Salicylic acid seed priming induces systemic acquired resistance to Fusarium fujikuroi in aromatic rice

Chhabra R., Sharma R., Hunjan M.S., Sharma P.

Bipolaris oryzae is the causative agent of rice brown spot disease, leading to serious economic yield losses all over the world. The present study aims to manage brown spot disease on susceptible rice cultivar PR 124 using botanical extracts of three meliaceae species. In vitro screening of botanical extracts revealed the dose dependent response of botanical extracts, though maximum mycelial inhibition occurred at 50% concentration. Ethanolic extracts showed a better inhibition percentage when compared to the aqueous extracts. Azadirachta indica exhibited the highest antifungal ability, followed by Melia azedarach and Toona ciliata. A field experiment conducted during two consecutive years (2021–22) revealed that pre-treatment with promising doses of botanical extracts followed by challenge inoculation 48 h after a foliar spray led to a significant reduction in the severity index, incidence, mean spot count, AUDPC and r value of treated plants over control. A significant increase in yield parameters was also recorded with the foliar application of botanical extracts as compared to un-inoculated (C1) and inoculated (C2) plots. The number of filled grains, test weight, panicle weight and fertility also increased as compared to control plants. The stimulation of the defence system and the antioxidant potential (phenols and lignin) coincided with the induction of endurance in susceptible cultivars. The results demonstrate that defence enzymes like PAL, TAL, glucanase and chitinase are expressed during the activation of a defence response induced by botanical extracts. A. indica was most effective at managing the disease development followed by M. azedarach and T. ciliata. Findings from this research are important for producing a biocide that is safe for the environment and could result in the effective management of this disease.

Gaur K., Kaur N., Sharma A.B., Choudhary A.

In this study, we evaluated the potential of seed priming with plant growth substances on morpho-physiological and yield attributes of bakanae infected seedlings of aromatic rice varieties Pusa Basmati 1121 and Pusa Basmati 1509. The study involved laboratory and field examination of seed priming for 6 h in SA at 5 mg L−1 &10 mg L−1 concentration, MeSA at 50 mg L−1 and 100 mg L−1 concentration, Prohexadione-Ca at 100 mg L−1 and 200 mg L−1 concentration, BABA at 5 mg L−1 and 10 mg L−1 concentration, followed by inoculation with the fourteen-day-old pure culture of Fusarium fujikuroi. The positive (inoculated hydroprimed seeds) and negative controls (un-inoculated untreated seeds) were maintained for both varieties simultaneously and the growth and physiological observations were made at 7, 14 and 21 DAG. The seed priming treatments recorded enhancement in germination, fresh and dry weight, vigour index, total chlorophyll and carotenoid content over the positive control. The optimum dose of each plant growth substance from laboratory evaluation was selected for the field studies. The procedure for priming and inoculation was same for field trial and observations were recorded at tillering, booting and anthesis stage. The seed priming treatments resulted in reduction in percent disease incidence and visible symptomology of bakanae concomitant with an enhancement in field emergence, biochemical attributes under field conditions. The study can be further used to decipher the biochemical and molecular mechanism underlying cross talk of plant growth substances during bakanae infestation in rice in order to design a disease management strategy.

Chhabra R., Sharma R., Hunjan M.S., Sharma V.K., Sharma P., Chauhan S.K.

Brown spot disease, caused by Bipolaris oryzae, is a dominant lethal rice disease that causes qualitative and quantitative crop damage. The current study sought to identify various histological and metabolic changes that occur during brown spot development in susceptible rice plants. We present a conceptual framework that shows B. oryzae suppresses the production of immune-related metabolites in a susceptible cultivar, PR 124 using a comparative metabolomics approach. Un-inoculated rice leaves have an epidermis followed by cortex parenchyma with large intercellular spaces, and no fungal hyphae or distortion were found. Following pathogen inoculation, fungus hyphae grow intercellularly in photosynthetic areas and intracellularly in the bundle sheath, resulting in the microcracks on the surface of the rice leaf. Cellular depositions could have produced the clogging, which disrupted water channels and induced distortion of vascular bundles, ultimately leading to cellular collapse and the withering of rice plants under field conditions. Silica on an infected leaf surface suggests a more robust defence response, thus providing some degree of endurance at the later stages of infection. A significant decline in the total chlorophyll and lignin content was observed in the inoculated leaves compared to the un-inoculated ones. Higher relative injury was recorded post-inoculation. Early oxidative responses like malondialdehyde, proline and hydrogen peroxide accumulation occurred in the flag leaves at various intervals after inoculation. Reduced salicylic acid, phenol and lignin content post-inoculation could be attributed to lowered phenylalanine ammonia lyase activity. Significant declines in the activities of catalase, peroxidase, chitinase and glucanase suggest that immune suppression by this biotrophic pathogen impacts specialised plant metabolism. Thus, these findings form the basis for additional studies focussed on the characterisation of metabolic components involved in pathogen perception during the early stages of intracellular signal transduction.

Chhabra R., Sharma R., Hunjan M.S., Sharma V.K., Thakur S., Chauhan S.

Chhabra R., Kaur N., Bala A.

Fusarium fujikuroi, the causal agent of foot rot disease, infects all the parts of rice, causing serious yield losses in different parts of the world. In order to understand the basis of disease tolerance, various biochemical and anatomical traits of ten days old healthy and infected basmati and non-basmati rice seedlings were compared. Total soluble sugar content was higher in healthy seedlings, while total phenols, orthodihydroxy phenols and total soluble proteins were higher in non-basmati rice seedlings post infection indicating the initiation of defence against the pathogen. Increase in the total antioxidant activity, enzymes and α-tocopherols in basmati rice cultivars post infection indicate the oxidative stress created by pathogen. Histopathological observations revealed hyphal and conidial growth in infected tissues of basmati cultivars whereas a complete and intact tissue was observed in healthy and infected counterparts of non-basmati cultivars. Bangladesh J. Bot. 51(1): 29-36, 2022 (March)

Ayesha M.S., Suryanarayanan T.S., Nataraja K.N., Prasad S.R., Shaanker R.U.

Pre-sowing seed treatment with systemic fungicides is a firmly entrenched practice for most agricultural crops worldwide. The treatment is intended to protect the crop against seed- and soil-borne diseases. In recent years, there is increasing evidence that fungicidal applications to manage diseases might inadvertently also affect non-target organisms, such as endophytes. Endophytes are ubiquitously present in plants and contribute to plant growth and development besides offering resistance to biotic and abiotic stresses. In seeds, endophytes may play a role in seed development, seed germination, seedling establishment and crop performance. In this paper, we review the recent literature on non-target effects of fungicidal applications on endophytic fungal community and discuss the possible consequences of indiscriminate seed treatment with systemic fungicide on seed endophytes. It is now well recognized that endophytes are ubiquitously present in all parts of the plant, including the seeds. They may be transmitted vertically from seed to seed as in many grasses and/or acquired horizontally from the soil and the environment. Though the origins and evolution of these organisms in plants are a matter of conjecture, numerous studies have shown that they symbiotically aid in plant growth and development, in nutrient acquisition as well in protecting the plants from abiotic and biotic stresses. Against this background, it is reasonable to assume that the use of systemic fungicides in seed treatment may not only affect the seed endophytes but also their attendant benefits to seedling growth and establishment. While there is evidence to indicate that fungicidal applications to manage plant diseases also affect foliar endophytes, there are only few studies that have documented the effect of seed treatment on seed-borne endophytes. Some of the convincing examples of the latter come from studies on the effect of fungicide application on rye grass seed endophyte AR37. More recently, experiments have shown that removal of seed endophytes by treatment with systemic fungicides leads to significant loss of seedling vigour and that such losses could be partially restored by enriching the seedlings with the lost endophytes. Put together, these studies reinforce the importance of seed endophytes to seedling growth and establishment and draw attention on how to trade the balance between the benefits of seed treatments and the direct and indirect costs incurred due to loss of endophytes. Among several approaches, use of reduced-risk fungicides and identifying fungicide-resistant endophytes are suggested to sustain the endophyte contribution to early seedling growth.

Peng Y., Yang J., Li X., Zhang Y.

Salicylic acid (SA) is an essential plant defense hormone that promotes immunity against biotrophic and semibiotrophic pathogens. It plays crucial roles in basal defense and the amplification of local immune responses, as well as the establishment of systemic acquired resistance. During the past three decades, immense progress has been made in understanding the biosynthesis, homeostasis, perception, and functions of SA. This review summarizes the current knowledge regarding SA in plant immunity and other biological processes. We highlight recent breakthroughs that substantially advanced our understanding of how SA is biosynthesized from isochorismate, how it is perceived, and how SA receptors regulate different aspects of plant immunity. Some key questions in SA biosynthesis and signaling, such as how SA is produced via another intermediate, benzoic acid, and how SA affects the activities of its receptors in the transcriptional regulation of defense genes, remain to be addressed.

Ignatenko A.A., Talanova V.V., Repkina N.S., Titov A.F.

The effect of salicylic acid (SA) on the reaction of cucumber plants (Cucumis sativus L.) on the effect of low positive temperatures has been studied. It was found that treatment with SA promotes a decrease in the release of electrolytes from the tissues of cotyledonous leaves of seedlings exposed to subdamaging (12°C) and damaging (4°C) temperatures. In the leaves of seedlings treated with SA and exposed to cold, a higher activity of superoxide dismutase, catalase, and guaiacol-specific peroxidase was recorded as well as an increase in the accumulation of transcripts of the genes encoding them (CuZnSOD and CAT) compared to the control variant (without SA treatment). Under the action of low temperatures (12 and 4°C) on seedlings, SA also reduced the formation of one of the end products of lipid peroxidation, malondialdehyde, in seedling leaves. It was concluded that SA has a protective effect on cucumber seedlings when exposed to low positive temperatures, which is expressed in the activation of antioxidant enzymes and a decrease in the level of oxidative stress.

Zhang Q.

Qifa Zhang describes how a 2008 publication in Nature Biotechnology on transgenic purple tomatoes inspired him to tackle food security by improving the nutrient and culinary value of black rice.

Jia H., Wang X., Wei T., Wang M., Liu X., Hua L., Ren X., Guo J., Li J.

Cadmium (Cd) is harmful to plant growth and can be easily transferred from soil to plants. Plant cell wall plays important role in preventing Cd from entering cells. Salicylic acid (SA) mediated defense response increases plant resistance to heavy metals. In this study, all tomato seedlings were pre-treated with 100 μM SA for 3 d, then seedlings were used to analyze the role of SA in regulating plant cell wall resistance to Cd stress. The results showed that exogenous SA significantly reduced Cd accumulation in tomato plants and changed Cd distribution. By analyzing the cell wall composition, it was found cellulose, hemicellulose, pectin, and lignin were induced by SA. Interestingly, the content of Cd in pectin decreased by SA pretreatment, however it was increased in cellulose. Gene expression analysis showed SA up-regulated the expression level of lignin and cellulose synthase genes, but down-regulated the expression of pectin methylesterase related genes. In addition, SA down-regulated the activity of pectin methylesterase. These results indicated that SA pretreatment up-regulated cell wall polysaccharide synthesis and related gene expression to thicken the cell wall and block Cd from passing through. Furthermore, SA decreased pectin methylesterase activity and content to reduce cell wall Cd accumulation and change the Cd partition ratio.

Chen C., Chen S., Liu C., Wu D., Kuo C., Lin C., Chou H., Wang Y., Tsai Y., Lai M., Chung C.

Bakanae disease in rice can cause abnormal elongation of the stem and leaves, development of adventitious roots, a larger leaf angle, and even death. Little is known about the infection, colonization, and distribution of Fusarium fujikuroi in rice plants across different growth stages. In this study, microscopic observation and quantitative real-time PCR were combined to investigate the pathogenesis of bakanae, using artificially inoculated seedlings of a susceptible rice cultivar, Zerawchanica karatals (ZK), a resistant cultivar, Tainung 67 (TNG67), naturally infected adult field plants (cultivars Kaohsiung 139, Taikeng 2, and Tainan 11), and an F. fujikuroi isolate expressing green fluorescent protein. In rice seedlings, F. fujikuroi hyphae were found to directly penetrate the epidermis of basal stems and roots, then extend inter- and intracellularly to invade the vascular bundles. Occlusion of vascular bundles and radial hyphal expansion from vascular bundles to surrounding parenchyma were observed in adult plants. Analysis of consecutive 3-cm segments of the whole plant revealed that F. fujikuroi was largely confined to the embryo, basal stem, and basal roots in seedlings, and distributed unevenly in the lower aerial parts (including nodes and internodes) of adult plants. The elongation and development of adventitious roots did not necessarily correlate with the amount of F. fujikuroi in diseased plants. Treatment of rice seeds with gibberellic acid-3 (GA3) at 0.5 mg/liter resulted in significantly more elongation of ZK than TNG67 seedlings, suggesting that the susceptibility of ZK to bakanae is associated with its higher sensitivity to GA3.

Chhabra R., Kaur N., Bala A.

Bakanae is the major fungal disease of rice posing threat to rice production all over the world. In order to understand the physiological and biochemical basis of resistance, the disease free and Fusarium fujikuroi infected seedlings of two aromatic (PUSA 1121 and PUSA 1509) and two non-aromatic (PR 121 and PR 126) rice cultivars were evaluated. The seeds of rice cultivars were inoculated with the virulent isolate of F. fujikuroi. Percent disease incidence, percent field emergence, seedling length and biochemical traits of 25 days old seedlings raised from healthy (disease free) and pathogen inoculated seeds in the nursery were compared. The experimental design was RBD and data was analysed using tukey’s HSD test. The infection led to a significant decline in the chlorophyll content in the infected seedlings of the aromatic cultivars, whereas non-significant variation between healthy and infected seedling of non-aromatic rice cultivars was observed. Seed inoculation with the fungus resulted in significant increase in length of the infected seedlings in aromatic rice cultivars as compared to non-aromatic indicating the resistance response to the production of Gibberellic acid in non-aromatic rice cultivars. The endogenous GA enhanced significantly postinfection in the aromatic PUSA 1121 and PUSA 1509 seedlings. Post pathogenesis, the infected seedlings of aromatic cultivars retained lower level of total phenols. The depletion of phenols post infection in infected seedlings of aromatic cultivars may be due sugar depletion. Our results show that the higher total antioxidant and enzymatic activity in aromatic rice cultivars post infection could be a useful biochemical marker for screening the tolerance potential of rice cultivars against bakanae disease.

Sunani S.K., Bashyal B.M., Kharayat B.S., Prakash G., Krishnan S.G., Aggarwal R.

Bakanae disease caused by Gibberella fujikuroi, Sawada, Wollenworth (teleomorph) (anamorph: Fusarium fujikuroi Nirenberg), is emerging as a serious disease of rice in India. Histopathological study of plant tissue during infection can provide valuable insight for understanding the pathogenesis and the mechanism of resistance. The early infection process of F. fujikuroi in rice was observed at microscopic level using light microscopes and scanning electron microscope (SEM). In seed, the pathogen was observed in the husk, embryo and endosperm with maximum colonization in the husk region. Different infectious structures such as swollen tip hyphae, infectious hyphae, appressorium and infection cushion were observed in F. fujikuroi-rice interactions. The estimates of per cent (%) transmission of inoculum (seed-seed; seedlings-seed and florets-seed) revealed maximum transmission of F. fujikuroi through florets followed by seedlings and seed. The present study established that the infection of rice plants by F. fujikuroi can occur through three routes namely seed, seedlings (soil) and florets (air). The information generated in the present study will be valuable for providing an understanding on the pathogenesis and help to device effective strategies for management of bakanae disease of rice.

Han X., Kahmann R.

Phytohormones regulate a large variety of physiological processes in plants. In addition, salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) are responsible for primary defense responses against abiotic and biotic stresses, while plant growth regulators, such as auxins, brassinosteroids (BRs), cytokinins (CKs), abscisic acid (ABA), and gibberellins (GAs), also contribute to plant immunity. To successfully colonize plants, filamentous pathogens like fungi and oomycetes have evolved diverse strategies to interfere with phytohormone pathways with the help of secreted effectors. These include proteins, toxins, polysaccharides as well as phytohormones or phytohormone mimics. Such pathogen effectors manipulate phytohormone pathways by directly altering hormone levels, by interfering with phytohormone biosynthesis, or by altering or blocking important components of phytohormone signaling pathways. In this review, we outline the various strategies used by filamentous phytopathogens to manipulate phytohormone pathways to cause disease.

Smirnoff N., Arnaud D.

Contents Summary 1197 I. Introduction 1198 II. Measurement and imaging of H2 O2 1198 III. H2 O2 and O2·- toxicity 1199 IV. Production of H2 O2 : enzymes and subcellular locations 1200 V. H2 O2 transport 1205 VI. Control of H2 O2 concentration: how and where? 1205 VII. Metabolic functions of H2 O2 1207 VIII. H2 O2 signalling 1207 IX. Where next? 1209 Acknowledgements 1209 References 1209 SUMMARY: Hydrogen peroxide (H2 O2 ) is produced, via superoxide and superoxide dismutase, by electron transport in chloroplasts and mitochondria, plasma membrane NADPH oxidases, peroxisomal oxidases, type III peroxidases and other apoplastic oxidases. Intracellular transport is facilitated by aquaporins and H2 O2 is removed by catalase, peroxiredoxin, glutathione peroxidase-like enzymes and ascorbate peroxidase, all of which have cell compartment-specific isoforms. Apoplastic H2 O2 influences cell expansion, development and defence by its involvement in type III peroxidase-mediated polymer cross-linking, lignification and, possibly, cell expansion via H2 O2 -derived hydroxyl radicals. Excess H2 O2 triggers chloroplast and peroxisome autophagy and programmed cell death. The role of H2 O2 in signalling, for example during acclimation to stress and pathogen defence, has received much attention, but the signal transduction mechanisms are poorly defined. H2 O2 oxidizes specific cysteine residues of target proteins to the sulfenic acid form and, similar to other organisms, this modification could initiate thiol-based redox relays and modify target enzymes, receptor kinases and transcription factors. Quantification of the sources and sinks of H2 O2 is being improved by the spatial and temporal resolution of genetically encoded H2 O2 sensors, such as HyPer and roGFP2-Orp1. These H2 O2 sensors, combined with the detection of specific proteins modified by H2 O2 , will allow a deeper understanding of its signalling roles.