TOWARDS THE SELECTION IN VITRO FOR RESISTANCE TO ALTERNARIA BRASSICICOLA (SCHW.) WILTS., IN BRASSICA NAPUS SSP. OLEIFERA (METZG.) SINSK., WINTER OILSEED RAPE

The current scenario and future perspectives of transgenic oilseed mustard by CRISPR-Cas9

PURPOSE

Production of a designer crop having added attributes is the primary goal of all plant biotechnologists. Specifically, development of a crop with a simple biotechnological approach and at a rapid pace is most desirable. Genetic engineering enables us to displace genes among species. The newly incorporated foreign gene(s) in the host genome can create a new trait(s) by regulating the genotypes and/or phenotypes. The advent of the CRISPR-Cas9 tools has enabled the modification of a plant genome easily by introducing mutation or replacing genomic fragment. Oilseed mustard varieties (e.g., Brassica juncea, Brassica nigra, Brassica napus, and Brassica carinata) are one such plants, which have been transformed with different genes isolated from the wide range of species. Current reports proved that the yield and value of oilseed mustard has been tremendously improved by the introduction of stably inherited new traits such as insect and herbicide resistance. However, the genetic transformation of oilseed mustard remains incompetent due to lack of potential plant transformation systems. To solve numerous complications involved in genetically modified oilseed mustard crop varieties regeneration procedures, scientific research is being conducted to rectify the unwanted complications. Thus, this study provides a broader overview of the present status of new traits introduced in each mentioned varieties of oilseed mustard plant by different genetical engineering tools, especially CRISPR-Cas9, which will be useful to improve the transformation system of oilseed mustard crop plants.

METHODS

This review presents recent improvements made in oilseed mustard genetic engineering methodologies by using CRISPR-Cas9 tools, present status of new traits introduced in oilseed mustard plant varieties.

RESULTS

The review highlighted that the transgenic oilseed mustard production is a challenging process and the transgenic varieties of oilseed mustard provide a powerful tool for enhanced mustard yield. Over expression studies and silencing of desired genes provide functional importance of genes involved in mustard growth and development under different biotic and abiotic stress conditions. Thus, it can be expected that in near future CRISPR can contribute enormously in improving the mustard plant's architecture and develop stress resilient oilseed mustard plant species.

Functional analysis of the Alternaria brassicicola non-ribosomal peptide synthetase gene AbNPS2 reveals a role in conidial cell wall construction

SUMMARY Alternaria brassicicola is a necrotrophic pathogen causing black spot disease on virtually all cultivated Brassica crops worldwide. In many plant pathosystems fungal secondary metabolites derived from non-ribosomal peptide synthetases (NPSs) are phytotoxic virulence factors or are antibiotics thought to be important for niche competition with other micro-organisms. However, many of the functions of NPS genes and their products are largely unknown. In this study, we investigated the function of one of the A. brassicicola NPS genes, AbNPS2. The predicted amino acid sequence of AbNPS2 showed high sequence similarity with A. brassicae, AbrePsy1, Cochliobolus heterostrophus, NPS4 and a Stagonospora nodorum NPS. The AbNPS2 open reading frame was predicted to be 22 kb in length and encodes a large protein (7195 amino acids) showing typical NPS modular organization. Gene expression analysis of AbNPS2 in wild-type fungus indicated that it is expressed almost exclusively in conidia and conidiophores, broadly in the reproductive developmental phase. AbNPS2 gene disruption mutants showed abnormal spore cell wall morphology and a decreased hydrophobicity phenotype. Conidia of abnps2 mutants displayed an aberrantly inflated cell wall and an increase in lipid bodies compared with wild-type. Further phenotypic analyses of abnps2 mutants showed decreased spore germination rates both in vitro and in vivo, and a marked reduction in sporulation in vivo compared with wild-type fungus. Moreover, virulence tests on Brassicas with abnps2 mutants revealed a significant reduction in lesion size compared with wild-type but only when aged spores were used in experiments. Collectively, these results indicate that AbNPS2 plays an important role in development and virulence.

Differences in cell death induction by Phytophthora Elicitins are determined by signal components downstream of MAP kinase kinase in different species of Nicotiana and cultivars of Brassica rapa and Raphanus sativus

Elicitins are small, secreted proteins produced by species of the plant-pathogenic oomycete Phytophthora. They induce hypersensitive cell death in most Nicotiana species and in some cultivars of Brassica rapa and Raphanus sativus. In this study, two true-breeding Fast Cycling B. rapa lines were established that showed severe necrosis (line 7-R) or no visible response (line 18-NR) after treatment with elicitin. Unexpectedly, microscopic examination revealed localized cell death in line 18-NR plants, and expression levels of various defense-marker genes were comparable in both lines. These results suggested that both "responsive" and "nonresponsive" plants responded to elicitin but differed in the extent of the cell death response. Expression of a constitutively active form of Arabidopsis (Arabidopsis thaliana) MAP kinase kinase 4 (AtMEK4(DD)) also induced rapid development of confluent cell death in line 7-R, whereas line 18-NR showed no visible cell death. Similarly, elicitin-responsive Nicotiana species and R. sativus cultivars showed significantly stronger cell death responses following expression of AtMEK4(DD) compared with nonresponsive species/cultivars. Line 7-R also showed higher sensitivity to toxin-containing culture filtrates produced by Alternaria brassicicola, and toxin sensitivity cosegregated with elicitin responsiveness, suggesting that the downstream responses induced by elicitin and Alternaria toxin share factors that control the extent of cell death. Interestingly, elicitin responsiveness was shown to correlate with greater susceptibility to A. brassicicola (a necrotroph) in B. rapa but less susceptibility to Phytophthora nicotianae (a hemibiotroph) in Nicotiana, suggesting a more extensive cell death response could cause opposite effects on the outcomes of biotrophic versus necrotrophic plant-pathogen interactions.

In vitro mutation and selection of doubled-haploid Brassica napus lines with improved resistance to Sclerotinia sclerotiorum

This paper describes a new protocol to develop doubled-haploid (DH) Brassica napus lines with improved resistance to Sclerotinia sclerotiorum. In this protocol, haploid seedlings derived from microspore cultures of B. napus were used to produce haploid calli for in vitro mutation-selection. For routine screening, mutation was induced by EMS (ethylmethane sulfonate) or occurred spontaneously, and screening for resistant mutants occurred on media with added oxalic acid (OA) as a selection agent. In tests with selected lines, the optimal concentration of EMS for mutation was determined to be 0.15%, and the optimal concentration of OA for in vitro screening was 3 mmol/l (half lethal dose was 3.1 mmol/l) for the first cycle of screening. There was an accumulated effect of OA toxicity on calli over two cycles of screening, but the growth and capacity of the surviving calli for regenerating seedlings were not affected by OA. Of the 54 DH lines produced from the in vitro mutation-selection, two DH lines of resistant mutants, named M083 and M004, were selected following seedling and glasshouse tests. The resistance of M083 and M004 to S. sclerotiorum following tests with both mycelial inoculum and OA was greater than that of their donor lines and the resistant control Zhongyou 821. In both glasshouse and field disease nurseries, disease indices on M083 and M004 were less than 50% of those of the control. The time required for M083 and M004 to mature was 14 days and 10 days shorter, respectively, than that of their donor lines. Furthermore, M083 had more pods per inflorescence, a greater 1,000 seed weight and higher yield than its donor line. Random amplified polymorphic DNA characterisation showed that M083 had DNA band patterns that differed from its donor line.

THE EFFECT OF A PHYTOTOXIN FROM ALTERNARIA BRASSICICOLA ON BRASSICA POLLEN

A methanol-soluble extract from the culture filtrate of Alternaria brassicicola (Schw.) Wilts., grey leaf spot, markedly inhibited pollen germination and pollen tube growth of Brassica napus L., Brassica campestris L. and Brassica oleracea L. in hanging drop culture at concentrations comparable to those causing death of B. napus secondary embryoids. The extract was partially purified by Sephadex column chromatography, and the fractions obtained were tested for their inhibitory capacity. Two fractions which killed secondary embryoids also inhibited pollen germination. The methanol-soluble extract and inhibitory fractions from Sephadex chromatography were further tested using a thin-layer chromatographic bioassay which showed that the original extract could be separated into at least four inhibitory compounds. The potential of the techniques used for screening and selection for resistance to compounds toxic to plants is briefly outlined.