Marine algal seaweed species are often regarded as an underutilized bioresource, many have been used as a source of food, industrial raw materials, and in therapeutic and botanical applications for centuries. Moreover, seaweed and seaweed-derived products have been widely used as amendments in crop production systems due to the presence of a number of plant growth-stimulating compounds. However, the biostimulatory potential of many of these products has not been fully exploited due to the lack of scientific data on growth factors present in seaweeds and their mode of action in affecting plant growth. This article provides a comprehensive review of the effect of various seaweed species and seaweed products on plant growth and development with an emphasis on the use of this renewable bioresource in sustainable agricultural systems.

Seaweed Extracts as Biostimulants of Plant Growth and Development

Senescence is a complex, highly regulated, developmental phase in the life of a leaf that results in the co-ordinated degradation of macromolecules and the subsequent mobilization of components to other parts of the plant. The application of molecular biology techniques to the study of leaf senescence has, in the last few years, enabled the isolation and characterization of a large range of cDNA clones representing genes that show increased expression in senescing leaves. The analysis of these genes and identification of the function of the encoded proteins will allow a picture of the complex processes that take place during senescence to be assembled. To date, genes encoding degradative enzymes such as proteases and nucleases, enzymes involved in lipid and carbohydrate metabolism and enzymes involved in nitrogen mobilization have all been identified as senescenceenhanced genes. A variety of other genes of no obvious senescence-related function have also been identified; their role in senescence may be less predictable and, possibly, more interesting. The combined action of several internal and external signals may be involved in the induction of senescence. Analysis of the regulatory mechanisms controlling the expression of senescence-induced genes will allow the signalling pathways that are involved in the regulation of senescence to be elucidated. Experiments with transgenic plants and mutants are already shedding light on the role played by cytokinins and ethylene in regulating senescence in leaves.

The molecular biology of leaf senescence

SUMMARY
Leaf senescence is a hiphly-controlled sequence of events comprising the final stage of development. Cells remain viable during the process and new gene expression is required. There is some similarity between senescence in plants and programmed cell death in animals. In this review, different classes of senescence-related genes are defined and progress towards isolating such genes is reported. A range of internal and external factors which appear to cause leaf senescence is considered and various models for the mechanism of senescence- initiation are described. The current understanding of senescence at the wrganelle and molecular levels is presented. Finally, same ideas are mooted as to why senescence occurs and why it should be studied further.

Gene expression during leaf senescence

The rate-limiting step of cytokinin biosynthesis in Arabidopsis thaliana Heynh. is catalyzed by ATP/ADP isopentenyltransferases, A. thaliana IsoPentenyl Transferase (AtIPT)1, and AtIPT4, and by their homologs AtIPT3, AtIPT5, AtIPT6, AtIPT7, and AtIPT8. To understand the dynamics of cytokinins in plant development, we comprehensively analyzed the expression of isopentenyltransferase genes of Arabidopsis. Examination of their mRNA levels and the expression patterns of the beta-glucuronidase (GUS) gene fused to the regulatory sequence of each AtIPT gene revealed a specific expression pattern of each gene. The predominant expression patterns were as follows: AtIPT1::GUS, xylem precursor cell files in the root tip, leaf axils, ovules, and immature seeds; AtIPT3::GUS, phloem tissues; AtIPT4::GUS and AtIPT8::GUS, immature seeds with highest expression in the chalazal endosperm (CZE); AtIPT5::GUS, root primordia, columella root caps, upper part of young inflorescences, and fruit abscission zones; AtIPT7::GUS, endodermis of the root elongation zone, trichomes on young leaves, and some pollen tubes. AtIPT1, AtIPT3, AtIPT5, and AtIPT7 were downregulated by cytokinins within 4 h. AtIPT5 and AtIPT7 was upregulated by auxin within 4 h in roots. AtIPT3 was upregulated within 1 h after an application of nitrate to mineral-starved Arabidopsis plants. The upregulation by nitrate did not require de novo protein synthesis. We also examined the expression of two genes for tRNA isopentenyltransferases, AtIPT2 and AtIPT9, which can also be involved in cytokinin biosynthesis. They were expressed ubiquitously, with highest expression in proliferating tissues. These findings are discussed in relation to the role of cytokinins in plant development.

Expression of cytokinin biosynthetic isopentenyltransferase genes in Arabidopsis: tissue specificity and regulation by auxin, cytokinin, and nitrate.

Lippia: traditional uses, chemistry and pharmacology: a review.

Antibacterial activity of South African plants used for medicinal purposes

Screening of Zulu medicinal plants for prostaglandin-synthesis inhibitors

Antibacterial and anti-inflammatory activities of some plants used for medicinal purposes in Kenya.

Fire is a major environmental selective force that influences plant communities in many parts of the world. Reproductive strategies have evolved as adaptations to the various factors generated by, and/or associated with, fire. This is particularly true for seeds, in which strategies have evolved that respond to both the physical and chemical germination cues that may be associated with fires. Some of these cues are clearly of a primary nature, while others may play a secondary and/or highly specific, or even permissive, role in the germination process. Clearly, there must be considerable interaction between the physical and chemical cues generated by wild fires. Smoke is the most striking chemical factor. Smoke released from burning vegetation contains a chemical signal that triggers germination of both fire climax and non-fire climax species from different parts of the world. It is used in horticulture to stimulate seed germination of wildflower species and can break dormancy and improve germination of vegetable crops, such as lettuce and celery. Smoke can be applied to seeds immediately before sowing, or the seeds may be pretreated and stored until conditions are appropriate for sowing. Both smoke and aqueous smoke-water are active in this respect. The active constituent(s) is volatile, thermostable, water soluble and long-lasting in aqueous solution and in the soil. Attempts to identify the active compound(s) and to determine the mechanism(s) of action have been unsuccessful. Smoke treatment is effective on many seeds that have a light requirement for germination. A very clear concentration effect, resembling that of hormonal responses has been established with aqueous smoke solutions. Smoke extracts interact with gibberellins, cytokinins, abscisic acid and ethylene in photoblastic and in thermodormant seeds. However, despite these interactions it remains questionable whether smoke acts via hormones in stimulating seed germination. There is a clear connection between smoke and life history traits. Smoke may well be the overriding trigger for germination in relation to specific growth habits, regeneration strategies, seed storage, seed sizes, dispersal modes and structures for a large number of species growing in fire-prone habitats. It is becoming increasingly clear that smoke as a germination (or growth regulating) cue must have evolved as part, or as a consequence of fire, as an evolutionary factor. As such, it is probably a very old development and serves as an additional protection mechanism to ensure germination at optimal times for seedling survival.

https://esj-journals.onlinelibrary.wiley.com/doi/pdf/10.1046/j.1442-1984.2000.00037.x

Johannes van Staden

Papers

Antibacterial activity of South African plants used for medicinal purposes

Screening of Zulu medicinal plants for prostaglandin-synthesis inhibitors

Antibacterial and anti-inflammatory activities of some plants used for medicinal purposes in Kenya.

Smoke as a germination cue

Screening of medicinal plants used in South African traditional medicine for genotoxic effects.