Effect of Arabinogalactans on Induction of White-Opaque Somatic Embryos of Avocado (Persea americana Mill.) cv. Duke-7

Abstract

The development of somatic embryogenesis in avocado (Persea americana Mill.) has been hampered by different chronic problems. One such problem is the low level of induction of white-opaque somatic embryos (WOSEs) during the process of obtaining full avocado plants. We detected the induction of multiple WOSEs promoted after the placement of three or four small WOSEs over the embryogenic callus of Duke-7. Among the other possible chemical inductors of the Arabinogalactans (AGPs), we identified a family of extracellular plant proteoglycans implicated in many aspects of the in vitro induction of somatic embryos (SE). We extracted AGPs directly from embryogenic cultures of avocado. When the induction/proliferation medium of embryogenic avocado calli (MS-0.1 mg L^-1 Picloram) was supplemented with 1-2 mg L^-1 AGP, the induction rate of good-quality WOSEs from the embryogenic callus increased significantly (more than ten times that of the control without AGP) and this effect persisted for at least five subcultures after the initial treatment with AGP. AGP also modified the texture and quality of the callus. The effect of AGP extends to other cultivars and proliferation media. Our objectives were to improve the induction of WOSEs and study the effect of AGP in the somatic embryogenesis of avocado.

Keywords: AGP extraction; WOSE induction; avocado cultivars; somatic embryogenesis; subtropical trees.

Figure 1

Induction of WOSEs by supplementing embryogenic callus explants of avocado cv. Duke-7 with (0, 1, 2, 3, 4 small WOSEs); isolated WOSE explants were used as control. Initiation of assay (A); end of assay after an incubation of 4 weeks (B). Arrows indicate the small WOSEs placed over the embryogenic calli.

Figure 2

Composition of sugars in the dialyzed medium of the different samples. Abbreviations: (EC + WS)—embryogenic callus plus small-sized WOSEs; (EC + WM)—embryogenic callus plus medium-sized WOSEs; (EC + WB)—embryogenic callus plus large-sized WOSEs; (WM)—medium-sized WOSEs alone; (EC)—embryogenic callus alone; Rha—rhamnose; Gal—galactose; Ara—arabinose; Fuc—fucose; Xyl—xylose; Man—mannose; Glc—glucuronic acid.

Figure 3

Test of WOSE induction by supplementing embryogenic callus explants of avocado cv. Duke-7 with different doses of AGP (0, 0.5, 1, 2, 3, 4 mg L⁻¹). Initiation of assay (A); end of assay (B).

Figure 4

Persistence of the effect of supplementation with AGP (1 mg L⁻¹) on WOSE proliferation and development after subculturing of explants to fresh medium without AGP.

Figure 5

Induction of WOSEs by supplementing embryogenic callus explants of avocado cv. Duke-7 with 1 mg L⁻¹ AGP plus one small-size WOSEs or one medium-size WOSE. Initiation of assay (A); end of assay after an incubation of 4 weeks (B).

Figure 6

Effect of supplementation with 1 mg L⁻¹ AGP over cv. Duke-7 explants consisting of 3 small-sized WOSEs and 3 medium-sized WOSEs, both without embryogenic calli. AGP was incorporated to the standard medium prior to autoclaving. Initiation of assay (A); end of assay after an incubation of 4 weeks (B).

Figure 7

Secondary WOSE development in cultures of cv. Duke-7 incubated in medium MS + 0.1 picloram supplemented with 1 mg L⁻¹ AGP on explants consisting of one medium-sized WOSE, without embryogenic callus (A) or with embryogenic callus (B), after 4 weeks of incubation.