Defense Responses of Saffron Plant
Saffron plant in its natural environment is constantly under siege by a multitude of disease-causing organisms including bacteria, fungi, viruses and nematodes. These phytopathogens invade into the plant apoplast and proliferate by assimilating nutrients from plant cells, hence provoking important economic damage to saffron around the world. Due to the high value of the saffron flowers, it’s important to be familiar with its potential threads. Saffron Express team is honored to invite you to continue this article.
Different Types of Diseases
Most pathogenic species affect the bulb, causing preand post-development of this organ, which in turn affects saffron viability, propagation and yield. However, only a relatively small proportion of these pathogens is capable of invading the host plant successfully and causing disease. Plants depend on sophisticated defensive strategies to resist this invasion, using both preformed and inducible defense responses. This ability to resist disease also depends on soil conditions such as structure, compaction, drainage, temperature and level of biological activity, along with farming practices that influence plant development, such as planting date and application of fertilizers or herbicides. Our ability to exert sustainable control over saffron plant’s diseases relies on a two-fold understanding of the development and defense mechanisms.
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Disease in Plants
The meristematic cells of a healthy saffron plant divide and differentiate as needed, while different types of specialized cells absorb water and nutrients from the soil; translocate these to all plant parts; carry on photosynthesis, move, metabolize or store photosynthetic products; and produce new reproductive structures for survival and multiplication. When a pathogenic organism interferes with the ability of cells or a plant part to carry out one or more of these essential functions, the activities of the cells are disrupted, altered, or inhibited and the plant becomes diseased. This disease, which is the outcome of a successful infection, rarely kills a plant, if the plant is not infected by a necrotrophy pathogen. At first, the infection is localized in one or a few cells and is invisible. Soon, however, the reaction becomes more widespread and affects plant parts, developing changes that are visible to the naked eye. These visible changes are the symptoms of the disease. The visible or otherwise measurable adverse changes in a plant produced by the pathogen infection are a measure of the degree of disease in the plant.
Saffron plant is cultivated for its red style branches, which once dry constitute the saffron spice. Saffron plant is a triploid sterile, propagated by bulbs. As a subterranean organ, the corm is susceptible to diseases caused by fungi, bacteria, nematodes and viruses. Infected plants die off early, resulting in reduction of corm yield, quality and flower and stigma production. In the following sections we will cover the different pathogens that have been isolated and identified in saffron plant and also the strategies which the plant has developed to deal with them.
Plant cells have a robust cell wall which viruses cannot penetrate them unaided. Most plant viruses are therefore transmitted by a vector organism that feeds on the plant or (in some diseases) are introduced through wounds made, for example, during farming operations. The largest and most significant vector group of plant viruses are insects, especially aphids, which transmit viruses from many different genera, including the Potyvirus and Cucumovirus viruses that infect saffron plant. Symptoms of viral infection include tissue yellowing (chlorosis) or browning (necrosis), mosaic patterns, and plant stunting. Plant viruses are biotrophs that all face the same three basic challenges: how to replicate in the cell initially infected; how to move into adjacent cells and the vascular system; and how to suppress host defenses and thereby colonize the entire plant.
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More than 20 genera of nematodes, i.e. round worms approximately 1 mm in length, cause plant disease. All the sedentary endoparasites’ nematodes have evolved the ability to induce morphological changes in plant cells to form feeding cells. The juvenile form migrates through the soil towards the root system. Once it reaches the root epidermis, it enters the plant preferentially in the differentiation and elongation zone by perforating cells using a combination of intensive stylet thrusting, enzymatic softening of the cell walls and mechanical force. The nematode then begins migration towards a site in the plant for suitable feeding. At this time, these nematodes become immobile and completely dependent on the successful induction and maintenance of specialized feeding cells. This intimate relationship persists for approximately two months until the end of the nematode’s life cycle. These parasites are therefore biotrophic: they do not kill the cells they feed from but instead modify them into efficient food sources. The mechanism of feeding cell formation is different and specific for each infecting nematode.
Preformed or Passive Defense Mechanisms in Saffron
Preformed defense is the first obstacle a pathogen faces when invading a plant. For example, the plant cell wall, and in the case of saffron the presence of the fibrous tunic, protects the corm from pathogens, insects and water loss. But plants constitutively produce a plethora of secondary metabolites, many of which can act as antimicrobial compounds during defense against microorganisms. These compounds may be present in their biologically active forms or may be stored as inactive precursors that are converted to their active forms by host enzymes in response to pathogen attack or tissue damage. These compounds include saponins, phenolics, cyclic hydroxamic acids, cyanogenic glycosides, isoflavonoids, sesquiterpenes, Sulphur-containing indole derivatives and many others. Altogether, these compounds represent the first chemical barriers to infection and are associated with non-host resistance. Saponins are glycosylated triterpenoid, steroid, or steroidal alkaloid molecules with antifungal activity.
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Inducible Defense Mechanisms in Saffron
The second obstacle an invading pathogen has to face is the inducible plant defense mechanisms. The induced mechanisms are associated with local changes at the site of pathogen infection, such as the hypersensitive response (HR), one of the most efficient forms of plant defenses. Besides causing accumulation of antimicrobial compounds, such as phenolic compounds, phytoalexins and antimicrobial peptides, HR also leads to an increase in the activity of peroxidases and polyphenol oxidase enzymes involved in defense responses. The response to a pathogen also involves transcriptional activation of numerous defense-related genes, opening of ion channels, modifications of protein phosphorylation status, and activation of preformed enzymes to undertake specific modifications to primary and secondary metabolism. In addition, a range of secondary signaling molecules are generated to ensure coordination of the defense response both temporally and spatially, resulting in rapid containment of the pathogen.
Saffron Plant's General Behavior
Many different classes of pathogens including viruses, nematodes and especially fungi severely affect saffron yield and quality. While new pathogenic fungi, e.g. Fusarium, are currently being described as important saffron pathogens, there are no recent reports on detection associated to important losses caused by historic fungi, such as Rhizoctonia. Surprisingly, pathogenic bacteria have not been detected as responsible for important saffron losses, suggesting the presence of important defense barriers in saffron plant, which prevent bacteria colonization. Genomic approaches have permitted the identification of several defense genes in saffron, although a number of important previously known genes are still missing, as is the case of elicitor-receptors for pathogen perception and recognition. Future research on saffron pathogenesis will aim at identifying more of these genes as well as their pathogen-derived elicitors, thus enabling the discovery of their plant receptors.
- This article was inspired and gathered from Oussama Ahrazem, Àngela Rubio-Moraga, Raquel Castillo-Lopez, Almudena Trapeo Mozos, Lourdes Gomez-Gomez, “Crocsu sativus Pathogens and Defence Responses”.