Global warming leads to the concurrence of a number of abiotic and biotic stresses, thus affecting agricultural productivity. Occurrence of abiotic stresses can alter plant-pest interactions by enhancing host plant susceptibility to pathogenic organisms, insects, and by reducing competitive ability with weeds. On the contrary, some pests may alter plant response to abiotic stress factors. Therefore, systematic studies are pivotal to understand the effect of concurrent abiotic and biotic stress conditions on crop productivity. However, to date, a collective database on the occurrence of various stress combinations in agriculturally-prominent areas is not available. This review attempts to assemble published information on this topic, with a particular focus on the impact of combined drought and pathogen stresses on crop productivity. In doing so, this review highlights some agriculturally important morpho-physiological traits that can be utilized to identify genotypes with combined stress tolerance. In addition, this review outlines potential role of recent genomic tools in deciphering combined stress tolerance in plants. This review will, therefore, be helpful for agronomists and field pathologists in assessing the impact of the interactions between drought and plant-pathogens on crop performance. Further, the review will be helpful for physiologists and molecular biologists to design agronomically relevant strategies for the development of broad spectrum stress tolerant crops.

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Impact of Combined Abiotic and Biotic Stresses on Plant Growth and Avenues for Crop Improvement by Exploiting Physio-morphological Traits

Contents Summary 67 I. Introduction 68 II. Silicon transport in plants: to absorb or not to absorb 69 III. The role of silicon in plants: not just a matter of semantics 71 IV. Silicon and biotic stress: beyond mechanical barriers and defense priming 76 V. Silicon and abiotic stress: a proliferation of proposed mechanisms 78 VI. The apoplastic obstruction hypothesis: a working model 79 VII. Perspectives and conclusions 80 Acknowledgements 81 References 81 SUMMARY: Silicon (Si) is not classified as an essential plant nutrient, and yet numerous reports have shown its beneficial effects in a variety of species and environmental circumstances. This has created much confusion in the scientific community with respect to its biological roles. Here, we link molecular and phenotypic data to better classify Si transport, and critically summarize the current state of understanding of the roles of Si in higher plants. We argue that much of the empirical evidence, in particular that derived from recent functional genomics, is at odds with many of the mechanistic assertions surrounding Si's role. In essence, these data do not support reports that Si affects a wide range of molecular-genetic, biochemical and physiological processes. A major reinterpretation of Si's role is therefore needed, which is critical to guide future studies and inform agricultural practice. We propose a working model, which we term the 'apoplastic obstruction hypothesis', which attempts to unify the various observations on Si's beneficial influences on plant growth and yield. This model argues for a fundamental role of Si as an extracellular prophylactic agent against biotic and abiotic stresses (as opposed to an active cellular agent), with important cascading effects on plant form and function.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/nph.15343

The controversies of silicon's role in plant biology.

Elemental silicon (Si), after oxygen, is the second most abundant element in the earth’s crust, which is mainly composed of silicates. Si is not considered essential for plant growth and development, however increasing evidence in the literature shows that this metalloid is beneficial to plants, especially under stress conditions. Indeed Si alleviates the toxic effects caused by abiotic stresses, e.g. salt stress, drought, heavy metals, to name a few. Biogenic silica is also a deterrent against herbivores. Additionally, Si ameliorates the vigour of plants and improves their resistance to exogenous stresses. The protective role of Si was initially attributed to a physical barrier fortifying the cell wall (e.g. against fungal hyphae penetration), however several studies have shown that the action of this element on plants is far more complex, as it involves a cross-talk with the cell interior and an effect on plant metabolism. In this study the beneficial role of Si on plants will be discussed, by reviewing the available data in the literature. Emphasis will be given to the protective role of Si during (a)biotic stresses and in this context both priming and the effects of Si on endogenous phytohormones will be discussed. A whole section will be devoted to the use of silica (SiO2) nanoparticles, in the light of the interest that nanotechnology has for agriculture. The paper also discusses the potential technological aspects linked to the use of Si in agriculture and to modify/improve the physical parameters of plant fibers. The study indeed provides perspectives on

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Silicon and Plants: Current Knowledge and Technological Perspectives.

Over the past few decades, heavy metal contamination in soil and water has increased due to anthropogenic activities. The higher exposure of crop plants to heavy metal stress reduces growth and yield, and affect the sustainability of agricultural production. In this regard, the use of silicon (Si) supplementation offers a promising prospect since numerous studies have reported the beneficial role of Si in mitigating stresses imposed by biotic as well as abiotic factors including heavy metal stress. The fundamental mechanisms involved in the Si-mediated heavy metal stress tolerance include reduction of metal ions in soil substrate, co-precipitation of toxic metals, metal-transport related gene regulation, chelation, stimulation of antioxidants, compartmentation of metal ions, and structural alterations in plants. Exogenous application of Si has been well documented to increase heavy metal tolerance in numerous plant species. The beneficial effects of Si are particularly evident in plants able to accumulate high levels of Si. Consequently, to enhance metal tolerance in plants, the inherent genetic potential for Si uptake should be improved. In the present review, we have discussed the potential role and mechanisms involved in the Si-mediated alleviation of metal toxicity as well as different approaches for enhancing Si-derived benefits in crop plants.

Role of Silicon in Mitigation of Heavy Metal Stresses in Crop Plants.

Recent advancement in the natural fiber polymer composites: A comprehensive review

Review on mechanical properties of natural fiber composites.

The controversy surrounding silicon (Si) benefits and essentiality in plants is exacerbated by the differential ability of species to absorb this element. This property is seemingly enhanced in species carrying specific nodulin 26-like intrinsic proteins (NIPs), a subclass of aquaporins. In this work, our aim was to characterize plant aquaporins to define the features that confer Si permeability. Through comparative analysis of 985 aquaporins in 25 species with differing abilities to absorb Si, we were able to predict 30 Si transporters and discovered that Si absorption is exclusively confined to species that possess NIP-III aquaporins with a GSGR selectivity filter and a precise distance of 108 amino acids (AA) between the asparagine-proline-alanine (NPA) domains. The latter feature is of particular significance since it had never been reported to be essential for Si selectivity. Functionality assessed in the Xenopus oocyte expression system showed that NIPs with 108 AA spacing exhibited Si permeability, while proteins differing in that distance did not. In subsequent functional studies, a Si transporter from poplar mutated into variants with 109- or 107-AA spacing failed to import, and a tomato NIP gene mutated from 109 to 108 AA exhibited a rare gain of function. These results provide a precise molecular basis to classify higher plants into Si accumulators or excluders.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/tpj.12904

A precise spacing between the NPA domains of aquaporins is essential for silicon permeability in plants

Summary
Tritrophic interactions involving a biocontrol agent, a pathogen and a plant have been analyzed predominantly from the perspective of the biocontrol agent. We have conducted the first comprehensive transcriptomic analysis of all three organisms in an effort to understand the elusive properties of Pseudozyma flocculosa in the context of its biocontrol activity against Blumeria graminis f.sp. hordei as it parasitizes Hordeum vulgare.
After inoculation of P. flocculosa, the tripartite interaction was monitored over time and samples collected for scanning electron microscopy and RNA sequencing.
Based on our observations, P. flocculosa indirectly parasitizes barley, albeit transiently, by diverting nutrients extracted by B. graminis from barley leaves through a process involving unique effectors. This brings novel evidence that such molecules can also influence fungal–fungal interactions. Their release is synchronized with a higher expression of powdery mildew haustorial effectors, a sharp decline in the photosynthetic machinery of barley and a developmental peak in P. flocculosa. The interaction culminates with a collapse of B. graminis haustoria, thereby stopping P. flocculosa growth, as barley plants show higher metabolic activity.
To conclude, our study has uncovered a complex and intricate phenomenon, described here as hyperbiotrophy, only achievable through the conjugated action of the three protagonists.

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Effectors involved in fungal-fungal interaction lead to a rare phenomenon of hyperbiotrophy in the tritrophic system biocontrol agent-powdery mildew-plant.

Review on metal matrix composites for marine applications

A simple formulation of Trichoderma viride Pers ex. S.F. Grey for seed pelleting was developed by mixing tale powder with Trichoderma biomass obtained by growing the fungus in molasses-yeast medium for 15 days and drying. For effective adherence to the seeds CMC was added @ 0.5 % leveJ after drying. This formulation packed in polythene bags yielded 8 x 10 7 colonies/g after 120 days. Seed treatment with this product effectively reduced the root rot diseases of urd bean (Vigna mungo {L.} Hepper caused by Macrophomina phaseolina (Tassi) Goid. both in the artificially infested soil and natural soil in farmers fields.

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G. Ramakrishnan

Papers

Review on mechanical properties of natural fiber composites.

A precise spacing between the NPA domains of aquaporins is essential for silicon permeability in plants

Effectors involved in fungal-fungal interaction lead to a rare phenomenon of hyperbiotrophy in the tritrophic system biocontrol agent-powdery mildew-plant.

Review on metal matrix composites for marine applications

Talc-Based formulation of Trichoderma viride for Biocontrol of Macrophomina phaseolina