High frequency production of rapeseed transgenic plants via combination of microprojectile bombardment and secondary embryogenesis of microspore-derived embryos

Efficient transformation of the isolated microspores of Chinese cabbage (Brassica rapa L. ssp. pekinensis) by particle bombardment

BACKGROUND

The low efficiency of genetic transformation in Chinese cabbage (Brassica rapa L. ssp. pekinensis) is the key problem affecting functional verification. Particle bombardment is a widely used method along with the Agrobacterium-mediated method. As a physical means, it has almost no restrictions on the type of host and a wide range of receptor types, which largely avoids the restriction of explants. The bombardment parameters, which include the number of bombardments, the bombardment pressure, and the bombardment distance, may affect the microspores' genetic transformation efficiency.

RESULTS

The transformation efficiency was improved using the particle bombardment method under the combination of bombardment shot times (3, 4, 5) × bombardment pressure (900, 1100, 1350 psi) × bombardment distance (3, 6, 9 cm). The average viability of microspores in the treatment group ranged from 74.76 to 88.55%, while the control group was 88.09%. When the number of shot times was 4, the number of embryos incubated in the treatment group ranged from 16 to 236 per dish, and the control group had 117 embryos per dish. When the bombardment parameters of the biolistic method were 4 shot times-1350 psi-3 cm, 4 times-1100 psi-3 cm, and 4 times-900 psi-3 cm, they had high transient expression efficiency, and the average number of transformed microspores was 21.67, 11.67, and 11.67 per dish (3.5 mL), respectively. When the bombardment parameters were 4 times, 900 psi, and 6 cm, the highest genetically transformed embryos were obtained, and the transformation efficiency reached 10.82%.

CONCLUSION

A new genetic transformation system with proper parameters for Chinese cabbage microspores was established using particle bombardment. This proper transformation system could provide a useful tool for the improvement of cultivar quality and the investigation of functional genes in Chinese cabbage.

Factor affecting the endogenous β-glucuronidase activity in rapeseed haploid cells: how to avoid interference with the Gus transgene in transformation studies

The gus gene is one of the most frequently used reporter genes in transgenic plants. However, this gene can only be used if the selected plant species does not show endogenous GUS activity. Rapeseed (Brassica napus) microspores and microspore-derived embryos (MDEs) were found to exhibit high activity of endogenous β-glucuronidase which interferes with the expression of bacterial β-glucuronidase that was transferred into these tissues by biolistic transformation. In order to eliminate this background activity from rapeseed MDEs, different pHs of the assay buffer (5.8, 7 and 8) with or without methanol in the reaction buffer and incubation of these tissues at different temperatures (24°C, 38°C and 55°C) were investigated. To avoid this problem in microspores, two incubation temperatures (38°C and 55°C) at different periods after GUS assay (4, 24 and 48h) and in the presence of 1mM potassium ferricyanide and 1mM potassium ferrocyanide were tested. The endogenous GUS activity was significantly decreased in transformed and untransformed MDEs, when the phosphate buffer was adjusted to pH 8 and 28% methanol in the reaction solution was used. In rapeseed microspores, use of 1mM potassium ferricyanide and 1mM potassium ferrocyanide in the reaction buffer enhanced the expression rate of gus transgene rather than endogenous GUS activity where the high levels of gus transgene expression was observed 4h after histochemical GUS assay. Incubation of rapeseed microspores and MDEs at 55°C completely eliminated the endogenous GUS activity. In this study, we also examined changes in endogenous GUS activity in rapeseed MDEs at several stages including the globular, heart, torpedo and cotyledonary stages. The level of endogenous GUS activity was increased 4.33 folds in heart embryos, 6.54 folds in torpedo embryos and 8.5 folds in cotyledonary embryos. Furthermore, the level of GUS activity increased 1.72 folds in MDEs of B. napus in 12-h treatment with 2μM gibberellic acid.

Oilseed rape seeds with ablated defence cells of the glucosinolate-myrosinase system. Production and characteristics of double haploid MINELESS plants of Brassica napus L

Oilseed rape and other crop plants of the family Brassicaceae contain a unique defence system known as the glucosinolate-myrosinase system or the 'mustard oil bomb'. The 'mustard oil bomb' which includes myrosinase and glucosinolates is triggered by abiotic and biotic stress, resulting in the formation of toxic products such as nitriles and isothiocyanates. Myrosinase is present in specialist cells known as 'myrosin cells' and can also be known as toxic mines. The myrosin cell idioblasts of Brassica napus were genetically reprogrammed to undergo controlled cell death (ablation) during seed development. These myrosin cell-free plants have been named MINELESS as they lack toxic mines. This has led to the production of oilseed rape with a significant reduction both in myrosinase levels and in the hydrolysis of glucosinolates. Even though the myrosinase activity in MINELESS was very low compared with the wild type, variation was observed. This variability was overcome by producing homozygous seeds. A microspore culture technique involving non-fertile haploid MINELESS plants was developed and these plants were treated with colchicine to produce double haploid MINELESS plants with full fertility. Double haploid MINELESS plants had significantly reduced myrosinase levels and glucosinolate hydrolysis products. Wild-type and MINELESS plants exhibited significant differences in growth parameters such as plant height, leaf traits, matter accumulation, and yield parameters. The growth and developmental pattern of MINELESS plants was relatively slow compared with the wild type. The characteristics of the pure double haploid MINELESS plant are described and its importance for future biochemical, agricultural, dietary, functional genomics, and plant defence studies is discussed.