The transformations of nitrogenous compounds by soil bacteria have beenstudied for over a century. In terms of the global fluxes between aerial and terrestrial-aquatic systems, the simplified nitrogen cycle can be envisioned as a triangle where the only biologically reversible reaction occurs between ammonium and nitrate. The reverse reactions of dinitrogen fixation or denitrification by biological means do not occur in nature. Hence, the reductive process of denitrification, defined as the reduction of nitrate or nitrite to gaseous nitrogen (usually N2), is intimately associated with the oxidative process of nitrification.

Biochemical Ecology of Nitrification and Denitrification

Both micro- and macroalgae have long been used to augment plant productivity and food production in various regions of the world through their beneficial effects when applied to soils. Interactions of algae with the soil community undoubtedly are complex and benefits are dependent on the crop and the local environmental conditions. This has resulted in much speculation as to mechanisms involved as well as the validity of the results reported. It is now 60 years since the first commercial seaweed extract was manufactured for agricultural use. These aqueous extracts allowed for the first time the direct application of soluble seaweed constituents to specific plant organs such as leaves and roots. The earlier concept that benefits of seaweeds and their extracts were due mainly to their manurial value or to their micronutrient suites is no longer tenable. Seaweeds likewise have been used for millennia as fodder supplements to improve animal nutrition and productivity. Recent research is focusing on their mode of action, specific health benefits, and the mechanisms of action in animals. Improved analytical techniques and instrumentation coupled with the use of molecular genetic tools are establishing that seaweed extracts can modify plant and animal responses at a fundamental level. It therefore seems appropriate to review key developments over the years and to remark on novel findings. A new and exciting vista has opened for seaweed extracts in both plant and animal applications.

Seaweed extract stimuli in plant science and agriculture

Gibberellins control a range of growth and developmental processes in higher plants and have been widely used in the agricultural industry. By binding to a nuclear receptor, GIBBERELLIN INSENSITIVE DWARF1 (GID1), gibberellins regulate gene expression by promoting degradation of the transcriptional regulator DELLA proteins, including GIBBERELLIN INSENSITIVE (GAI). The precise manner in which GID1 discriminates and becomes activated by bioactive gibberellins for specific binding to DELLA proteins remains unclear. Here we present the crystal structure of a ternary complex of Arabidopsis thaliana GID1A, a bioactive gibberellin and the amino-terminal DELLA domain of GAI. In this complex, GID1A occludes gibberellin in a deep binding pocket covered by its N-terminal helical switch region, which in turn interacts with the DELLA domain containing DELLA, VHYNP and LExLE motifs. Our results establish a structural model of a plant hormone receptor that is distinct from the mechanism of the hormone perception and effector recognition of the known auxin receptors.

Gibberellin-induced DELLA recognition by the gibberellin receptor GID1

A biostimulant is an organic material that, when applied in small quantities, enhances plant growth and development such that the response cannot be attributed to the application of traditional plant nutrients. This review is aimed at highlighting developments in the processing of macroalgae for agricultural biostimulants (AB), summarising the biologically active components of brown macroalgae and examining the factors supporting the use of macroalgal AB for managing abiotic and biotic stresses in crop plants. The policy drivers supporting the use of macroalgal-derived ABs in agriculture are also emphasised. We examine the use of macroalgal ABs in crop production and evaluated the benefits of seed priming, foliar application, soil drenches and hydroponic treatments. The use of macroalgal ABs on crop plants can generate multiple benefits with reported effects including enhanced rooting, higher crop and fruit yields, freezing, drought and salt tolerance, enhanced photosynthetic activity and resistance to fungi, bacteria and virus. ABs can be applied as an alternative, or used in conjunction with synthetic crop protection products and plant growth regulators, and may have a role in maintaining crop production levels, health and quality in the future when many active ingredients will be lost to the industry due to changes in European Union regulations. Worldwide, macroalgae remain largely unexploited, we highlight some of the future research and development priorities.

Plant biostimulants: a review on the processing of macroalgae and use of extracts for crop management to reduce abiotic and biotic stresses

Publisher Summary The chapter presents a discussion on plant growth-regulating (PGR) substances in the rhizosphere, with emphasis on microbial production and functions. The chapter discusses the rhizosphere as a site of plant-microbe interactions; plant growth-regulating substances and their sources; biochemistry of microbial production of PGRs; production of PGRs by rhizosphere microorganisms; metabolism of PGRs in soil; and ecological significance of PGRs produced in the rhizosphere. The chapter provides a better understanding of the mechanisms of actions of microbially derived PGRs and their interactions with plants. Moreover, development of hormone-deficient plant mutants can provide opportunities to clearly define the role of microbially produced PGRs in plan-microbe interactions. An understanding of these aspects can aid in the utilization of microbial PGRs for the betterment and benefit of sustainable agriculture.

Plant growth-regulating substances in the rhizosphere: microbial production and functions

Gibberellic acid is a metabolic product of the fungus Gibberella fujikuroi, similar in physiological properties to the gibberellins described by Japanese investigators. Supplied in a nutrient solution to wheat plants growing in water culture, it causes increased growth of the shoots, as a result of increased length of stem internodes and leaves. The leaves are narrower and paler than those of untreated plants. Under similar conditions pea seedlings develop much elongated stem internodes, but the leaves are little changed in size. These morphological responses are similar to those seen in etiolation.

The total dry weight of both peas and wheat so treated is increased; if shoots alone are considered the increase in dry weight is even greater; the dry weight of roots is reduced, though this does not occur if the gibberellic acid is applied to the shoots in a lanolin paste. The increased dry weight is mainly attributable to increased carbon assimilation. Treated plants contain more soluble carbohydrate than controls, but this accounts only for a small part of the increased carbon assimilation. The increase in glucose content is particularly striking.

Gibberellic acid is rapidly biologically degraded in soil.

The plant‐growth‐promoting properties of gibberellic acid, a metabolic product of the fungus gibberella fujikuroi

A Physiological Comparison of Gibberellic Acid with Some Auxins

The Distribution of Substances similar to Gibberellic Acid in Higher Plants

Some strains of Gibberella fujikuroi (Saw.) Wr. (conidial state Fusarium moniliforme (Sheld.) emend. Snyder & Hansen) produce metabolic products with plant-growth-promoting properties. The capacity to produce such substances is common in rice-infecting strains of G. fujikuroi; isolates from other hosts only infrequently produce such metabolites. From a selected rice-infecting strain a plant-growth-promoting substance has been isolated in pure form. This substance, gibberellic acid, is distinct in chemical properties from the gibberellins described by Japanese investigators. In surface culture on Raulin-Thom medium (2.5% sucrose) yields of 40 mg./1. are obtained in 14 days at 25°. In submerged aerated culture yields approaching 200 mg./1. are obtained on Raulin-Thom medium (4% sucrose) in 18 days at 25°. Under such conditions, although gibberellic acid begins to appear in the medium during the latter part of the phase of active growth, at least 75%, and probably more, of the total final yield is produced after maximum dry weight has been reached and rapid uptake of sugar and ammonia-nitrogen has ceased.

Gibberellic acid, a metabolic product of the fungus Gibberella fujikuroi: Some observations on its production and isolation

IF the fungal metabolite gibberellic acid is applied to dwarf varieties of pea, french bean (Phaseolus), broad bean (Vicia) or maize, growth is greatly accelerated and plants similar to the normal tall phenotype are produced; the tall varieties themselves are much less affected by gibberellic acid, if at all1,2. This suggests that this acid, or a substance with similar physiological properties, is a normal plant-growth regulator, produced less abundantly in dwarf varieties than in tall ones. The growth-promoting substance obtained from immature bean seeds by Mitchell, Skaggs and Anderson3 was probably of this type; more recently, West and Phinney4 have reported the probable occurrence of substances like gibberellic acid in seeds of several plant species; I have obtained evidence of such a substance in the shoots of dwarf and tall peas, and this is summarized below.

Margaret Radley

Papers

The plant‐growth‐promoting properties of gibberellic acid, a metabolic product of the fungus gibberella fujikuroi

A Physiological Comparison of Gibberellic Acid with Some Auxins

The Distribution of Substances similar to Gibberellic Acid in Higher Plants

Gibberellic acid, a metabolic product of the fungus Gibberella fujikuroi: Some observations on its production and isolation

Occurrence of Substances Similar to Gibberellic Acid in Higher Plants