This review presents a comprehensive and systematic study of the field of plant biostimulants and considers the fundamental and innovative principles underlying this technology. The elucidation of the biological basis of biostimulant function is a prerequisite for the development of science-based biostimulant industry and sound regulations governing these compounds. The task of defining the biological basis of biostimulants as a class of compounds, however, is made more complex by the diverse sources of biostimulants present in the market, which include bacteria, fungi, seaweeds, higher plants, animals and humate-containing raw materials, and the wide diversity of industrial processes utilized in their preparation. To distinguish biostimulants from the existing legislative product categories we propose the following definition of a biostimulant as ‘a formulated product of biological origin that improves plant productivity as a consequence of the novel or emergent properties of the complex of constituents, and not as a sole consequence of the presence of known essential plant nutrients, plant growth regulators, or plant protective compounds’. The definition provided here is important as it emphasizes the principle that biological function can be positively modulated through application of molecules, or mixtures of molecules, for which an explicit mode of action has not been defined. Given the difficulty in determining a ‘mode of action’ for a biostimulant, and recognizing the need for the market in biostimulants to attain legitimacy, we suggest that the focus of biostimulant research and validation should be upon proof of efficacy and safety and the determination of a broad mechanism of action, without a requirement for the determination of a specific mode of action. While there is a clear commercial imperative to rationalize biostimulants as a discrete class of products, there is also a compelling biological case for the science-based development of, and experimentation with biostimulants in the expectation that this may lead to the identification of novel biological molecules and phenomenon, pathways and processes, that would not have been discovered if the category of biostimulants did not exist, or was not considered legitimate.

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Biostimulants in Plant Science: A Global Perspective.

Ferulic acid is the most abundant hydroxycinnamic acid in the plant world and maize bran with 3.1% (w/w) ferulic acid is one of the most promising sources of this antioxidant. The dehydrodimers of ferulic acid are important structural components in the plant cell wall and serve to enhance its rigidity and strength. Feruloyl esterases are a subclass of the carboxylic acid esterases that hydrolyze the ester bond between hydroxycinnamic acids and sugars present in plant cell walls and they have been isolated from a wide range of microorganisms, when grown on complex substrates such as cereal brans, sugar beet pulp, pectin and xylan. These enzymes perform a function similar to alkali in the deesterification of plant cell wall and differ in their specificities towards the methyl esters of cinnamic acids and ferulolylated oligosaccharides. They act synergistically with xylanases and pectinases and facilitate the access of hydrolases to the backbone of cell wall polymers. The applications of ferulic acid and feruloyl esterase enzymes are many and varied. Ferulic acid obtained from agricultural byproducts is a potential precursor for the production of natural vanillin, due to the lower production cost.

Ferulic acid: an antioxidant found naturally in plant cell walls and feruloyl esterases involved in its release and their applications.

Past research into flooding tolerance and oxygen shortages in plants has been motivated largely by cultivation problems of arable crops. Unfortunately, such species are unsuitable for investigating the physiological and biochemical basis of anoxia-tolerance as selection has reduced any tolerance of anaerobiosis and anaerobic soil conditions that their wild ancestors might have possessed, Restoration of anoxia-tolerance to species that have lost this property is served better by physiological and molecular studies of the mechanisms that are employed in wild species that still possess long-term anoxia-tolerance. Case studies developing these arguments are presented in relation to a selection of crop and wild species, The flooding sensitivity and metabolism of maize is compared in relation to rice in its capacity for anaerobic germination, The sensitivity of potato to flooding is related to its disturbed energy metabolism and inability to maintain functioning membranes under anoxia and post-inoxia, By contrast, long-term anoxia-tolerance in the American cranberry (Vaccinium macrocarpon) and the arctic grass species Deschampsia beringensis can be related to the provision and utilization of carbohydrate reserves. Among temperate species, the sweet flag (Acorus calamus) shows a remarkable tolerance of anoxia in both shoots and roots and is also able to mobilize carbohydrate and maintain ATP levels during anoxia as well as preserving membrane lipids against anoxic and post-anoxic injury. Phragmites australis and Spartina alterniflora, although anoxia-tolerant, are both sulphide-sensitive species which can pre-dispose them to the phenomenon of die-back in stagnant, nutrient-rich water. Glyceria maxima adapts to flooding through phenological adaptations with a seasonal metabolic tolerance of anoxia confined to winter and spring which, combined with a facility for root aeration and early spring growth, allows rapid colonization of sites with only shallow flooding. The diversity of responses to flooding in wild plants suggests that, depending on the life strategy and habitat of the species, many different mechanisms may be involved in adapting plants to survive periods of inundation and no one mechanism on its own is adequate for ensuring survival.

Oxygen deprivation stress in a changing environment

Natural product isolation

Salt-tolerant plants grow in a wide variety of saline habitats, from coastal regions, salt marshes and mudflats to inland deserts, salt flats and steppes. Halophytes living in these extreme environments have to deal with frequent changes in salinity level. This can be done by developing adaptive responses including the synthesis of several bioactive molecules. Consequently, several salt marsh plants have traditionally been used for medical, nutritional, and even artisanal purposes. Currently, an increasing interest is granted to these species because of their high content in bioactive compounds (primary and secondary metabolites) such as polyunsaturated fatty acids, carotenoids, vitamins, sterols, essential oils (terpenes), polysaccharides, glycosides, and phenolic compounds. These bioactive substances display potent antioxidant, antimicrobial, anti-inflammatory, and anti-tumoral activities, and therefore represent key-compounds in preventing various diseases (e.g. cancer, chronic inflammation, atherosclerosis and cardiovascular disorder) and ageing processes. The ongoing research will lead to the utilisation of halophytes as a new source of healthy products as functional foods, nutraceuticals or active principles in several industries. This contribution focuses on the ethnopharmacological uses of halophytes in traditional medicine and reviews recent investigations on their biological activities and nutraceuticals. The work is distributed according to the different families of nutraceuticals (lipids, vitamins, proteins, glycosides, phenolic compounds, etc.) discussing the analytical techniques employed for their determination. Information about the claimed health promoting effects of the different families of nutraceuticals is also provided together with data on their application.

Medicinal halophytes: potent source of health promoting biomolecules with medical, nutraceutical and food applications.

Purification and identification of active antibacterial components in Carpobrotus edulis L.

In Ethiopia extracts from specific plants are used traditionally as natural fungicides in small scale farming systems where synthetic chemicals are out of reach of the average subsistence farmer while no scientific base exists for this practice. Subsequently, methanolic crude extracts from Dolichos kilimandscharicus and Maerua subcordata roots as well as Phytolacca dodecandra berries were screened in vitro for antifungal activity against Botrytis cinerea Pers.:Fr., Fusarium oxysporum Schlechtend.:Fr., Sclerotium rolfsii Sacc., Rhizoctonia solani Kuhn,, Botryosphaeria dothidea (Moug.:Fr.) Ces & De Not. and Pythium ultimum Trow, using an agar diffusion method. Compared to the other plants as well as specific standard fungicides for each pathogen, the root extract of D. kilimandscharicus showed the highest broad-spectrum in vitro antifungal activity by inhibiting the mycelial growth of three of the six test organisms. Additionally, in vivo antifungal activity of dry powdered material from these plants against sorghum covered (Sporisorium sorghi) and loose kernel (Sporisorium cruentum) smuts was screened under field conditions by artificially inoculating separate sets of sorghum seed with smut spores before treating with plant material or two traditionally used fungicides, Thiram® and Apron Plus® that served as positive controls. Although the incidence of both loose and covered kernel smuts were significantly reduced by material from all plant species, the dry powdered berries of P. dodecandra were most effective. Compared to the untreated control, treatment with the plant material as well as standard fungicides resulted in significant yield increases. It was concluded that a rationale has been established for further investigation into the structured utilization of natural vegetation indigenous to Ethiopia in the agricultural industry.

In vitro and in vivo antifungal activity of crude extracts and powdered dry material from Ethiopian wild plants against economically important plant pathogens

The antimicrobial properties of crude methanol extracts of above- and below-soil parts of Tulbaghia violacea were quantified by means of an agar diffusion method against 6 plant pathogenic bacteria and 7 fungi. The growth of 3 out of the 6 bacteria, Clavibacter michiganensis, Ralstonia solanacearum, and Xanthomonas campestris, was significantly inhibited by crude extracts of both below-soil and aerial parts of T. violacea, whereas the growth of Pseudomonas syringae, Erwinia carotovora, and Agrobacterium tumefaciens was unaffectedl. Compared with the standard fungicide, both the aerial and below-soil extracts of T. violacea significantly inhibited the mycelial growth of 6 of the 7 test fungi, Botrytis cinerea, Sclerotium rolfsii, Rhizoctonia solani, Mycosphaerella pinodes, Botryosphaeria dothidea, and P. ultimum, whereas only the below-soil extract inhibited the mycelial growth of Fusarium oxysporum significantly. The broad-spectrum antifungal activity shown by the crude T. violaceae extracts supplied a rationale for a further investigation into the in vivo activity of the extracts under glasshouse and field conditions.

Tulbaghia violacea L. I: In vitro antimicrobial properties towards plant pathogens

In vitro antifungal activity of crude extracts from Tulbaghia violacea against 6 economically important plant pathogenic fungi was previously reported. The in vivo control of Mycosphaerella pinodes, causative of Ascochyta blight, by different concentrations of a crude aerial part extract of T. violacea was subsequently followed qualitatively and quantitatively in terms of lesions that developed over a 6-day period at 20°C on detached pea (Pisum sativum) leaves. Infection by M. pinodes spores was prevented when the extract was applied both before and after inoculation, confirming complete inhibition of spore germination, whereas no phytotoxic effect was observed on the leaves. Additionally, the control of sorghum covered (Sporisorium sorghi) and loose (S. cruentum) kernel smuts by seed treatment with the crude extract was tested under field conditions. Before planting, different sorghum seed lots were inoculated separately with spores from the 2 pathogens at a rate of 0.5% per kg (w/w), followed by treatment with the crude extract, at a concentration of 2.0 mg/mL, 24 h later. A standard fungicide, Thiram (65 W), was applied as a positive control at a rate of 0.25% per kg (v/w). Disease incidence was quantified during harvest and expressed as percentage infected plants. Seed treatment with the extract significantly (P < 0.05) reduced the incidence of both sorghum loose and covered smut diseases, compared favourably with the standard fungicide, and resulted in significant yield increases compared to the untreated control.

Tulbaghia violacea L. II: In vivo antifungal properties towards plant pathogens

Natural products from plants: commercial prospects in terms of antimicrobial, herbicidal and bio-stimulatory activities in an integrated pest management system.

Johan C. Pretorius

Papers

Purification and identification of active antibacterial components in Carpobrotus edulis L.

In vitro and in vivo antifungal activity of crude extracts and powdered dry material from Ethiopian wild plants against economically important plant pathogens

Tulbaghia violacea L. I: In vitro antimicrobial properties towards plant pathogens

Tulbaghia violacea L. II: In vivo antifungal properties towards plant pathogens

Natural products from plants: commercial prospects in terms of antimicrobial, herbicidal and bio-stimulatory activities in an integrated pest management system.