Manganese (Mn) is an important micronutrient for plant growth and development and sustains metabolic roles within different plant cell compartments. The metal is an essential cofactor for the oxygen-evolving complex (OEC) of the photosynthetic machinery, catalyzing the water-splitting reaction in photosystem II (PSII). Despite the importance of Mn for photosynthesis and other processes, the physiological relevance of Mn uptake and compartmentation in plants has been underrated. The subcellular Mn homeostasis to maintain compartmented Mn-dependent metabolic processes like glycosylation, ROS scavenging, and photosynthesis is mediated by a multitude of transport proteins from diverse gene families. However, Mn homeostasis may be disturbed under suboptimal or excessive Mn availability. Mn deficiency is a serious, widespread plant nutritional disorder in dry, well-aerated and calcareous soils, as well as in soils containing high amounts of organic matter, where bio-availability of Mn can decrease far below the level that is required for normal plant growth. By contrast, Mn toxicity occurs on poorly drained and acidic soils in which high amounts of Mn are rendered available. Consequently, plants have evolved mechanisms to tightly regulate Mn uptake, trafficking, and storage. This review provides a comprehensive overview, with a focus on recent advances, on the multiple functions of transporters involved in Mn homeostasis, as well as their regulatory mechanisms in the plant's response to different conditions of Mn availability.

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Manganese in Plants: From Acquisition to Subcellular Allocation.

Clubroot caused by Plasmodiophora brassicae affects the Brassicaceae family of plants, including many important vegetable and broadacre crops. In the last 20 years increasing intensity of vegetable production and the rapid growth in popularity of oilseed rape as a broadacre or arable break crop have increased the severity of clubroot and the area of land affected in both the vegetable and broadacre industries. Resting spores of P. brassicae are long-lived in soil, but the number of spores can be reduced through crop rotation, fallowing, chemical application, and management of brassica weeds. The host-pathogen system is responsive to a range of control measures, including calcium and boron amendments, manipulation of soil pH, and fungicide application. Molecular tests have been developed to predict disease and resistant cultivars are available for some crops. Increasingly, a multifaceted or integrated approach is being used to manage clubroot. This approach has been particularly successful in vegetable production systems.

Integrated Control of Clubroot

SUMMARY
Plasmodiophora brassicae causes clubroot disease in cruciferous plants, and is an emerging threat to Canadian canola (Brassica napus) production. This review focuses on recent studies into the pathogenic diversity of P. brassicae populations, mechanisms of pathogenesis and resistance, and the development of diagnostic tests for pathogen detection and quantification.

Taxonomy:Plasmodiophora brassicae is a soil-borne, obligate parasite within the class Phytomyxea (plasmodiophorids) of the protist supergroup Rhizaria.

Disease symptoms: Clubroot development is characterized by the formation of club-shaped galls on the roots of affected plants. Above-ground symptoms include wilting, stunting, yellowing and premature senescence.

Disease cycle:Plasmodiophora brassicae first infects the root hairs, producing motile zoospores that invade the cortical tissue. Secondary plasmodia form within the root cortex and, by triggering the expression of genes involved in the production of auxins, cytokinins and other plant growth regulators, divert a substantial proportion of plant resources into hypertrophic growth of the root tissues, resulting in the formation of galls. The secondary plasmodia are cleaved into millions of resting spores and the root galls quickly disintegrate, releasing long-lived resting spores into the soil. A serine protease, PRO1, has been shown to trigger resting spore germination.

Physiological specialization: Physiological specialization occurs in populations of P. brassicae, and various host differential sets, consisting of different collections of Brassica genotypes, are used to distinguish among pathotypes of the parasite.

Detection and quantification: As P. brassicae cannot be cultured, bioassays with bait plants were traditionally used to detect the pathogen in the soil. More recent innovations for the detection and quantification of P. brassicae include the use of antibodies, quantitative polymerase chain reaction (qPCR) and qPCR in conjunction with signature fatty acid analysis, all of which are more sensitive than bioassays.

Resistance in canola: Clubroot-resistant canola hybrids, recently introduced into the Canadian market, represent an important new tool for clubroot management in this crop. Genetic resistance must be carefully managed, however, as it has been quickly overcome in other regions. At least three resistance genes and one or two quantitative trait loci are involved in conferring resistance to P. brassicae. Root hair infection still occurs in resistant cultivars, but secondary plasmodia often remain immature and unable to produce resting spores. Fewer cell wall breakages occur in resistant hosts, and spread of the plasmodium through cortical tissue is restricted. More information on the genetics of clubroot resistance in canola is needed to ensure more effective resistance stewardship.

Useful websites:http://www.canolacouncil.org/clubroot/resources.aspx, http://tu-dresden.de/die_tu_dresden/fakultaeten/fakultaet_mathematik_und_naturwissenschaften/fachrichtung_biologie/botanik/pflanzenphysiologie/clubroot, http://www.ohio.edu/people/braselto/plasmos/

Plasmodiophora brassicae: a review of an emerging pathogen of the Canadian canola (Brassica napus) crop.

https://www.cell.com/trends/plant-science/pdf/S1360-1385(16)00074-1.pdf

Manganese Deficiency in Plants: The Impact on Photosystem II

Clubroot, caused by the obligate parasite Plasmodiophora brassicae, is an economically important disease affecting plants in the family Brassicaceae worldwide. In Canada, it has mainly been a problem on cruciferous vegetables in the traditional production areas of Ontario, Quebec, British Columbia and the Atlantic Provinces. In the Prairie Provinces, clubroot has been reported sporadically in a few home gardens and commercial vegetable fields in Alberta and Manitoba over the past 80 years; however, this situation changed dramatically with the discovery of 12 infected canola (Brassica napus) fields near Edmonton, AB in 2003. Annual surveys carried out since 2003 have revealed that clubroot is a much more widespread and serious disease in Alberta than initially thought. By 2008, it had been detected in about 410 canola, mustard and vegetable fields in central and southern areas of the province. The Alberta Clubroot Management Plan was developed to guide farmers, agribusinesses, oil and gas companie...

Clubroot of cruciferous crops – new perspectives on an old disease†

A nested polymerase chain reaction (PCR) method was developed for detection of DNA from Plasmodiophora brassicae in naturally infested field soil samples. The target sequences 389 bp and 507 bp were amplified from Swedish populations of P. brassicae. The protocols described enabled detection of DNA in various soil classes with an inoculum level of P. brassicae corresponding to a disease severity index (DSI) higher than 21 in a greenhouse bioassay. Three sequenced Swedish P. brassicae isolates had identical sequence in the 18S/ITS 1 region, but differed by a few nucleotides from an isolate sequenced in the UK. The results indicate that the primers used are general for P. brassicae, and consequently the nested PCR assay has a potential to be developed as a routine diagnostic test.

Detection of Plasmodiophora Brassicae By PCR in Naturally Infested Soils

Brassica oilseed crops have been grown by Swedish farmers since the early 1940s. Years of high market prices for vegetable oils resulted in intensive cultivation of Brassica oilseeds in various regions, which led to problems with soilborne pathogens including Plasmodiophora brassicae. This pathogen most likely was present in Swedish soils prior to the Brassica oilseed boom after World War II. Currently, reports of clubroot disease outbreaks in Sweden are frequent, with a trend of increasing incidence. Since 2012, DNA-based soil analyses for the presence of P. brassicae have been offered to farmers in order to improve their crop rotation planning. Other means of limiting the damage caused by P. brassicae also are presently under study, such as the effects of boron or different sources of nitrogen. The distribution and identification of different pathotypes/races of P. brassicae in Sweden could so far not be verified. To aid in race diagnostics, resistance breeding efforts, and in understanding the ...

Clubroot, a persistent threat to Swedish oilseed rape production

Cultivation of forage maize is increasing in Sweden. Sole maize is low in protein and supplementation of protein feed is needed. This study investigated whether forage maize and legumes can be intercropped in a system suitable for farmers (simultaneous sowing and harvest) under Swedish climate conditions, and whether intercropping maize and faba bean improves the forage quality compared with sole maize. Two field experiments were performed in which maize intercropped with faba bean (Vicia faba L.) in alternate rows with 0 or 60 kg N/ha was compared with sole maize crop with 120 kg N/ha. Maize dry matter (DM) yield decreased by intercropping (from mean 14,171 kg DM/ha for sole maize to 8,888–10,791 kg DM/ha for intercropped maize). Mean yield of faba bean ranged from 2,907 to 2,966 kg DM/ha. Compared with sole maize, intercropped treatments slightly increased forage protein content (mean increase 10–15 g/kg DM). Furthermore, intercropping increased forage in vitro organic matter digestibility by 4 % units (from 80.8 to 84.7 %), while the starch concentration slightly decreased from 316 to 236–254 g/kg DM. Intercropping increased neutral detergent fibre concentration from 435 to 478–497 g/kg DM and the sugar concentration from 57.3 to 61.5–72.3 g/kg DM. The results showed that intercropping silage maize and faba beans is possible under Swedish climate conditions.

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Ann-Charlotte Wallenhammar

Papers

Detection of Plasmodiophora Brassicae By PCR in Naturally Infested Soils

Clubroot, a persistent threat to Swedish oilseed rape production

Intercropping Maize and Faba Bean for Silage Under Swedish Climate Conditions

Special Issue: Clubroot, a persistent threat to Swedish oilseed rape production

Seed distance in relation to row distance: Effect on grain yield and weed biomass in organically grown winter wheat, spring wheat and spring oats