Papaya (Carica papaya L.) is a popular and economically important fruit tree of tropical and subtropical countries. The fruit is consumed world-wide as fresh fruit and as a vegetable or used as processed products. This review focuses primarily on two aspects. Firstly, on advances in in vitro methods of propagation, including tissue culture and micropropagation, and secondly on how these advances have facilitated improvements in papaya genetic transformation. An account of the dietary and nutritional composition of papaya, how these vary with culture methods, and secondary metabolites, both beneficial and harmful, and those having medicinal applications, are discussed. An overview of papaya post-harvest is provided, while ‘synseed’ technology and cryopreservation are also covered. This is the first comprehensive review on papaya that attempts to integrate so many aspects of this economically and culturally important fruit tree that should prove valuable for professionals involved in both research and commerce.

/pdf/papaya-carica-papaya-l-biology-and-biotechnology-54wxiq1w3e.pdf

Papaya (Carica papaya L.) Biology and Biotechnology

Immature zygotic embryos from open-pollinated and selfed Carica papaya L fruits, 90 to 114 days post-anthesis, produced 2 to 20 somatic embryos on apical domes, cotyledonary nodes, and radicle meristems after culture for three weeks on half-strength Murashige and Skoog (MS) medium supplemented with 01 to 25 mg l(-1) 2,4-dichlorophenoxyacetic acid (2,4-D), 400 mg l(-1) glutamine, and 6% sucrose After six weeks of culture, about 40 to 50% of the zygotic embryos had become embryogenic, and each embryogenic embryo yielded hundreds of somatic embryos within five months of culture on media supplemented with 2,4-D Somatic embryos matured on half-strength MS medium, germinated on MS medium containing 5 mg l(-1) kinetin, and grew large enough for greenhouse culture on MS medium Shoots were rooted in vermiculite and grown in the greenhouse

Somatic embryogenesis and plant regeneration from immature zygotic embryos of papaya (Carica papaya L.)

High frequency somatic embryogenesis in papaya (Carica papaya L.) tissue cultures was achieved by culturing hypocotyl sections from ten-day-old seedlings on half-strength Murashige and Skoog salts (MS) medium containing modified MS vitamins, 2.3 to 112.5 μM 2,4-dichlorophenoxyacetic acid (2,4-d), 400 mg l-1 glutamine, and 6% sucrose. Four hermaphroditic Hawaiian cultivars produced embryogenic calluses after ten to 14 weeks of culture at 27°C in the dark. Efficiency in embryogenic response of genotypes differed, ‘Kapoho’ > ‘Sunset’ > ‘Sunrise’ > ‘Waimanalo’. The frequency of embryogenesis in induction medium containing 4.5 μM 2,4-d was lowest with 3% sucrose and highest with 7% sucrose. Somatic embryos developed directly from embryogenic calluses on induction medium, or, more often, they differentiated from calluses subcultured on a medium devoid of growth regulators. Between 50 and 500 embryos were produced from each 2-mm hypocotyl section after at least two months on induction medium and two months on maturation medium. Embryos subsequently developed into normal-looking plants on MS medium. Shoot cuttings from germinated embryos and micropropagated plants were rooted with 5.0 μM indole-3-butyric acid (IBA), grown in the greenhouse, and transferred to the field.

High frequency somatic embryogenesis and plant regeneration from papaya hypocotyl callus

Since the first reported induction of somatic embryogenesis in carrot cell cultures more than 25 years ago (131, 151), there has been significant progress and numerous reviews (3–6, 8, 17, 25, 35, 40, 41, 48, 51, 98, 103, 123, 126, 128, 135, 145, 166, 167). Consequently, this review will emphasize the more recent work on woody perennials, including conifers and woody monocotyledonous plants. Reviewed elsewhere in this series are somatic embryogenesis in endosperm (Lakshmi Sita), haploids (Rohr, Chen), and the role of ammonia in somatic embryogenesis (Durzan).

Somatic Embryogenesis in Woody Perennials

Generation of transgenic papaya (Carica papaya L.) has been hampered by the low rates of transformation achieved by conventionalAgrobacterium infection or microprojectile bombardment. We describe an efficientAgrobacterium-mediated transformation method based on wounding of cultured embryogenic tissues with carborundum in liquid phase. Embryogenic tissues were obtained from cultured immature zygotic embryos collected 75–90 days after pollination. The expressible coat protein (CP) gene of a Taiwan strain of papaya ringspot virus (PRSV) was constructed in a Ti binary vector pBGCP, which contained the NPT-II gene as a selection marker. The embryogenic tissues were vortexed with 600 mesh carborundum in sterile distilled water for 1 min before treating with the disarmedA. tumefaciens containing the pBGCP. Transformed cells were cultured on kanamycin-free medium containing 2,4-D and carbenicillin for 2–3 weeks and then on the kanamycin medium for 3–4 months. The developed somatic embryos were transferred to the medium containing NAA, BA and kanamycin and subsequently regenerated into normal-appearing plants. Presence of the PRSV CP gene in the putative transgenic lines was detected by PCR and the expression of the CP was verified by Western blotting. The transgene was nuclearly inherited as revealed by segregation analysis in the backcrossed R1 progeny. From five independent experiments, the average successful rate of transformation was 15.9% of the zygotic embryos treated (52 transgenic somatic embryo clusters out of 327 zygotic embryos treated), about 10–100 times higher than the available methods previously reported. Thus, wounding highly regenerable differentiating tissues by carborundum vortexing provides a simple and efficient way for papaya transformation mediated byAgrobacterium.

Efficient transformation of papaya by coat protein gene of papaya ringspot virus mediated byAgrobacterium following liquid-phase wounding of embryogenic tissues with caborundum.

Two methods of in vitro culture were employed to regenerate papaya plants. One involved regeneration of plants from callus and the other, production of multiple plants from single shoot-tip explants. Callus was induced from stem sections of papaya seedlings in a medium containing 1 mg per 1 NAA and 0.1 mg per 1 kinetin. The callus regenerated shoots and/or embryoids when transferred to a medium of lower auxin, 0 to 0.05 mg per 1 IAA, and higher cytokinin, 1 to 2 mg per 1 kinetin. Multiple shoots were produced when the excised shoot-tip explants were cultured in a medium supplemented with 0.05 mg per 1 IAA and either 5 mg per 1 kinetin or 0.5 to 1.0 mg per 1 benzyladenine. Root formation of the shoots or embryoids that derived from callus or shoot tips occurred in a medium containing 5 mg per 1 IAA and in a light intensity of 3000 to 4000 lx. The rooted plants could be established in soil and under standard greenhouse conditions after they had been acclimated by initially growing them in moist vermiculite contained in polyethylene-covered pots.

Shi-Tao Yie

Papers

Plant regeneration from shoot tips and callus of papaya