RESUMO
Solid mutant induction using specialized habituation and PBR (Plant bio-regulator) autotrophy-mediated suspension-based ISE system was the prime aim of present investigation. Based on survival of cell clumps after mutagen treatment, the probit analysis was calculated. The result revealed LD50 at 54.31 Gy in gamma, while for EMS (ethyl methanesulfonate), it was 0.1% for 3 h and 0.5% for 1 h. Based on embryogenesis efficiency, a dose rate of 100 Gy and 0.1% EMS for a 3-h exposure were selected for regeneration. As compared to control, significant decrease in the embryogenesis efficiency was recorded at 100 Gy (85.92%) with similar reduction trends in embryo production (79.49%), germination (13.43%), conversion (2.48%), establishment (15.78%) and acclimatization (60.92%). The growth-related parameters such as root and shoot length and number of leaves/regenerant were also significantly reduced to 67.29%, 30.19% and 5.03%, respectively, in the regenerated plants after gamma irradiation as compared to control. In the EMS treatment, at the dose rate of 0.1% for 3-h, the embryogenesis efficiency was reduced to 43.67% with similar diminution trends in embryo production (59.49%), germination (8.95%), conversion (1.94%), establishment (4.37%) and acclimatization (29.9%). The growth-related parameters in the EMS treatment, decreased to 91.00% (root length), 71.34% (shoot length) and 35.03% (no. of leaves). The molecular marker based varied amplifications confirmed the occurrence of mutations in both gamma and EMS induced M1 regenerants. The study highlights the alternative high frequency in vitro mutagenesis protocol for induction of solid mutants in Kinnow mandarin and related citrus species.
RESUMO
Kinnow (Citrus nobilis Lour. × Citrus deliciosa Ten.) needs to be genetically improved for traits such as seedlessness using biotechnological tools. Indirect somatic embryogenesis (ISE) protocols have been reported for citrus improvement. However, its use is restricted due to frequent occurrences of somaclonal variation and low recovery of plantlets. Direct somatic embryogenesis (DSE) using nucellus culture has played a significant role in apomictic fruit crops. However, its application in citrus is limited due to the injury caused to tissues during isolation. Optimization of the explant developmental stage, explant preparation method, and modification in the in vitro culture techniques can play a vital role in overcoming the limitation. The present investigation deals with a modified in ovulo nucellus culture technique after the concurrent exclusion of preexisting embryos. The ovule developmental events were examined in immature fruits at different stages of fruit growth (stages I-VII). The ovules of stage III fruits (>21-25 mm in diameter) were found appropriate for in ovulo nucellus culture. Optimized ovule size induced somatic embryos at the micropylar cut end on induction medium containing Driver and Kuniyuki Walnut (DKW) basal medium with kinetin (KIN) 5.0 mg L-1 and malt extract (ME) 1,000 mg L-1. Simultaneously, the same medium supported the maturation of somatic embryos. The matured embryos from the above medium gave robust germination with bipolar conversion on Murashige and Tucker (MT) medium + gibberellic acid (GA3) 2.0 mg L-1 + ά-naphthaleneacetic acid (NAA) 0.5 mg L-1 + spermidine 100 mg L-1 + coconut water (CW) 10% (v/v). The bipolar germinated seedlings established well upon preconditioning in a plant bio regulator (PBR)-free liquid medium under the light. Consequently, a cent percent survival of emblings was achieved on a potting medium containing cocopeat:vermiculite:perlite (2:1:1). Histological studies confirmed the single nucellus cell origin of somatic embryos by undergoing normal developmental events. Eight polymorphic Inter Simple Sequence Repeats (ISSR) markers confirmed the genetic stability of acclimatized emblings. Since the protocol can induce rapid single-cell origin of genetically stable in vitro regenerants in high frequency, it has potential for the induction of solid mutants, besides crop improvement, mass multiplication, gene editing, and virus elimination in Kinnow mandarin.