Plants as sessile organisms are exposed to persistently changing stress factors. The primary stresses such as drought, salinity, cold and hot temperatures and chemicals are interconnected in their effects on plants. These factors cause damage to the plant cell and lead to secondary stresses such as osmotic and oxidative stresses. Plants cannot avoid the exposure to these factors but adapt morphologically and physiologically by some other mechanisms. Almost all stresses induce the production of a group of proteins called heat-shock proteins (Hsps) or stress-induced proteins. The induction of transcription of these proteins is a common phenomenon in all living things. These proteins are grouped in plants into five classes according to their approximate molecular weight: (1) Hsp100, (2) Hsp90, (3) Hsp70, (4) Hsp60 and (5) small heat-shock proteins (sHsps). Higher plants have at least 20 sHsps and there might be 40 kinds of these sHsps in one plant species. It is believed that this diversification of these proteins reflects an adaptation to tolerate the heat stress. Transcription of heat-shock protein genes is controlled by regulatory proteins called heat stress transcription factors (Hsfs). Plants show at least 21 Hsfs with each one having its role in regulation, but they also cooperate in all phases of periodical heat stress responses (triggering, maintenance and recovery). There are more than 52 plant species (including crop ones) that have been genetically engineered for different traits such as yield, herbicide and insecticide resistance and some metabolic changes.In conclusion, major heat-shock proteins have some kind of related roles in solving the problem of misfolding and aggregation, as well as their role as chaperones.

Agricultural biotechnology is very familiar with the properties of nanomaterial and their potential uses. Therefore, the present experiment was conducted to test the beneficial effects of nanosilicon dioxide (nSiO2: size- 12 nm) on the seed germination of tomato (Lycopersicum esculentum Mill. cv Super Strain B). Application of nSiO2 significantly enhanced the characteristics of seed germination. Among the treatments, 8 g L(-1) of nSiO2 improved percent seed germination, mean germination time, seed germination index, seed vigour index, seedling fresh weight and dry weight. Therefore, it is very clear that nSiO2 has a significant impact on the seed germination potential. These findings could provide that alternative source for fertilizer that may improve sustainable agriculture.

Nitric oxide (NO) has now gained significant place in plant science, mainly due to its properties (free radical, small size, no charge, short-lived, and highly diffusible across biological membranes) and multifunctional roles in plant growth, development, and regulation of remarkable spectrum of plant cellular mechanisms. In the last few years, the role of NO in tolerance of plants to abiotic stress has established much consideration. As it is evident from the present review, recent progress on NO potentiality in tolerance of plants to environmental stresses has been impressive. These investigations suggest that NO, itself, possesses antioxidant properties and might act as a signal in activating ROS-scavenging enzyme activities under abiotic stress. NO plays an important role in resistance to salt, drought, temperature (high and low), UV-B, and heavy metal stress. Rapidly increasing evidences indicate that NO is essentially involve in several physiological processes; however, there has been much disagreement regarding the mechanism(s) by which NO reduces abiotic stress.

Research into nanotechnology, an emerging science, has advanced in almost all fields of technology. The aim of the present study was to evaluate the role of nano-silicon dioxide (nano-SiO2 ) in plant resistance to salt stress through improvement of the antioxidant system of squash (Cucurbita pepo L. cv. white bush marrow). Seeds treated with NaCl showed reduced germination percentage, vigor, length, and fresh and dry weights of the roots and shoots. However, nano-SiO2 improved seed germination and growth characteristics by reducing malondialdehyde and hydrogen peroxide levels as well as electrolyte leakage. In addition, application of nano-SiO2 reduced chlorophyll degradation and enhanced the net photosynthetic rate (Pn ), stomatal conductance (gs ), transpiration rate, and water use efficiency. The increase in plant germination and growth characteristics through application of nano-SiO2 might reflect a reduction in oxidative damage as a result of the expression of antioxidant enzymes, such as catalase, peroxidase, superoxide dismutase, glutathione reductase, and ascorbate peroxidase. These results indicate that nano-SiO2 may improve defense mechanisms of plants against salt stress toxicity by augmenting the Pn , gs , transpiration rate, water use efficiency, total chlorophyll, proline, and carbonic anhydrase activity in the leaves of plants.

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