Seaweed-Based Compounds and Products for Sustainable Protection against Plant Pathogens

Sustainable agricultural practices increasingly demand novel, environmentally friendly compounds which induce plant immunity against pathogens. Stimulating plant immunity using seaweed extracts is a highly viable strategy, as these formulations contain many bio-elicitors (phyco-elicitors) which can significantly boost natural plant immunity. Certain bioactive elicitors present in a multitude of extracts of seaweeds (both commercially available and bench-scale laboratory formulations) activate pathogen-associated molecular patterns (PAMPs) due to their structural similarity (i.e., analogous structure) with pathogen-derived molecules. This is achieved via the priming and/or elicitation of the defense responses of the induced systemic resistance (ISR) and systemic acquired resistance (SAR) pathways. Knowledge accumulated over the past few decades is reviewed here, aiming to explain why certain seaweed-derived bioactives have such tremendous potential to elicit plant defense responses with considerable economic significance, particularly with increasing biotic stress impacts due to climate change and the concomitant move to sustainable agriculture and away from synthetic chemistry and environmental damage. Various extracts of seaweeds display remarkably different modes of action(s) which can manipulate the plant defense responses when applied. This review focuses on both the similarities and differences amongst the modes of actions of several different seaweed extracts, as well as their individual components. Novel biotechnological approaches for the development of new commercial products for crop protection, in a sustainable manner, are also suggested.     

Tasco®: a product of Ascophyllum nodosum enhances immune response of Caenorhabditis elegans against Pseudomonas aeruginosa infection

The effects of Tasco^®, a product made from the brown seaweed (Ascophyllum nodosum) were tested for the ability to protect Caenorhabditis elegans against Pseudomonas aeruginosa infection. A water extract of Tasco^® (TWE) reduced P. aeruginosa inflicted mortality in the nematode. The TWE, at a concentration of 300 µg/mL, offered the maximum protection and induced the expression of innate immune response genes viz.; zk6.7 (Lypases), lys-1 (Lysozyme), spp-1 (Saponin like protein), f28d1.3 (Thaumatin like protein), t20g5.7 (Matridin SK domain protein), abf-1 (Antibacterial protein) and f38a1.5 (Lectin family protein). Further, TWE treatment also affected a number of virulence components of the P. aeuroginosa and reduced its secreted virulence factors such as lipase, proteases and toxic metabolites; hydrogen cyanide and pyocyanin. Decreased virulence factors were associated with a significant reduction in expression of regulatory genes involved in quorum sensing, lasI, lasR, rhlI and rhlR. In conclusion, the TWE-treatment protected the C. elegans against P. aeruginosa infection by a combination of effects on the innate immunity of the worms and direct effects on the bacterial quorum sensing and virulence factors.     

Combined effects of plant growth‐promoting rhizobacteria and genistein on nitrogen fixation in soybean at suboptimal root zone temperatures

Application of plant growth‐promoting rhizobacteria (PGPR) or the plant to bacteria signal molecule genistein has been shown to increase nodulation and nitrogen (N) fixation by soybean [Glycine max (L.) Merr.] over a range of root zone temperatures (RZTs) and, specifically, off‐sets at least some of the ill‐effects of low RZTs. Two sets of controlled‐environment experiments, one on a growth bench and the other in a greenhouse, were conducted to examine the combined ability of both PGPR and genistein to reduce the negative effects of low RZT on soybean nodulation and N fixation. Each of two the PGPR strains, Serratia proteamaculans 1–102 and Serratia liquefaciens 2–68 were co‐inoculated with Bradyrhizobium japonicum USDA 110 preincubated with 17.5 (somewhat inhibitory), and 15°C (very inhibitory). At RZTs of 25 and 17.5°C PGPR strains and genistein in combination increased the number of nodules and the amount of Nn fixed. The most stimulatory effect was observed at 17.5°C for the combination: S. proteamaculans 1–102 plus B. japonicum USDA 110 pre‐incubated in 15 μM genistein under greenhouse conditions. For most treatment combinations the stimulatory effects of PGPR and genistein were additive at RZTs of 17.5 and 25°C. Surprisingly, the combination of these two factors resulted in antagonism at the very inhibitory RZT of 15°C. The results suggest that the negative effects of certain low RZTs could be more effectively off‐set by combined treatments of PGPR plus geneistin pre‐incubation of rhizobial cultures than by their individual treatment.     

Critical Leaf Area Index in Pigeonpea

Experiment conducted with six pigeonpea cultivars over three seasons revealed that the critical leaf area index was 5.3 which coincided with the maximum crop growth rate and optimum net assimilation rate. It was also evident that the crop growth rate was influenced more by NAR rather than LAI. This study also suggests that by maintaining higher photosynthesis upto harvest, there is ample possibility to increase the crop growth rate till harvest.     

Intercropping of Corn with Soybean, Lupin and Forages: Silage Yield and Quality

Intercropping of corn with legumes is an alternative to corn monocropping and has a number of advantages, for example, lower levels of inputs, lower cost of production and better silage quality than monocrop systems. An experiment was carried out at two sites in 1993 and 1994 to investigate the effects of seeding soybean or lupin alone or in combination with one of three forages (annual ryegrass, Lolium multiflorum Lam.; perennial ryegrass, Lolium perenne L.; red clover, Trifolium pratense L.) on silage yield and quality. The intercrop plots received 90 kg ha⁻¹ less nitrogen fertilizer than monocrop plots, which received 180 kg ha⁻¹. Corn biomass yield had a variable response to the treatments, but showed no change at most site-years. Soybean and lupin biomass yields were decreased by intercropping (80–98 % for soybean, and 94–100 % for lupin). However, when corn growth was limited due to poor establishment at one site in 1994, soybean was able to grow well and produce yields similar to those of monocropped soybean. The three underseeded forages did not grow well during the period examined (up to silage harvest) and had no effect on the yield of any crop. Total silage yields were similar to corn monocrop biomass yields even during 1994 at the site with low corn population densities because soybean was able to compensate for reduced corn growth.     

Root and Rhizosphere Colonization of Soybean [Glycine max (L.) Merr.] by Plant-Growth-Promoting Rhizobacteria at Low Root Zone Temperatures and under Short-Season Conditions

Co-inoculation of plant-growth-promoting rhizobacteria (PGPR) with B. japonicum has been shown to increase soybean [ Glycine max (L.) Merr.] nodulation, nitrogen fixation, growth and physiological activity at suboptimal root zone temperatures (RZTs). We studied the survival and growth of seven PGPR inoculated on soybean in a sterile rooting medium at three RZTs (25, 17.5 and 15 °C) on a growth bench. The survival of the two most promising strains ( Serratia liquefaciens 2-68 and S. proteamaculans 1-102) was studied under field conditions in methyl bromide fumigated and non-fumigated soils. In general, population densities varied with temperature. PGPR strains generally colonized the rhizosphere and root surface effeciently at higher RZTs; however, S. proteamaculans 1-102 colonized best at a low RZT (15 °C). The population of PGPR applied to the rhizosphere either with or without addition of B. japonicum increased over time in fumigated soil as compared to non-fumigated soil, indicating that the PGPR survive and proliferate better under fumigated conditions. S. liquefaciens 2-68 had higher population densities both on the root and in the rhizosphere, demonstrating, their ability to colonize under short-season conditions. The possible interactions between the two cultivars (Maple Glen and AC Bravor) and the PGPR were generally not significant, despite observations that growth and yield of AC Bravor respond more strongly to PGPR inoculation.     

Physiology of phytoplankton in relation to metal concentration Effect of cadmium on Scenedesmus bijugatus and Nitzschia palea

The effect of Cd at sublethal concentrations on pure cultures of S. bijugatus and N. palea was studied with special reference to their physiology during exponential growth phase. Photosynthetic productivity, respiration, pH, biomass, pigments, carbohydrate, protein, lipid as well as nutrient uptake were selected to study the comprehensive effect of the metal. The physiology of the species was significantly affected by Cd during the exponential growth phase and altered the ratio of carbohydrate, protein and lipid in the test species.