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Capsicum annuum L.: Comparison
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Please note this is a comparison between Version 1 by Abdul Gafur and Version 2 by Vicky Zhou.

Capsicum annuum L. is a significant horticulture crop known for its pungent varieties and used as a spice. The pungent character in the plant, known as capsaicinoid, has been discovered to have various health benefits. However, its production has been affected due to various exogenous stresses, including diseases caused by a soil-borne pathogen, Pythium spp. Predominantly affecting the Capsicum plant in younger stages and causing damping-off, this pathogen can incite root rot in later plant growth stages. Due to the involvement of multiple Pythium spp. and their capability to disperse through various routes, their detection and diagnosis have become crucial. However, the quest for a point-of-care technology is still far from over. The use of an integrated approach with cultural and biological techniques for the management of Pythium spp. can be the best and most sustainable alternative to the traditionally used and hazardous chemical approach. The lack of race-specific resistance genes against Pythium spp. can be compensated with the candidate quantitative trait loci (QTL) genes in C. annuum L.

  • control strategies
  • diagnosis
  • epidemiology
  • post-emergence damping-off
  • pre-emergence damping-off
  • Pythium
  • resistant cultivars
  • root rot

1. Introduction

Cultivars of Capsicum annuum L.(Solanaceae) are an essential part of many cuisines worldwide as a spice. C. annuum L. is a semi-perennial herbaceous plant that is usually grown as an annual crop worldwide, including in India [1]. The genus Capsicum is split into three gene complexes based on the crossing ability between species [2], of which the species of C. annuum complex (C. annuum, C. frutescens, and C. chinense) is commercially most important [3]. In 2018, worldwide production of dry chilies and peppers was estimated to be up to almost 4.2 million tons over an area of 1.8 million hectares. India is the biggest producer of dry chilies and pepper grown worldwide. Capsaicinoids are distinctive of the Capsicum genus and are produced entirely inside the fruit placenta [4], which causes the characteristics of fruit pungency in this genus [5]. Capsaicinoids are a mixture of at least 23 alkaloids (vanillylamines), a major representative of which are dihydrocapsaicin and capsaicin, which collectively account for almost 90% of the total capsaicinoid content in the fruit [6]. A mildly or non-pungent analog of the capsaicinoids, namely capsinoid, is also present in pungent varieties in trace amounts and shares a similar structure with capsaicinoids [7]. Fresh Capsicum fruits are also rich in phenolic content, including flavonoids, phenolic acids, and tannins [8]. Mature fruits are abundant in carotenoids such as capsanthin, capsorubin, β-carotene, etc., of which capsanthin is the major contributor [9]. Minerals such as potassium, phosphorous, magnesium, calcium, sodium, iron, manganese, boron, selenium, copper, and zinc are commonly found in Capsicum; however, their content is dependent on variables such as the fruits’ variety, growth stage, environmental factors, and the cultivation practices [10].

Various health benefits have been found to be related to different Capsicum cultivars. Fresh green peppers and red peppers have ample amounts of vitamin A, C, and antioxidant compounds [6][11][12][13]. Srinivasan [14], in his review on the biological activities of capsaicin, documented the various potential anti-carcinogenic, anti-inflammatory, pain relief, weight loss, gastrointestinal, and cardioprotective effects of capsaicin in detail.

Small-fruited cultivars were observed to form a more divergent phylogenetic group than the large-fruited varieties, making the small-fruited cultivars genetically more distant [15]. Although various breeding programs have led to the creation of cultivars with favorable traits, this has still not found full application in making disease-resistant varieties because of many interspecific crossing barriers [6][16], including many factors both pre and post-fertilization [2]. Production of C. annuum is restricted by many fungal, bacterial, viral, and nematode diseases associated with it worldwide [17]. The damping-off caused by various Pythium spp. is a significant disease of C. annuum cultivars that predominantly occurs in nursery beds affecting seeds and young seedlings [18][19]. When the plant is in the seed or seedling stage of its life cycle, infection by Pythium species causes pre-emergence and post-emergence damping-off, decaying the seeds and seedlings before emergence and after the emergence of the plant from the soil surface, respectively. However, infected mature plants have also been found to show root rot symptoms. Pythium spp. mainly affect the younger or juvenile tissues, which have not yet developed any secondary thickenings; thus, the infection is limited to seeds, seedlings, and the younger roots. The post-emergence damping-off of seedlings is often associated with symptoms such as reduced growth, water soaking, wilting, black or brown discoloration, and root rot [20][21][22]. In more mature plants, water-soaked roots and lesions of stem at the soil line, stunted growth, and brown discoloration of roots are prevalent [23].

In India, chili was first reported as the host of Pythium spp. over 100 years ago [24]. Since then, there have been repeated records of damping-off and root rot incidences caused by Pythium spp. in chili [25][26]. Pre-emergence and post-emergence incidences ranging from 7% to 90% of crops have been reported in several states of India [27][28][29][30][31]. In Pakistan, many occurrences of damping-off and root rot disease incidences ranging from 13% to 46% have been reported [20][22][32], and an estimated loss of Rs. 70,000–100,000/acre, in the case of hot pepper, has been observed [33]. Aside from damaging crops directly, Pythium spp. have also been observed to break resistance to nematodes in chili plants [34]. Financial losses due to Pythium infections are not limited to direct damage to crops; instead, they also include the re-sowing costs [35].

Although Pythium spp. has been found to infect both sweet and pungent varieties, the current study is centered on the pungent cultivars. Figure 1 presents a brief summary of the components reviewed in this study. This study focuses on the techniques that have been followed to detect and diagnose Pythium spp., the management strategies studied to control Pythium spp.-related diseases in Capsicum since 2010, and resistant breeding against Pythium spp.

2. Virulence Mechanism of Pythium spp. and Challenges in Resistant Breeding against Pythium spp.

Disease development by

Pythium

spp. involves an extensive repertoire of carbohydrate-active enzymes (CAZymes), including glycoside hydrolases, polysaccharide lyases, carbohydrate esterases, proteases, etc., which help in plant cell wall penetration and further colonization

[36][37]

. Lévesque et al.

[37]

, in their study of the

P. ultimum

genome, observed high sequence similarity and synteny with another phytopathogenic oomycete,

Phytophthora

, sharing genes and encoding enzymes involved in the metabolism of carbohydrates. However, the absence or underrepresentation of many notable genes in

Pythium

encoding for cutinases, xylanases, pectinases, etc., that are present in the

Phytophthora

genome suggests some difference in the methods of virulence between these oomycetes

[38]

. Even the expression of cellulases and pectinases in

Pythium

spp. are limited; they are sufficient for hyphal penetration but not for complete saccharification. Furthermore, the absence of these complex carbohydrate-degrading enzymes and the presence of α-glucosidase, α-amylases, α-glucoamylases, and invertases, degrading the plant starch and sucrose, establishes the affirmation of phytopathogenic

Pythium

towards young plant tissues with no secondary growth

[36]

. The presence of genes encoding proteases families such as subtilisin-related proteases, metalloproteases, and E3 ligases, inducing necrosis and cell-wall degradation in the core

Pythium spp.

genes and the absence or lack of RXLR effectors in

Pythium

support its non-host specificity and necrotrophic lifestyle

[37][38][39]

. Unlike

Phytophthora

,

Pythium

spp. have been observed to lack RXLR effectors with avirulence activities

[38][39]

. The absence of these avirulence factors has been correlated with the lack of gene-for-gene resistance against

Pythium

spp. Thus, the absence of a virulence factor producing RXLR effectors makes the identification of race-specific resistance genes against

Pythium

very difficult to explore

[40]

. However, Ai et al.

[41]

have recently identified RXLR effectors in nine

Pythium

spp. that share a common ancestor with

Phytophthora

. RXLR effectors in

Pythium

spp. exhibited necrosis-inducing activities resulting in plant cell death. The difficulties in identifying race-specific resistance genes against

Pythium

spp. make quantitative gene expression a more comfortable approach for developing resistant varieties. Several major and minor quantitative trait loci (QTL) associated with root rot resistance have been identified in

Pythium

spp.-affected crops such as snap bean

[42]

and soybean

[40][43][44][45]

. The partial (horizontal) resistance achieved through the involvement of multiple QTLs in the soybean plants leading to transgressive segregation has been observed to confer a common response pattern against multiple

Pythium

spp.

[40][45]

. As

Pythium

spp.-affected crops are often inhabited by several

Pythium

spp., the development of a variety with resistance against multiple species would be more efficient.

3. Conclusions

As one of the significant horticulture crops in India and worldwide, improvements in Capsicum production yield are a primary target that needs to be achieved. Biotic stress due to different soil-borne pathogens poses a significant threat to that target. Different Pythium spp. have been reported to cause infections in Capsicum cultivars with a disparate reproductive behavior and growth environment. These factors make the detection of the pathogen and correct identification of the species of paramount importance. The technology for the detection and diagnosis of pathogens has evolved from polyclonal antibodies to the use of multiple species-specific primers for a single species. The use of LAMP technology has provided a possible escape from the use of sophisticated instruments and pure DNA isolation. However, the search for a point-of-care technology that does not require a specific skill set and can provide results in a shorter time frame is still far from over.

Although biological control has provided a viable alternative to chemical management, in a natural setup, the comparative disease control of Pythium spp. has not been achieved yet. Additionally, the successful expression of antagonism by the microbes partially depends upon the cultural and environmental conditions. An integrated system of cultural control and microbial control can provide the desired targets in Pythium spp. disease control. The use of disease-resistant varieties is another sustainable alternative for disease control. However, in this case, no research work could be found involving Pythium-resistance development in C. annuum cultivars. The candidate QTL genes for the resistance against multiple Pythium spp. could prove to be groundbreaking in resistance breeding in C. annuum L.

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