With global climate changes and the increased demand for food due to expected world population growth, genetic improvement programs have aimed at producing crops with increased yield and tolerance to environmental stresses, such as drought, salinity, and pathogens. On the other hand, genetic improvement programs via biotechnology require candidate genes that confer traits of interest to be incorporated into improved crops. In this regard, genes encoding transcription factors (TFs) can be promising since they are proteins that transcriptionally regulate the expression of target genes related to the most diverse roles in the plant, including defense against stresses. Among TFs, bZIP (basic leucine zipper) proteins regulate many developmental and physiological processes in the plant, such as seed formation, fruit ripening, nutrient assimilation, and defense response to abiotic and biotic stresses. In this review, we aim to highlight the main advances in the potential use of bZIP TFs in the genetic improvement of crops. We address this potential mainly regarding crop tolerance to stresses and other agricultural traits, such as increased yield and fruit features.

Chitinases are present in diverse form of organisms from bacteria, fungi, insects, plants. Plant chitinases are part of pathogenesis-related proteins. When plant (host) cells are under pathogen stress, plant chitinases are strongly expressed and hence plant chitinases play a critical part against fungal pathogens. Chitinases are also found to be involved in various abiotic stress responses like wounding, osmotic pressure, cold, heavy metal stress, salt in plants. Understanding of the plant chitinases will provide an insight for improving the pathogenic activity of various potential biocontrol strains and to develop novel pathogen resistant strategies for exploring their roles with regards to plant defense. The present review covers the detailed account of potential and relevance of plant chitinases for controlling pathogens infection in plant and prospecting to improve plant defense, growth and yield.

Phloem sieve element (SE) occlusion has been hypothesized for decades to be a mechanism of resistance against phloem sap-feeding insects. Few studies have tested this hypothesis although it is likely a widespread phenomenon. This review focuses on SE occlusion by callose and P-proteins. Both are reversible, which would allow the plant to defend itself against phloem sap-feeders when SEs are penetrated and resume normal function when the insects give up and withdraw their stylets. Callose (β-1,3 glucans with some β-1,6 branches) serves many roles in plant physiology in many different tissues, each being under the control of different callose synthase genes; only callose deposited in SE sieve pores is relevant to SE occlusion. The amount of callose in sieve pores (and consequently how much it impedes sap flow) is determined by the balance in activity between callose synthase and β-1,3 glucanase. Sieve pore callose deposition has been shown to provide resistance to some phloem sap-feeders in a few studies, and in one, the difference in resistance between a susceptible and resistant rice variety was due to the ability or inability of the insect to upregulate the plants\' β-1,3 glucanase that degrades the callose deposition. P-proteins occur only in dicotyledons and include a variety of proteins, not all of which are involved in SE occlusion. In some plants, P-proteins form distinct bodies in mature functional SEs. In papilionid legumes, these discrete bodies, called forisomes can expand and contract. In their expanded state, they effectively plug SEs and stop the flow of sap while in their contracted state, they provide negligible resistance to sap flow. Expansion of forisomes is triggered by an influx of Ca2+ into the SE. Penetration of a legume (Vicia faba) SE by a generalist aphid not adapted to legumes triggers forisome expansion which occludes the SE and prevents the aphid from ingesting sap. In contrast, a legume specialist aphid, Acyrthosiphon pisum, does not trigger forisome expansion and readily ingests sap from V. faba. P-protein bodies in SEs of non-legumes do not appear to be involved in SE occlusion. In most dicotyledons, P-proteins do not form discrete bodies, but rather occur as filamentous aggregations adhering to the parietal margins of the SE and in response to damage, are released into the lumen where they are carried by the flow of sap to the downstream sieve plate where they back up and clog the sieve pores. Their effectiveness at actually stopping the flow of sap is controversial. In one study, they seemed to provide little resistance to the flow of sap while in other studies, they provided considerable resistance. In response to injury in melon, they completely stop the flow of sap, and in an aphid-resistant melon, penetration of SEs by the melon aphid, Aphis gossypii, triggers P-protein occlusion which prevents the aphids from ingesting sap. The first P-protein described, PP1, occurs only in the genus Cucurbita, and although it has been often cited to function as a SE occlusion protein, experimental evidence suggests it does not play a significant role in SE occlusion. The most common strategy for phloem sap-feeders to mitigate P-protein occlusion seems to be avoid triggering it. A widely cited in vitro study suggested that aphid saliva can reverse P-protein occlusion, but a subsequent study demonstrated that saliva was ineffective at reversing P-protein occlusion in vivo. Lastly, SE callose deposition in wheat triggered by Russian wheat aphid has been hypothesized to create an artificial sink that benefits the aphid, but additional studies are needed to test that hypothesis.

Saponins, prominent secondary plant metabolites, are recognized for their roles in plant defense and medicinal benefits. Soyasaponins, commonly derived from legumes, are a class of triterpenoid saponins that demonstrate significant potential for plant and human health applications. Previous research and reviews largely emphasize human health effects of soyasaponins. However, the biological effects of soyasaponins and their implications for plants in the context of human health have not been well-discussed. This review provides comprehensive discussions on the biological roles of soyasaponins in plant defense and rhizosphere microbial interactions; biosynthetic regulation and compound production; immunological effects and potential for therapeutics; and soyasaponin acquisition attributed to processing effects, bioavailability, and biotransformation processes based on recent soyasaponin research. Given the multifaceted biological effects elicited by soyasaponins, further research warrants an integrated approach to understand molecular mechanisms of regulations in their production as well as their applications in plant and human health.

Numerous harmful microorganisms and insect pests have the ability to cause plant infections or damage, which is mostly controlled by toxic chemical agents. These chemical compounds and their derivatives exhibit hazardous effects on habitats and human life too. Hence, there\'s a need to develop novel, more effective and safe bio-control agents. A variety of microbes such as viruses, bacteria, and fungi possess a great potential to fight against phytopathogens and thus can be used as bio-control agents instead of harmful chemical compounds. These naturally occurring microorganisms are applied to the plants in order to control phytopathogens. Moreover, practicing them appropriately for agriculture management can be a way towards a sustainable approach. The MBCAs follow various modes of action and act as elicitors where they induce a signal to activate plant defense mechanisms against a variety of pathogens. MBCAs control phytopathogens and help in disease suppression through the production of enzymes, antimicrobial compounds, antagonist activity involving hyper-parasitism, induced resistance, competitive inhibition, etc. Efficient recognition of pathogens and prompt defensive response are key factors of induced resistance in plants. This resistance phenomenon is pertaining to a complex cascade that involves an increased amount of defensive proteins, salicylic acid (SA), or induction of signaling pathways dependent on plant hormones. Although, there\'s a dearth of information about the exact mechanism of plant-induced resistance, the studies conducted at the physiological, biochemical and genetic levels. These studies tried to explain a series of plant defensive responses triggered by bio-control agents that may enhance the defensive capacity of plants. Several natural and recombinant microorganisms are commercially available as bio-control agents that mainly include strains of Bacillus, Pseudomonads and Trichoderma. However, the complete understanding of microbial bio-control agents and their interactions at cellular and molecular levels will facilitate the screening of effective and eco-friendly bio-agents, thereby increasing the scope of MBCAs. This article is a comprehensive review that highlights the importance of microbial agents as elicitors in the activation and regulation of plant defense mechanisms in response to a variety of pathogens.

The second most abundant biological macromolecule, next to cellulose is Chitosan. It is a versatile naturally occurring hydrophilic polysaccharide, derived as a deacetylated form of chitin. Due to its biocompatibility, biodegradability and antimicrobial activity, it has become a significant area of research towards drug delivery system, plant growth promotion, anti-pathogenic potentiality, seed priming and in plant defense mechanism. Various synthetic strategies have been established in recent years that couples different metals with chitosan nanoparticles. Metals like silver, copper, zinc, iron and nickel are highly compatible to form chitosan metallic nanoparticles and are proved to be non-toxic to the agricultural plant system. This review highlights the mode of action of nanochitosan on Gram-positive and Gram-negative bacteria in a distinguished manner as well as its action on fungi. A prime focus has been given on the skeletal framework of the metallic nanochitosan particles. Our study also projects the antimicrobial mechanism of chitosan based on its physiochemical properties, environmental factors and the type of organism on which it acts. Moreover, the mechanism for stimulation of plant immunity by metallic nanochitosan has also been reviewed. Our study relies on the conclusion that chitosan metallic nanoparticles showed enhanced anti-pathogenic and plant growth promoting activity in comparison to bulk chitosan.

Rising atmospheric carbon dioxide, an important driver of climate change, has multifarious effects on crop yields and quality. Despite tremendous progress in understanding the mechanisms of plant responses to elevated CO2, only a few studies have examined the CO2-enrichment effects on tea plants. Tea [Camellia sinensis (L.)], a non-deciduous woody perennial plant, operates massive physiologic, metabolic and transcriptional reprogramming to adapt to increasing CO2. Tea leaves elevate photosynthesis when grown at CO2-enriched environment which is attributed to increased maximum carboxylation rate of RuBisCO and maximum rates of RuBP regeneration. Elevated CO2-induced photosynthesis enhances the energy demand which triggers respiration. Stimulation of photosynthesis and respiration by elevated CO2 promotes biomass production. Moreover, elevated CO2 increases total carbon content, but it decreases total nitrogen content, leading to an increased ratio of carbon to nitrogen in tea leaves. Elevated CO2 alters the tea quality by differentially influencing the concentrations and biosynthetic gene expression of tea polyphenols, free amino acids, catechins, theanine, and caffeine. Signaling molecules salicylic acid and nitric oxide function in a hierarchy to mediate the elevated CO2-induced flavonoid biosynthesis in tea leaves. Despite enhanced synthesis of defense compounds, tea plant defense to some insects and pathogens is compromised under elevated CO2. Here we review the physiological and metabolic responses of tea plants to elevated CO2. In addition, the potential impacts of elevated CO2 on tea yield and defense responses are discussed. We also show research gaps and critical research areas relating to elevated CO2 and tea quality for future study.

Plant-based secondary metabolites with medicinal potentialities such as defensins are small, cysteine-rich peptides that represent an imperative aspect of the inherent defense system. Plant defensins possess broad-spectrum biological activities, e.g., bactericidal and insecticidal actions, as well as antifungal, antiviral, and anticancer activities. The unique structural and functional attributes provide a nonspecific and versatile means of combating a variety of microbial pathogens, i.e., fungi, bacteria, protozoa, and enveloped viruses. Some defensins in plants involved in other functions include the development of metal tolerance and the role in sexual reproduction, while most of the defensins make up the innate immune system of the plants. Defensins are structurally and functionally linked and have been characterized in various eukaryotic microorganisms, mammals, plants, gulls, teleost species of fish, mollusks, insect pests, arachnidan, and crustaceans. This defense mechanism has been improved biotechnologically as it helps to protect plants from fungal attacks in genetically modified organisms (GMO). Herein, we review plant defensins as secondary metabolites with medicinal potentialities. The first half of the review elaborates the origin, structural variations, and mechanism of actions of plant defensins. In the second part, the role of defensins in plant defense, stress response, and reproduction are discussed with suitable examples. Lastly, the biological applications of plant defensins as potential antimicrobial and anticancer agents are also deliberated. In summary, plant defensins may open a new prospect in medicine, human health, and agriculture.

Urease (urea amidohydrolase, EC 3.5.1.5) is a nickel-containing enzyme produced by plants, fungi, and bacteria that catalyzes the hydrolysis of urea into ammonia and carbamate. Urease is of historical importance in Biochemistry as it was the first enzyme ever to be crystallized (1926). Finding nickel in urease\'s active site (1975) was the first indication of a biological role for this metal. In this review, historical and structural features, kinetics aspects, activation of the metallocenter and inhibitors of the urea hydrolyzing activity of ureases are discussed. The review also deals with the non-enzymatic biological properties, whose discovery 40 years ago started a new chapter in the study of ureases. Well recognized as virulence factors due to the production of ammonia and alkalinization in diseases by urease-positive microorganisms, ureases have pro-inflammatory, endocytosis-inducing and neurotoxic activities that do not require ureolysis. Particularly relevant in plants, ureases exert insecticidal and fungitoxic effects. Data on the jack bean urease and on jaburetox, a recombinant urease-derived peptide, have indicated that interactions with cell membrane lipids may be the basis of the non-enzymatic biological properties of ureases. Altogether, with this review we wanted to invite the readers to take a second look at ureases, very versatile proteins that happen also to catalyze the breakdown of urea into ammonia and carbamate.

Plants have evolved a variety of defense mechanisms to tackle virus attack. Endogenous plant proteins can function as virus suppressors. Different types of proteins mediate defense responses against plant viruses. Pathogenesis-related (PR) proteins are activated upon pathogen infections or in different stress situations and their production is one of many components in plant defense. Ribosome-inactivating proteins (RIPs) suppress translation by enzymatically damaging ribosomes and they have been found to have antiviral activity. RNA-binding proteins (RBPs) bind to target RNAs via specialized RNA-binding domain and can directly or indirectly function in plant defense system against RNA viruses. Proteins involved in silencing machinery, namely Dicer-like (DCL) proteins, Argonaute (AGO) proteins, and RNA-dependent RNA polymerases (RDRs) confer innate antiviral defense in plants as they are able to degrade foreign RNA of viral origin. This review aims to provide a comprehensive and up-to-date picture of plant proteins participating in antiviral defense. As a result we discuss proteins conferring plant antiviral resistance and their potential future applications in different fields of life including agriculture and medicine.