In vitro regeneration of selected Kenyan papaya (Carica papaya L.) lines through shoot tip culture

Papaya, an important fruit crop in Kenya is commercially propagated through seeds which lead to production of non-true-to-types plants. Conventional vegetative propagation methods are not carried out, hence, the need for micropropagation for mass multiplication of selected lines. An assessment for the effect of 6-benzylaminopurine (BAP) at 0.1, 0.5, 1.0 and 2.0 mg/l combined with α-naphthalene acetic acid (NAA) at 0.05, 0.1, 0.5 and 1.0 mg/l and a control on shoot multiplication and elongations, and indole-3-butyric acid (IBA) at 0, 0.1, 0.5, 1.0, 2.0, 2.5 and 3.0 mg/l on root induction were evaluated. Number of shoots and their length were recorded every three weeks for 12 weeks. Number of roots, root length and percentage rooting induction were recorded after eight weeks. The highest number of shoots was recorded in 0.5 mg/l BAP combined with 0.1 mg/l NAA and the longest shoots were recorded in 0.1 mg/l BAP combined with 0.05 mg/l NAA across the three lines. IBA at 2.5 mg/l produced the highest number of roots, root length and highest percentage of rooting induction. An in vitro regeneration of selected papaya lines through shoot tip culture was established.Key words: Carica papaya, in vitro regeneration, shoots multiplication, rooting

Metagenomic analysis of plant viruses associated with papaya ringspot disease in Carica papaya L. in Kenya

Carica papaya L. is an important fruit crop grown by small- and large-scale farmers in Kenya for local and export markets. However, its production is constrained by papaya ringspot disease (PRSD). The disease is believed to be caused by papaya ringspot virus (PRSV). Previous attempts to detect PRSV in papaya plants showing PRSD symptoms, using enzyme-linked immunosorbent assay (ELISA) and reverse transcriptase-polymerase chain reaction (RT-PCR) procedures with primers specific to PRSV, have not yielded conclusive results. Therefore, the nature of viruses responsible for PRSD was elucidated in papaya leaves collected from 22 counties through Illumina MiSeq next-generation sequencing (NGS) and validated by RT-PCR and Sanger sequencing. Viruses were detected in 38 out of the 48 leaf samples sequenced. Sequence analysis revealed the presence of four viruses: a Potyvirus named Moroccan watermelon mosaic virus (MWMV) and three viruses belonging to the genus Carlavirus. The Carlaviruses include cowpea mild mottle virus (CpMMV) and two putative Carlaviruses —closely related but distinct from cucumber vein-clearing virus (CuVCV) with amino acid and nucleotide sequence identities of 75.7–78.1 and 63.6–67.6%, respectively, in the coat protein genes. In reference to typical symptoms observed in the infected plants, the two putative Carlaviruses were named papaya mottle-associated virus (PaMV) and papaya mild mottle-associated virus (PaMMV). Surprisingly, and in contrast to previous studies conducted in other parts of world, PRSV was not detected. The majority of the viruses were detected as single viral infections, while a few were found to be infecting alongside another virus (for example, MWMV and PaMV). Furthermore, the NGS and RT-PCR analysis identified MWMV as being strongly associated with ringspot symptoms in infected papaya fruits. This study has provided the first complete genome sequences of these viruses isolated from papaya in Kenya, together with primers for their detection—thus proving to be an important step towards the design of long-term, sustainable disease management strategies