Читать книгу Biotechnology of Fruit and Nut Crops - Группа авторов - Страница 200

BREEDING OBJECTIVES. Increased variability in crop plants has been sought for pest resistance, stress tolerance, increased vigour and yield, improved fruit quality and functional analysis of the sex determination gene. Mutants created for variability have been created by irradiation and chemical treatments with colchicine and ethyl methanesulfonate (EMS). Irradiation causes damage to DNA by rearrangements and deletions while chemical treatments like EMS produce random point mutations in DNA by nucleotide substitution, particularly guanine alkylation, that result in G/C to A/T transitions (Bhat et al., 2007; Talebi et al., 2012). Colchicine is usually used to double chromosome number since it prevents the formation of microtubules during mitosis.

PROTOCOL. Data on radiation dosages that result in viable mutants have been reported (Chan, 2009; Husselman et al., 2016). Dosages ranging from 0 to 300 Gy were screened, although 525 Gy were also used (Chan, 2009). Mutagenesis by colchicine or EMS was utilized by Mahadevamma et al. (2012) and M.-L.Wang (2018, Hawaii, personal communication).

ACCOMPLISHMENTS. LD₅₀ gamma irradiation for papaya callus was 2.75 kR (27.5 Gy). Seeds of promising selections in South Africa did not germinate at 100 and 120 Gy (Husselman et al., 2016). A dosage of 80 Gy resulted in reduced plant height and stem circumference of ‘Bh65’, ‘Maradol’ and ‘Sunrise Solo’, although yield was not decreased except for ‘Shew Me’ and ‘V3’. Fruit size was smaller. Mutation breeding in Malaysia involved screening ‘Sekaki’ and ‘Eksotika’ with dosages of 0–300 Gy with dry and wet seeds (Chan, 2009). Wet seeds were presoaked overnight in water and surface dried. Dry seeds showed no loss of viability at 300 Gy, while 100 Gy was lethal for presoaked seeds. At a 525 Gy, presoaked seeds had 50% reduction in seedling elongation. Irradiation at 42.5 Gy was optimal for presoaked seeds that showed 50% reduction in germination and growth. ‘Eksotika’ dry (525 Gy) and presoaked (42.5 Gy) seeds resulted in M2 populations with stem splitting, leaf variegation and puckered, crinkled leaves on dwarfs. The M3 exhibited a wide variability of physiological defects of presoaked seeds, including shorter plants with vigorous leaf development compared to seedlings that resulted from dry seeds irradiated with 525 Gy and untreated seeds. The prospects of creating dwarf ‘Eksotika’ with low fruit bearing, higher total soluble solids and larger fruit size has potential, and several of the M2 and M3 plants had very good resistance to malformed top disease.

Colchicine at 0.05% for 20 min or at 0.025% for 40 min resulted in high rates of seed survival (Mahadevamma et al., 2012). According to M.-L. Wang, Hawaii, 2018, personal communication, papaya seeds were treated with 25–40 mM EMS for 6–18 h, detoxified with 0.1 M sodium thiosulfate, rinsed with distilled water and planted. After 2–3 weeks, c.25–80% of ‘SunUp’ and Australian ‘AU9’ germinated. Many of the ‘SunUp’ seedlings were sterile. One partial male sterile plant was identified. The goal of this ongoing work has been to identify the sex determination gene by functional analysis.

Altered characteristics of mutagenized papayas were identified; reduced tree height was observed (Chan, 2009). After EMS treatment, a single candidate, M1, appeared to be a seedless hermaphrodite, but subsequent M2 and M3 inbred seedlings included some females. Several thousand more mutants are being grown for further flowering in the field (M.-L. Wang, unpublished results).