Elucidating the interactions between the rust Hemileia vastatrix and a Calonectria mycoparasite and the coffee plant.

Calonectria hemileiae, a fungus associated with pustules of the coffee leaf rust (CLR, Hemileia vastatrix) in Brazil, was tested in vitro and in planta to assess its biocontrol potential. The fungus inhibited the germination of rust spores by over 80%. CLR severity was reduced by 93% when Calonectria was applied to coffee leaf discs inoculated with H. vastatrix, whilst a reduction of 70-90% was obtained for in planta experiments. Mycoparasitism was demonstrated through the fulfillment of Koch's postulates. Elucidation of the biochemical interaction between Calonectria and Hemileia on coffee plants indicated that the mycoparasite was able to increase plant resistance to rust infection. Coffee plants sprayed with Calonectria alone showed greater levels of chitinase, ß-1,3-glucanase, ascorbate peroxidase and peroxidase. Although effective in controlling the rust, fungicide applications damaged coffee photosynthesis, whereas no harm was caused by Calonectria. We conclude that C. hemileiae shows promise as a biocontrol agent of CLR.

Silicon's Role in Plant Stress Reduction and Why This Element Is Not Used Routinely for Managing Plant Health.

Numerous reviews and hundreds of refereed articles have been published on silicon's effects on abiotic and biotic stress as well as overall plant growth and development. The science for silicon is well-documented and comprehensive. However, even with this robust body of information, silicon is still not routinely used for alleviating plant stress and promoting plant growth and development. What is holding producers and growers back from using silicon? There are several possible reasons, which include: (i) lack of consistent information on which soil orders are low or limited in silicon, (ii) no universally accepted soil test for gauging the amounts of soluble silicon have been calibrated for many agronomic or horticultural crops, (iii) most analytical laboratories do not routinely assay plant tissue for silicon and current standard tissue digestion procedures used would render silicon insoluble, (iv) many scientists still state that plants are either silicon accumulators or non-accumulators when in reality all plants accumulate some silicon in their plant tissues, (v) silicon is not recognized as being necessary for plant development, (vi) lack of economic studies to show the benefits of applying silicon, and (vii) lack of extension outreach to present the positive benefits of silicon to producers and growers. Many of these issues mentioned above will need to be resolved if silicon is to become a standard practice to improve agronomic and horticultural crop production and plant health.

Physiological and biochemical insights into the basal level of resistance of two maize hybrids in response to Fusarium verticillioides infection.

Fusarium stalk rot (FSR), caused by Fusarium verticillioides, is one of the most destructive diseases impacting maize yield worldwide. In this study, net carbon assimilation rate (A), stomatal conductance to water vapor (gs), transpiration rate (E), and internal CO2 concentration (Ci) were evaluated on leaves and the activities of enzymes (chitinase (CHI), ß-1-3-glucanase (GLU), phenylalanine ammonia-lyase (PAL), polyphenoloxidase (PPO), catalase (CAT), ascorbate peroxidase (APX), peroxidase (POX)) as well the concentrations of total soluble phenolics (TSP), lignin-thioglycolic acid (LTGA) derivatives, and malondialdehyde (MDA) were evaluated in the internodes and nodes of plants from maize hybrids moderately resistant (BRS 1035) and susceptible (30F35Y) to FSR. The upward relative lesion length (URLL) and radial fungal colonization (RFC) were 46 and 29% lower for the BRS 1035 hybrid in comparison to 30F35Y hybrid, respectively, at 30 after inoculation (dai). For both hybrids, A, gs, and E values significantly decreased while the Ci values increased on infected leaves compared to noninoculated plants. Inoculated plants from BRS 1035 hybrid showed an increase in A compared to inoculated plants from 30F35Y hybrid, and the increase in Ci values was greater for plants from 30F35Y hybrid at 30 dai compared to plants from BRS 1035 hybrid. The CHI, GLU, PPO, CAT, APX, and POX activities increased for inoculated plants from both hybrids compared to the noninoculated plants. In the internodes region, the increase in the activities of CHI (during the infection process of F. verticillioides) and GLU (at earlier stages of F. verticillioides infection) was more pronounced for plants from BRS 1035 hybrid than for plants from 30F35Y hybrid. In the region of the nodes, activities of CHI (during the infection process of F. verticillioides), PAL (at 20 dai), PPO (at 30 dai), and CAT and POX (both at three dai) were more pronounced for plants from BRS 1035 hybrid than for plants from 30F35Y hybrid. In the internodes region, the lower TSP concentration at 30 dai was linked to a high concentration of LTGA derivatives for inoculated plants from BRS 1035 hybrid compared to inoculated plants from 30F35Y hybrid. Taking together, the results of the present study allowed to conclude that the infection by F. verticillioides triggered physiological and biochemical changes on the stalk of maize plants influencing photosynthesis on leaves. A more robust antioxidative metabolism for reactive oxygen species removal in association with an efficient and strong activity of defense enzymes helped to minimize the cellular damage caused by F. verticillioides infection resulting, therefore, in an increase in maize resistance to FSR.

Effect of glutamate on Pyricularia oryzae infection of rice monitored by changes in photosynthetic parameters and antioxidant metabolism.

Considering the importance of blast caused by Pyricularia oryzae in the decrease of rice yield worldwide, this study aimed to assess the photosynthetic performance [leaf gas exchange and chlorophyll (Chl) a fluorescence parameters as well as the photosynthetic pigments concentration], the activities of antioxidant enzymes [ascorbate peroxidase, catalase (CAT), peroxidase (POX), superoxide dismutase (SOD), glutathione peroxidase (GPX), glutathione reductase (GR) and glutathione-S-transferase] and concentrations of hydrogen peroxide (H2 O2 ) and malondialdehyde (MDA) in the leaves of rice plants non-supplied (-Glu) or supplied (+Glu) with glutamate (Glu) and non-infected or infected by P. oryzae. Blast severity was reduced in the leaves of +Glu plants. On the infected leaves of +Glu plants, the values for internal CO2 concentration were lower while the values for net carbon assimilation rate, stomatal conductance as well as for the concentrations of Chl a, Chl b and carotenoids were higher in comparison to infected leaves of -Glu plants. The functionality of the photosynthetic apparatus was preserved in the infected leaves of +Glu plants. The activities of CAT, GPX, GR, POX and SOD increased in the infected leaves of both -Glu and +Glu plants compared to their non-inoculated counterparts, but their activities were lower for +Glu plants. The lower activity of these antioxidative enzymes was triggered by the reduced hydrogen peroxide concentration in the infected leaves of +Glu plants resulting in lower MDA concentration. It can be concluded that photosynthesis was less impaired in infected plants supplied with glutamate due to the lower biochemical constraints for CO2 fixation. Moreover, there was a need for lower activity of reactive oxygen species scavenging enzymes in infected leaves of plants supplied with glutamate due to the lower oxidative stress imposed by P. oryzae infection.

Water relation, leaf gas exchange and chlorophyll a fluorescence imaging of soybean leaves infected with Colletotrichum truncatum.

Considering the potential of anthracnose to decrease soybean yield and the need to gain more information regarding its effect on soybean physiology, the present study performed an in-depth analysis of the photosynthetic performance of soybean leaflets challenged with Colletotrichum truncatum by combining chlorophyll a fluorescence images with gas-exchange measurements and photosynthetic pigment pools. There were no significant differences between non-inoculated and inoculated plants in leaf water potential, apparent hydraulic conductance, net CO2 assimilation rate, stomatal conductance to water vapor and transpiration rate. For internal CO2 concentration, significant difference between non-inoculated and inoculated plants occurred only at 36 h after inoculation. Reductions in the values of the chlorophyll a fluorescence parameters [initial fluorescence (F0), maximal fluorescence (Fm), maximal photosystem II quantum yield (Fv/Fm), quantum yield of regulated energy dissipation (Y(NPQ))] and increases in effective PS II quantum yield (Y(II)), quantum yield of non-regulated energy dissipation Y(NO) and photochemical quenching coefficient (qP) were noticed on the necrotic vein tissue in contrast to the surrounding leaf tissue. It appears that the impact of the infection by C. truncatum on the photosynthetic performance of the leaflets was minimal considering the preference of the fungus to colonize the veins.

Silicon's Role in Abiotic and Biotic Plant Stresses.

Silicon (Si) plays a pivotal role in the nutritional status of a wide variety of monocot and dicot plant species and helps them, whether directly or indirectly, counteract abiotic and/or biotic stresses. In general, plants with a high root or shoot Si concentration are less prone to pest attack and exhibit enhanced tolerance to abiotic stresses such as drought, low temperature, or metal toxicity. However, the most remarkable effect of Si is the reduction in the intensities of a number of seedborne, soilborne, and foliar diseases in many economically important crops that are caused by biotrophic, hemibiotrophic, and necrotrophic plant pathogens. The reduction in disease symptom expression is due to the effect of Si on some components of host resistance, including incubation period, lesion size, and lesion number. The mechanical barrier formed by the polymerization of Si beneath the cuticle and in the cell walls was the first proposed hypothesis to explain how this element reduced the severity of plant diseases. However, new insights have revealed that many plant species supplied with Si have the phenylpropanoid and terpenoid pathways potentiated and have a faster and stronger transcription of defense genes and higher activities of defense enzymes. Photosynthesis and the antioxidant system are also improved for Si-supplied plants. Although the current understanding of how this overlooked element improves plant reaction against pathogen infections, pest attacks, and abiotic stresses has advanced, the exact mechanism(s) by which it modulates plant physiology through the potentiation of host defense mechanisms still needs further investigation at the genomic, metabolomic, and proteomic levels.

Silicon improves rice grain yield and photosynthesis specifically when supplied during the reproductive growth stage.

Silicon (Si) has been recognized as a beneficial element to improve rice (Oryza sativa L.) grain yield. Despite some evidence suggesting that this positive effect is observed when Si is supplied along the reproductive growth stage (from panicle initiation to heading), it remains unclear whether its supplementation during distinct growth phases can differentially impact physiological aspects of rice and its yield and the underlying mechanisms. Here, we investigated the effects of additions/removals of Si at different growth stages and their impacts on rice yield components, photosynthetic performance, and expression of genes (Lsi1, Lsi2 and Lsi6) involved in Si distribution within rice shoots. Positive effects of Si on rice production and photosynthesis were manifested when it was specifically supplied during the reproductive growth stage, as demonstrated by: (1) a high crop yield associated with higher grain number and higher 1000-grain weight, whereas the leaf area and whole-plant biomass remained unchanged; (2) an increased sink strength which, in turn, exerted a feed-forward effect on photosynthesis that was coupled with increases in both stomatal conductance and biochemical capacity to fix CO2; (3) higher Si amounts in the developing panicles (and grain husks) in good agreement with a remarkable up-regulation of Lsi6 (and to a lesser extent Lsi1). We suggest that proper levels of Si in these reproductive structures seem to play an as yet unidentified role culminating with higher grain number and size.

Silicon nutrition increases grain yield, which, in turn, exerts a feed-forward stimulation of photosynthetic rates via enhanced mesophyll conductance and alters primary metabolism in rice.

Silicon (Si) is not considered to be an essential element for higher plants and is believed to have no effect on primary metabolism in unstressed plants. In rice (Oryza sativa), Si nutrition improves grain production; however, no attempt has been made to elucidate the physiological mechanisms underlying such responses. Here, we assessed crop yield and combined advanced gas exchange analysis with carbon isotope labelling and metabolic profiling to measure the effects of Si nutrition on rice photosynthesis, together with the associated metabolic changes, by comparing wild-type rice with the low-Si rice mutant lsi1 under unstressed conditions. Si improved the harvest index, paralleling an increase in nitrogen use efficiency. Higher crop yields associated with Si nutrition exerted a feed-forward effect on photosynthesis which was fundamentally associated with increased mesophyll conductance. By contrast, Si nutrition did not affect photosynthetic gas exchange during the vegetative growth phase or in de-grained plants. In addition, Si nutrition altered primary metabolism by stimulating amino acid remobilization. Our results indicate a stimulation of the source capacity, coupled with increased sink demand, in Si-treated plants; therefore, we identify Si nutrition as an important target in attempts to improve the agronomic yield of rice.

Deficiency in silicon uptake affects cytological, physiological, and biochemical events in the rice--Bipolaris oryzae interaction.

This study investigated how a defect in the active uptake of silicon (Si) affects rice resistance to brown spot. Plants from a rice mutant (low silicon 1 [lsi1]) and its wild-type counterpart (cv. Oochikara), growing in hydroponic culture with (+Si; 2 mM) or without (-Si) Si, were inoculated with Bipolaris oryzae. Si concentration in leaf tissue of cv. Oochikara and the lsi1 mutant increased by 381 and 263%, respectively, for the +Si treatment compared with the -Si treatment. The incubation period was 6 h longer in the presence of Si. The area under brown spot progress curve for plants from cv. Oochikara and the lsi1 mutant was reduced 81 and 50%, respectively, in the presence of Si. The reduced number of brown epidermal cells on leaves from cv. Oochikara and the lsi1 mutant supplied with Si contributed to the lower lipid peroxidation and electrolyte leakage. The concentration of total soluble phenolics in cv. Oochikara supplied with Si (values of 4.2 to 15.4 µg g(-1) fresh weight) was greater compared with plants not supplied with Si (values of 1.9 to 11.5 µg g(-1) fresh weight). The concentration of lignin was also important to the resistance of cv. Oochikara and the lsi1 mutant. Polyphenoloxidase activity did not contribute to the resistance of cv. Oochikara and the lsi1 mutant to brown spot, regardless of Si supply. Peroxidase and chitinase activities were higher in cv. Oochikara and the lsi1 mutant supplied with Si. These results bring novel evidence of the involvement of Si in a more complex defense mechanism than simply the formation of a physical barrier to avoid or delay fungal penetration.

Defective active silicon uptake affects some components of rice resistance to brown spot.

Rice is known to accumulate high amounts of silicon (Si) in plant tissue, which helps to decrease the intensity of many economically important rice diseases. Among these diseases, brown spot, caused by the fungus Bipolaris oryzae, is one of the most devastating because it negatively affects yield and grain quality. This study aimed to evaluate the importance of active root Si uptake in rice for controlling brown spot development. Some components of host resistance were evaluated in a rice mutant, low silicon 1 (lsi1), defective in active Si uptake, and its wild-type counterpart (cv. Oochikara). Plants were inoculated with B. oryzae after growing for 35 days in a hydroponic culture amended with 0 or 2 mMol Si. The components of host resistance evaluated were incubation period (IP), relative infection efficiency (RIE), area under brown spot progress curve (AUBSPC), final lesion size (FLS), rate of lesion expansion (r), and area under lesion expansion progress curve (AULEPC). Si content from both Oochikara and lsi1 in the +Si treatment increased in leaf tissue by 219 and 178%, respectively, over the nonamended controls. Plants from Oochikara had 112% more Si in leaf tissue than plants from lsi1. The IP of brown spot from Oochikara increased approximately 6 h in the presence of Si and the RIE, AUBSPC, FLS, r, and AULEPC were significantly reduced by 65, 75, 33, 36, and 35%, respectively. In the presence of Si, the IP increased 3 h for lsi1 but the RIE, AUBSPC, FLS, r, and AULEPC were reduced by only 40, 50, 12, 21, and 12%, respectively. The correlation between Si leaf content and IP was significantly positive but Si content was negatively correlated with RIE, AUBSPC, FLS, r, and AULEPC. Single degree-of-freedom contrasts showed that Oochikara and lsi1 supplied with Si were significantly different from those not supplied with Si for all components of resistance evaluated. This result showed that a reduced Si content in tissues of plants from lsi1 dramatically affected its basal level of resistance to brown spot, suggesting that a minimum Si concentration is needed. Consequently, the results of this study emphasized the importance of an active root Si uptake system for an increase in rice resistance to brown spot.

Silicon enhances the accumulation of diterpenoid phytoalexins in rice: a potential mechanism for blast resistance.

ABSTRACT Although several reports underscore the importance of silicon (Si) in controlling Magnaporthe grisea on rice, no study has associated this beneficial effect with specific mechanisms of host defense responses against this fungal attack. In this study, however, we provide evidence that higher levels of momilactone phytoalexins were found in leaf extracts from plants inoculated with M. grisea and amended with silicon (Si(+)) than in leaf extracts from inoculated plants not amended with silicon (Si(-) ) or noninoculated Si(+) and Si(-) plants. On this basis, the more efficient stimulation of the terpenoid pathway in Si(+) plants and, consequently, the increase in the levels of momilactones appears to be a factor contributing to enhanced rice resistance to blast. This may explain the lower level of blast severity observed on leaves of Si(+) plants at 96 h after inoculation with M. grisea. The results of this study strongly suggest that Si plays an active role in the resistance of rice to blast rather than the formation of a physical barrier to penetration by M. grisea.

Ultrastructural and cytochemical aspects of silicon-mediated rice blast resistance.

ABSTRACT Although exogenous application of silicon (Si) confers efficient control of rice blast, the probable hypothesis underlying this phenomenon has been confined to that of a mechanical barrier resulting from Si polymerization in planta. However, in this study, we provide the first cytological evidence that Si-mediated resistance to Magnaporthe grisea in rice correlates with specific leaf cell reaction that interfered with the development of the fungus. Accumulation of an amorphous material that stained densely with toluidine blue and reacted positively to osmium tetroxide was a typical feature of cell reaction to infection by M. grisea in samples from Si+ plants. As a result, the extent of fungal colonization was markedly reduced in samples from Si+ plants. In samples from Si- plants, M. grisea grew actively and colonized all leaf tissues. Cytochemi-cal labeling of chitin revealed no difference in the pattern of chitin localization over fungal cell walls of either Si+ or Si- plants at 96 h after inoculation, indicating limited production of chitinases by the rice plant as a mechanism of defense response. On the other hand, the occurrence of empty fungal hyphae, surrounded or trapped in amorphous material, in samples from Si+ plants suggests that phenolic-like compounds or phytoalexins played a primary role in rice defense response against infection by M. grisea. This finding brings new insights into the complex role played by Si in the nature of rice blast resistance.

Effect of rice growth stages and silicon on sheath blight development.

ABSTRACT The objective of this study was to determine the effect of silicon (Si) and rice growth stages on tissue susceptibility to sheath blight (Rhizoctonia solani Kühn) under controlled conditions. Rice plants (cv. Rio Formoso) were grown in pots containing low-Si soil amended with Si at 0, 0.48, 0.96, 1.44, and 1.92 g pot(-1) and inoculated with R. solani at the following days after emergence: 45 (four-leaf stage), 65 (eight-leaf stage), 85 (tillering), 117 (booting), and 130 (panicle exsertion). For plants inoculated with R. solani at all growth stages, Si concentration in straw increased as rate of Si increased from 0 to 1.92 g pot(-1). Concentration of calcium in the straw did not differ among plant growth stages. Although incubation period was not affected by the amount of Si added to the soil, this variable was shorter at booting and panicle exsertion stages. As the rates of Si increased in the soil, the total number of sheath blight lesions on sheaths and total area under the relative lesion extension curve decreased at all plant growth stages. The severity of sheath blight was lower at booting and panicle exsertion stages as the rates of Si increased in the soil. In general, plants grown in Si-nonamended pots and inoculated with R. solani were more vulnerable to infection at all growth stages, but especially at 45 days after emergence. Plant dry weights for inoculated plants increased as the Si rates increased from 0 to 1.92 g pot(-1). The greatest dry weight increases occurred for plants inoculated at booting and panicle exsertion stages. Si fertilization is a promising method for controlling sheath blight in areas where soil is Si deficient and when cultivars that exhibit an acceptable level of resistance to sheath blight are not available for commercial use.