Plants have innate immune systems or defense mechanisms that respond to the attack of pathogenic microorganisms. Unlike mammals, they lack mobile defense cells, so defense processes depend on autonomous cellular events with a broad repertoire of recognition to detect pathogens, which compensates for the lack of an adaptive immune system. These defense mechanisms remain inactive or latent until they are activated after exposure or contact with inducing agents, or after the application of the inductor; they remain inactive only until they are affected by a pathogen or challenged by an elicitor from the same. Resistance induction represents a focus of interest, as it promotes the activation of plant defense mechanisms, reducing the use of chemical synthesis pesticides, an alternative that has even led to the generation of new commercial products with high efficiency, stability and lower environmental impact, which increase productivity by reducing not only losses but also increasing plant growth. Considering the above, the objective of this review is to address the issue of resistance induction with a focus on the potential of the use of oligosaccharides in agriculture, how they are recognized by plants, how they can be used for commercial products and perspectives.

Various diseases, including bacterial panicle blight (BPB) and sheath rot, threaten rice production. It has been established that Burkholderia glumae (B. glumae) is the causative agent of the above mentioned pathologies. In the present study, antagonistic activity, growth promotion, and the metabolite profiles of two rhizobacteria, isolated in different paddy fields, were assessed against B. glumae. Strains were identified based on 16S rRNA gene sequences, and the phylogenetic analyses showed that both strains belong to the genus Enterobacter, with high similarity to the strain Enterobacter tabaci NR146667.2 (99%). The antagonistic activity was assessed with the disc diffusion method. Active fractions were isolated through a liquid/liquid extraction with ethyl acetate (EtOAc) from the fermentation media, and their antibacterial activities were evaluated following the Clinical and Laboratory Standards Institute (CLSI) guidelines. The Pikovskaya modified medium was used to test the ability of in vitro inorganic phosphorus solubilization, and BSB1 proved to be the best inorganic phosphorus solubilizer, with a solubilization index (SI) of 4.5 ± 0.2. The glass-column fractionation of the EtOAc extracted from BCB11 produced an active fraction (25.9 mg) that inhibited the growth of five B. glumae strains by 85–95%. Further, metabolomic analysis, based on GC–MS, showed 3-phenylpropanoic acid (3-PPA) to be the main compound both in this fraction (46.7%), and in the BSB1 extract (28.6%). This compound showed antibacterial activity against all five strains of B. glumae with a minimum inhibitory concentration (MIC) of 1000 mg/L towards all of them. The results showed that rice rhizosphere microorganisms are a source of compounds that inhibit B. glumae growth and are promising plant growth promoters (PGP).

Phytophthora cinnamomi causes one of the most important diseases in avocado crop and its chemical management represents 25% of the production cost per year. Induction of plant defense responses by elicitors is a promising strategy that is compatible with sustainable agriculture. This study aimed to evaluate the effect of curdlan application on the induction of defense responses in avocado plants against P. cinnamomi. The trials were conducted under greenhouse conditions, and curdlan leaf spraying was performed one day before the inoculation of the pathogen. The results showed that the application of elicitor significantly increased the protection of avocado plants against P. cinnamomi, decreasing the injury and wilting. The Curd + Phy treatment improved the defenses of plants by increasing the enzymes peroxidase (POD) and polyphenol oxidase (PPO) in the first 3 h after inoculation and increasing the enzymes superoxide dismutase (SOD) and phenylalanine ammonium lyase (PAL) 144 h after inoculation (p < 0.05). Also, chlorophyll and carotenoid content increased or remained stable in Curd + Phy treatment. Therefore, these results suggest that curdlan increases the protection against P. cinnamomi and its protection could be due to an increase in the activity of the enzymes related to the phenylpropanoid pathway as well as the effect on chlorophyll and carotenoids.

The objective of this study was to determine the potential of different filamentous fungi to degrade cellulose in rice husk pre-treated with steam explosion or alkaline hydrolysis. A preliminary test performed with carboxymethyl cellulose and nine fungi (Trichoderma 1, 2, 3, 4, 5; Trichoderma reesei; Aspergillus niger; Rhizopus oryzae and an isolated fungus from rice husk) allowed the selection of the fungi that can degrade cellulose the most. Subsequently, the fastest growing fungi on the substrate (carboxymethyl cellulose) were subjected to a fermentation bioreactor (18 mL of the fungus with 2 mL of conidial suspension at a concentration of 5 x 106 conidia mL-1). Their potential to degrade cellulose was determined. This was done by measuring the amount of total carbohydrate and reducing sugars using the anthrone method and 3,5 dinitrosalicylic acid respectively. On the other hand, the endoglucanase, exoglucanase and β-glucosidase activity of the two most promising fungus (Trichoderma sp. 1 and Aspergillus sp.) was evaluated. Statistical analysis revealed no significant differences; however, the rice husk pre-treated with steam explosion before the fungal strains had the greatest amount of total carbohydrates; it produces 2.9 and 1.4 times more than those not treated with alkaline hydrolysis. Moreover, fungi Trichoderma sp. 1 and Aspergillus sp. had higher number of total released carbohydrate and reducing respectively, which demonstrated the difference in the enzyme system of the two microorganisms. Endoglucanase and exoglucanase activities had similar performance for Aspergillus sp., and Trichoderma sp. 1, during the 288 h of the test. Likewise, β-glucosidase activity was similar. After 192 h, values of 0.150 and 0.140 IU mL-1 were obtained for Aspergillus sp. and Trichoderma sp. 1, respectively. Finally, the applicability of rice husk in agribusiness as a raw material for subsequent fermentation and for obtaining added-value compounds is shown. Key words: Enzymatic activity, rice husk, fermentable sugar, agroindustrial wastes, filamentous fungi.

The disease caused by the oomycete Phytophthora cinnamomi Rands affects most avocado (Persea americana) varieties, requiring additional tools for the selection of resistant materials. In the present study, induction of defense responses related to the oxidative metabolism in avocado was evaluated as a support for establishing its role in the defense responses during infection by this oomycete. Infection trials were carried out with the avocado clones ‘Duke 7’ (tolerant) and ‘Hass’ (susceptible), which had their stem base inoculated with the pathogen and whole leaves sampled at 0, 3, 24, 48 and 120 hours after infection (hai), for subsequent evaluation of the activities of the enzymes superoxide dismutase (SOD) and ascorbate peroxidase (APX), as well as for assessment of free radical scavenging capacity, total polyphenol content and caspase 3-like activity. The area under the disease progress curve was 1.3 and 14.8 (cm.day-1) for ‘Duke 7’ and ‘Hass’, respectively, and the former showed high resistance to the oomycete, which correlated with higher APX activity, higher free radical scavenger activity, and expression of caspase 3-like activity. Resistance or susceptibility in avocado during interaction with Phytophthora cinnamomi is suggested to be mediated by the participation of induced and innate antioxidants and by the induction of apoptosis responses in the resistant genotype ‘Duke 7’, whereas there is lack of such responses in ‘Hass’, the susceptible genotype.