Plants are exposed to a variety of abiotic stresses in nature and exhibit unique and complex responses to these stresses depending on their degree of plasticity involving many morphological, cellular, anatomical, and physiological changes. Phytohormones are known to play vital roles in the ability of plants to acclimatize to varying environments, by mediating growth, development, source/sink transitions and nutrient allocation. These signal molecules are produced within the plant, and also referred as plant growth regulators. Although plant response to salinity depends on several factors; nevertheless, phytohormones are thought to be the most important endogenous substances that are critical in modulating physiological responses that eventually lead to adaptation to salinity. Response usually involves fluctuations in the levels of several phytohormones, which relates with changes in expression of genes involved in their biosynthesis and the responses they regulate. Present review described the potential role of different phytohormones and their balances against salinity stress and summarized the research progress regarding plant responses towards salinity at physiological and molecular levels. We emphasized the role of abscisic acid, indole acetic acid, cytokinins, gibberellic acid, salicylic acid, brassinosteroids, jasmonates, ethylene and triazoles in mediating plant responses and discussed their crosstalk at various baseline pathways transduced by these phytohormones under salinity. Current progress is exemplified by the identification and validation of several significant genes that enhanced crops tolerance to salinity, while missing links on different aspects of phytohormone related salinity tolerance are pointed out. Deciphering mechanisms by which plant perceives salinity and trigger the signal transduction cascades via phytohormones is vital to devise salinity related breeding and transgenic approaches.

Phytohormones and plant responses to salinity stress: a review

Synthetic nitrogen (N) fertilizer has played a key role in enhancing food production and keeping half of the world's population adequately fed. However, decades of N fertilizer overuse in many parts of the world have contributed to soil, water, and air pollution; reducing excessive N losses and emissions is a central environmental challenge in the 21st century. China's participation is essential to global efforts in reducing N-related greenhouse gas (GHG) emissions because China is the largest producer and consumer of fertilizer N. To evaluate the impact of China's use of N fertilizer, we quantify the carbon footprint of China's N fertilizer production and consumption chain using life cycle analysis. For every ton of N fertilizer manufactured and used, 13.5 tons of CO2-equivalent (eq) (t CO2-eq) is emitted, compared with 9.7 t CO2-eq in Europe. Emissions in China tripled from 1980 [131 terrogram (Tg) of CO2-eq (Tg CO2-eq)] to 2010 (452 Tg CO2-eq). N fertilizer-related emissions constitute about 7% of GHG emissions from the entire Chinese economy and exceed soil carbon gain resulting from N fertilizer use by several-fold. We identified potential emission reductions by comparing prevailing technologies and management practices in China with more advanced options worldwide. Mitigation opportunities include improving methane recovery during coal mining, enhancing energy efficiency in fertilizer manufacture, and minimizing N overuse in field-level crop production. We find that use of advanced technologies could cut N fertilizer-related emissions by 20-63%, amounting to 102-357 Tg CO2-eq annually. Such reduction would decrease China's total GHG emissions by 2-6%, which is significant on a global scale.

/pdf/new-technologies-reduce-greenhouse-gas-emissions-from-3hk403x0vk.pdf

New technologies reduce greenhouse gas emissions from nitrogenous fertilizer in China

Plants are sessile beings, so the need of mechanisms to flee from unfavorable circumstances has provided the development of unique and sophisticated responses to environmental stresses. Depending on the degree of plasticity, many morphological, cellular, anatomical, and physiological changes occur in plants in response to abiotic stress. Phytohormones are small molecules that play critical roles in regulating plant growth and development, as well as stress tolerance to promote survival and acclimatize to varying environments. To congregate the challenges of salinity, temperature extremes, and osmotic stress, plants use their genetic mechanism and different adaptive and biological approaches for survival and high production. In the present attempt, we review the potential role of different phytohormones and plant growth-promoting rhizobacteria in abiotic stresses and summarize the research progress in plant responses to abiotic stresses at physiological and molecular levels. We emphasized the regulatory circuits of abscisic acid, indole acetic acid, cytokinins, gibberellic acid, salicylic acid, brassinosteroids, jasmonates, ethylene, and triazole on exposure to abiotic stresses. Current progress is exemplified by the identification and validation of several significant genes that enhanced crop tolerance to stress in the field. These findings will make the modification of hormone biosynthetic pathways for the transgenic plant generation with augmented abiotic stress tolerance and boosting crop productivity in the coming decades possible.

Potential role of phytohormones and plant growth-promoting rhizobacteria in abiotic stresses: consequences for changing environment.

Phytohormones are chemical messengers produced in one part of plant and translocated to the other parts, where they play critical roles in regulating plant responses to stress at extremely low concentration. Phytohormones are natural products and they called plant growth regulators, when they are synthesized chemically. Plants are usually subjected to environmental factors such as drought or high soil and water salinity. The reduction in plant growth exposed to saline environments could be due to either the effects of specific ions on metabolism or adverse water relations. Different strategies are being employed to maximize plant growth under saline conditions. One of them is to produce salt tolerant genotypes of different crops. Attempts to improve tolerance to salinity through conventional plant breeding methods are time consuming, laborious and depended on existing genetic variability. In addition, many attempts have been made to overcome this disorder, including proper management and exogenous application of plant growth regulators. This article presents a review of the role of abscisic acid (ABA), indole acetic acid (IAA), cytokinins (CK), gibberellic acid (GA), brassinosteroids (BR), jasmonates (JA), salicylic acid (SA) and triazoles (TR) in alleviating salt stress in crops.

/pdf/the-role-of-phytohormones-in-alleviating-salt-stress-in-crop-18bwxs10gc.pdf

The Role of Phytohormones in Alleviating Salt Stress in Crop Plants

Plant hormones play vital roles in the ability of plants to acclimatize to varying environments by mediating growth, development and nutrient allocation. Hormones move through specific pathways to regulatory sites where they respond to stress at awfully low concentration. All biological activities are directly or indirectly affected by both phytohormones. Here we review the role of hormones against abiotic tolerance in crop plants. The main findings are: (1) abscisic acid act as a mediator in plant responses to many stresses, including salt stress. (2) Stress modifies the level of indole acetic acid (IAA) thus reducing growth. (3) Functional analysis of cytokinin receptor mutants show that cytokinin receptors of Arabidopsis act as negative regulators in abscisic acid (ABA) signaling and in osmotic stress response. (4) The mechanisms by which gibberellic acid (GA) priming could induce salt tolerance in plants are not yet clear. Salinity perturbs the hormonal balance in plants. Under salt stress hormonal homeostasis might be the possible mechanism of GA3-induced plant salt tolerance. (5) A low level of salicylic acid and jasmonate is effective against abiotic stress by enhancing physiological processes and improving tolerance. (6) Role of brassinosteroids and triazole during environmental stress is emerging. (7) Ethylene is considered as a stress hormone; however, the role of ethylene in salt stress is equivocal. The present review focus on abscisic acid, indole acetic acid, cytokinins, gibberellic acid, salicylic acid, brassinosteroids, jasmonates, ethylene and triazole.

Crop Plant Hormones and Environmental Stress

Nitrogen (N), auxin and cytokinins (CTKs) play important roles in regulating the growth of rice tiller buds. However, little is known about the underlying mechanisms and the relationships between them in the regulation of tiller bud growth in rice. In the present study, N and auxin indole-3-acetic acid (IAA) were used to regulate tiller bud growth in Nanjing 44 rice, a japonica cultivar. The tiller buds grew rapidly after external application of 40 mg L -1 N; significantly elevated CTK levels were found in tiller buds and nodes preceding bud growth. However, external application of IAA inhibited the growth of tiller buds and reversed the increase in CTK levels in tiller buds and nodes induced by N. According to previous studies, the rice adenosine phosphate isopentenyltransferase (OsIPT ) genes encode a key enzyme in CTK biosynthesis. To determine the site of CTK synthesis involved in bud growth, we examined the expression patterns of the OsIPT genes in tiller buds, tiller nodes and roots. Our results indicate that the CTKs that promote tiller bud growth are biosynthesised mainly in the tiller nodes, and there is little or no contribution from the roots. N and IAA regulated the expression of the OsIPT genes and inhibited the biosynthesis of CTKs in the tiller nodes, thus regulating the growth of rice tiller buds.

/pdf/the-relationship-between-nitrogen-auxin-and-cytokinin-in-the-5bazatszex.pdf

The relationship between nitrogen, auxin and cytokinin in the growth regulation of rice (Oryza sativa L.) tiller buds

Auxin and cytokinin (CTK) play important roles in regulating the growth of rice tiller buds. Auxins inhibit bud growth and CTK, which are regulated by auxins, promote growth. However, little is known about the underlying molecular mechanisms. Here, we studied auxin/CTK regulation in Nanjing 44 rice, a japonica cultivar. After full heading, we cut off the roots and removed the panicles. The tiller buds grew rapidly after panicle removal; significantly elevated CTK levels were found in tiller buds and nodes preceding bud growth. However, external application of the auxin indole-3-acetic acid (IAA) inhibited the growth of tiller buds and reversed the increase of CTK levels in tiller buds and nodes induced by panicle removal. According to previous studies, the rice gene adenosine phosphate isopentenyltransferase (OsIPT) encodes a key enzyme in CTK biosynthesis. To determine the site of CTK synthesis involved in bud growth, we examined the expression pattern of OsIPT in tiller buds and nodes. Our results demonstrated that tiller bud growth is not regulated by CTK derived from roots; rather, CTK is biosynthesised mainly in the nodes and subsequently delivered to tiller buds. Auxin negatively regulates the expression of OsIPT and the biosynthesis of CTK in nodes, thus inhibiting the growth of tiller buds in rice.

/pdf/auxin-inhibits-the-outgrowth-of-tiller-buds-in-rice-oryza-3bcn2vkrl3.pdf

Auxin Inhibits the Outgrowth of Tiller Buds in Rice (Oryza sativa L.) by Downregulating Osipt Expression and Cytokinin Biosynthesis in Nodes

In the present study, GA3 and NAA were used to inhibit the growth of unproductive tillers of rice plants, and we measured the tiller occur and the grain yield of different treatments, the object of this study was to determine the effect of external hormones on the tiller growth and grain yield, and whether we can regulate the tiller growth by some manipulations of the hormone levels. The results indicated that external 10 mg L-1 GA3 and 1000 mg L-1 NAA significantly inhibit the growth of unproductive tillers, and the elimination of unproductive tillers promoted the growth of productive tillers at the middle and late growth stages, and promoted the development of heavy panicles, and finally increased the grain yield. However, the external 100 mg L-1 GA3 (G100) significantly decreased the grain yield of Yangdao 6. The reason was that the most of the plants of G100 treatment of Yangdao 6 were lodging at filling stage, and the lodging significantly decreased the spikelet filling and grain weight, and finally decreased the grain yield. These results suggested that the application of GA3 in field production may lead to lodging, especially for the indica cultivars. So in field production, GA3 should be used carefully.

	 

	Key words: Rice (Oryza sativa L.), gibberellic acid (GA3), α-naphthalene acetic acid (NAA), tiller, yield.

Effect of Gibberellic acid (GA3) and -naphthalene acetic acid (NAA) on the growth of unproductive tillers and the grain yield of rice (Oryza sativa L.)

A field plot comparison experiment was conducted to study the strong stem effect and physiological characteristics of rice plant under rice-duck farming. The results showed that under rice-duck farming, the morphology of rice plant changed obviously, and the carbohydrate content and C/N ratio of the plant as well as the dry matter output from rice stem increased significantly, due to the activities of duck in paddy field. In the meantime, the length of basal internodes decreased by 2. 88%, while the stem diameter, stem mechanical strength, and stem lodging resistant index increased by 64.90%, 11.78%, and 10.95%, respectively. Rice-duck farming increased the root mass and root vitality in deeper soil layers, and decreased the proportion of black roots by 16.63%. It was indicated that rice-duck farming benefited the formation of strong rice stem and increased the stress resistance of rice plant, which would be favorable to the stable and high-yielding of rice.

[Strong stem effect and physiological characteristics of rice plant under rice-duck farming].

A field experiment was conducted to study the characteristics of paddy field nitrogen (N) leakage and runoff under rice-duck farming (MRD), conventional farming (MR), and conventional farming with flooding (CK). Comparing with that under MR, the paddy field under MRD had a notable decrease of N (especially NO3- -N) concentration in its leaked liquid; but this concentration was tended to be increased, compared with that under CK. After 7-9 days of fertilization, the NH4+ -N and NO3- -N concentrations in paddy field surface water were higher under MRD than under MR. However, owing to the no draining and the higher band, the paddy field under MRD had a notable reduction of drainage, resulting in a marked decrease of N runoff than that under MR. Comparing with MR, the paddy field under MRD had an addition of nitrogen supply from duck dung, a reduction of N leakage and runoff, a lesser application of chemical nitrogen fertilizer, and more nitrogen uptake by rice plant. Both the reduction of N input and that of N output in rice-duck farming system were nearly equal in quantity.

[Characteristics of paddy field nitrogen leakage and runoff in rice-duck farming system].

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