Somatic embryogenesis in Carica papaya as affected by auxins and explants, and morphoanatomical-related aspects.

JAMILE L.D. CIPRIANO,ANA CLÁUDIA F. CRUZ,EDILSON R. SCHMILDT,JOSÉ CARLOS LOPES,RODRIGO S. ALEXANDRE,WAGNER C. OTONI,KARINA C. MANCINI

doi:10.1590/0001-3765201820160252

Abstract

The aim of this study was to evaluate somatic embryogenesis in juvenile explants of the THB papaya cultivar. Apical shoots and cotyledonary leaves were inoculated in an induction medium composed of different concentrations of 2,4-D (6, 9, 12, 15 and 18 µM) or 4-CPA (19, 22, 25, 28 and 31 µM). The embryogenic calluses were transferred to a maturation medium for 30 days. Histological analysis were done during the induction and scanning electron microscopy after maturing. For both types of auxin, embryogenesis was achieved at higher frequencies with cotyledonary leaves incubated in induction medium than with apical shoots; except for callogenesis. The early-stage embryos (e.g., globular or heart-shape) predominated. Among the auxins, best results were observed in cotyledonary leaves induced with 4-CPA (25 µM). Histological analyses of the cotyledonary leaf-derived calluses confirmed that the somatic embryos (SEs) formed from parenchyma cells, predominantly differentiated via indirect and multicellular origin and infrequently via synchronized embryogenesis. The secondary embryogenesis was observed during induction and maturation phases in papaya THB cultivar. The combination of ABA (0.5 µM) and AC (15 g L-1) in maturation medium resulted in the highest somatic embryogenesis induction frequency (70 SEs callus-1) and the lowest percentage of early germination (4%).

Highlights

The papaya (Carica papaya L.) is a very important fruit crop species worldwide
In virtue of the scarcity of studies on THB cultivar and the significant interest of somatic embryogenesis applied to papaya, histological analyses are required to understand the induction and maturation stages of the somatic embryos. This present study aimed to evaluate the embryogenic induction of papaya THB cultivar from the explant type and auxin (2,4-D and 4-CPA), establish the maturation medium favorable to the normal formation of the embryos, as well as analyzing the morphoanatomical aspects of its development
The capacity of 2,4-D to activate embryogenic routes may be related to its ability to induce stress genes that contribute to the cellular reprogramming of somatic to embryogenic cells (Fehér 2005, 2015)

Summary

Introduction

The papaya (Carica papaya L.) is a very important fruit crop species worldwide. In Brazil, the state of Espírito Santo is one of the largest producers and exporters of papaya fruits. Several commercial genotypes of this species exist; for instance the THB cultivar, a selection within the ‘Solo’ group of fruits. This cultivar has been widely accepted among producers of northern Espírito Santo for its high productivity and small stature. Zygotic embryos (Ascencio-Cabral et al 2008, Farzana et al 2008, Malabadi et al 2011, Anandan et al 2012) or segments of seedlings (Yu et al 2003, Saha et al 2004, Farzana et al 2008, Homhuan et al 2008) can be used in tissue culture to produce explants and somatic embryos via somatic embryogenesis. This technique produces large numbers of somatic embryos that can be transformed into plantlets, identified as hermaphrodite ex vitro and subsequently rescued in vitro and used in the field

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