recovery during rehydration. We review the role of the dehydration‐induced protection mechanisms in chloroplasts and how photosynthesis is restored during rehydration.

Chloroembryos: A unique photosynthesis system

The embryos of some angiosperm taxa contain chlorophyll and this chlorophyllous stage is persisting until the embryo matures (further referred as chloroembryos). Besides being chlorophyllous, these embryos seem to have the ability to photosynthesize. This suggests that the chlorophyllous state of the embryo has an important role in seed development. The photosynthesis of chloroembryos is highly shade adaptive in nature as it is embedded within the supporting tissues (several layers of pod wall, seed coat and endosperm). Moreover, these chloroembryos are developing in a highly osmotic environment, and contain various components of the photosynthetic machinery. Detailed studies were performed in these chloroembryos in order to elucidate the structure of the chloroplasts, pigment composition, the photochemical activities, the rate of carbon assimilation and also the shade adaptive features. It has been shown that the respired CO2 within these chloroembryos is recycled by the efficient photosynthetic components of the chloroembryos and thus potentially influences the seeds carbon economy. Thus, the major role of embryonic photosynthesis is to produce both energy-rich molecules and oxygen, of which the former can be directly used for biosynthesis. During embryogenesis oxygen production is especially important, in a situation wherein the oxygen is limited within the enclosed seed. As these chloroembryos grow in an environment of a sugar rich endosperm, it requires some adaptive mechanisms in this high osmotic environment. The additional polypeptides found in the thylakoids of chloroembryo chloroplasts in comparison to the thylakoids of leaf chloroplast have been suggested to have a role in protecting the photosynthetic components in the chloroembryos in an environment of high osmotic strength. An attempt to understand osmotic stress tolerance existing in these chloroembryos may lead to a better understanding of tolerance of photosynthesis to osmotic stress.

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

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.

Heavy metal detoxification mechanisms in halophytes: an overview

Heavy metals are among the major pollutants from anthropogenic inputs that reach mangrove ecosystem by urban and agricultural runoff, industrial effluents, boating, mining and other processes. To minimize the detrimental effects of heavy metal exposure and their accumulations, plants in general have evolved biological detoxification mechanisms, which include avoidance or exclusion, excretion and accumulation. To protect the cellular components from oxidative damage by heavy metal contamination, biological systems have developed enzymatic and non-enzymatic antioxidant mechanisms. Another detoxification mechanisms produced in plants are osmoprotectants, which are the compatible solutes which maintain a favourable water potential gradient and protect cellular structures from toxic ions. Besides these mechanisms, another heavy metal detoxification system in plants involves the chelation of metals by metal binding molecules like metallothioneins (MTs) and phytochelatins (PCs). To limit the heavy metal toxicity from mangrove ecosystem, it was found that phytoremediation is a most useful technology where in plants are used to remove pollutants from the environment and it is considered as a comparatively new, low-cost and highly promising technology for the remediation of heavy metal. Rhizofiltration, phytovolatilization, phytoextraction and phytostabilization are the important phytoremediation techniques. Among these phytoextraction and phytostabilization are found highly important in the case of mangroves and are promising means of phytoremediation.

Enhanced phytostabilization of cadmium by a halophyte - Acanthus ilicifolius L.

ABSTRACT Heavy metal pollution in mangrove wetlands has become a growing matter of concern as it serves as sink and source for toxic heavy metals including cadmium (Cd). The present study evaluates the phytostabilization potential of a halophyte, Acanthus ilicifolius L., toward Cd under hydroponic culture conditions. Accumulation, translocation, and effects of Cd on the antioxidant system of A. ilicifolius were studied. Results indicated that A. ilicifolius accumulated Cd mainly in roots (96.4%) as compared to stem (1.4%) and leaves (0.6%) and the accumulated Cd is retained in root rather than being translocated to shoots as indicated by TF < 0.26. Moreover, malondialdehyde (MDA) content increased upon Cd treatment, which is further detoxified by the enzymatic and nonenzymatic antioxidant mechanism. Antioxidants like proline, ascorbate, and amino acid recorded an increased accumulation in the Cd-treated plants followed by the upregulation of antioxidant enzymes like superoxide dismutase (SOD), catalase (CAT), guaiacol peroxidase (GPX), and ascorbate peroxidase (APX). Therefore, the rate of sugar accumulation was found to be decreased in plants treated with Cd as compared to the control plants. Thus, having relatively high BCFroot (69.3) and low TFshoot (0.26) values, A. ilicifolius can be suggested as a potential candidate for phytostabilization of Cd in mangrove wetlands.

Arbuscular mycorrhizal association enhances drought tolerance potential of promising bioenergy grass (Saccharum arundinaceum retz.)

The influence of arbuscular mycorrhizal fungi (AMF) (Glomus spp.) on some physiological and biochemical characteristics of bioenergy grass Saccharum arundinaceum subjected to drought stress was studied. The symbiotic association of Glomus spp. was established with S. arundinaceum, a potential bioenergy grass as evident from the increase in percentage of root infection and distribution frequency of vesicles when compared with non-arbuscular mycorrhizal plants. AMF-treated plants exhibited an enhanced accumulation of osmolytes such as sugars and proline and also increased protein content under drought. AMF association significantly increased the accumulation of non-enzymatic antioxidants like phenols, ascorbate and glutathione as well as enhanced the activities of antioxidant enzymes such as SOD (superoxide dismutase), APX (ascorbate peroxidase) and GPX (guaiacol peroxidase) resulting in reduced lipid peroxidation in S. arundinaceum. AMF symbiosis also ameliorated the drought-induced reduction of total chlorophyll content and activities of photosystem I and II. The maximum quantum efficiency of PS II (Fv/Fm) and potential photochemical efficiency (Fv/Fo) were higher in AMF plants as compared to non-AMF plants under drought stress. These results indicate that AMF association alleviate drought stress in S. arundinaceum by the accumulation of osmolytes and non-enzymatic antioxidants and enhanced activities of antioxidant enzymes, and hence, the photosynthetic efficiency is improved resulting in increased biomass production. AMF association with energy grasses also improves the acclimatization of S. arundinaceum for growing in marginal lands of drought-affected soils.

Seed halopriming outdo hydropriming in enhancing seedling vigor and osmotic stress tolerance potential of rice varieties

Seed priming improves the seed performance and also helps the seedlings to alleviate the detrimental effects of various stresses. Seed priming is believed to bring about some biochemical changes in the metabolism within the seed, which ultimately favors germination and the further growth stages of the seedlings even under stressed conditions. The present investigation was carried out to study the effect of hydropriming and halopriming in three rice varieties (Neeraja, Vaisakh, and Vytilla 6), with varied abiotic stress tolerance potential under NaCl and PEG stress. In general, the application of both stresses, NaCl and PEG induced retardation of growth and metabolism of the seedlings. However, seed priming treatments could reduce the extent of decrease in these biological attributes. Both hydro- and halopriming resulted in the enhancement of protein, carbohydrate, and photosynthetic pigment content, modulated antioxidant enzyme activities, reduced the lipid peroxidation of biomembranes, and enhanced the photochemistry and mitochondrial activities in rice seedlings subjected to NaCl and PEG stress as compared to non-primed ones. According to the various morphological, physiological, and biochemical characteristics studied in the rice seedlings raised from primed and non-primed seeds, we confirmed that both hydropriming and halopriming had a positive influence on stimulating metabolism in rice seeds, which ultimately resulted in improved seedling vigor and tolerance under NaCl and PEG stress. Halopriming was found to be more efficient than hydropriming in enhancing the seedling vigor, overall growth, and stress tolerance potential of rice varieties.