Plant regeneration via direct and indirect organogenesis in some cultivars of the sweet potato (Ipomoea batatas (L.) Lam.)

Abstract

This study investigated regeneration of shoots from node and shoot tip explants from three drought tolerant sweet potato cultivars KEM10, KSP11 and KSP20 with the aim of providing planting material for farmers in the ASALS where rainfall is inadequate and crop failures frequent. The explants were cultured on Murashige and Skoog (MS) medium supplemented with cytokinins kinetin (KIN), benzylaminopurine (BAP) and zeatin (ZEA) each at 0.5, 1.0 2.0, 3.0, 4.0 and 5.0mg1-1 concentration levels. This study revealed that the node explant was efficient in producing shoots with the highest frequency of nodes; ZEA at 4.0mg1-1 induced the growth of shoots with higest mean number of nodes than did KIN or BAP. The high frequency of nodes produced per shoot is important for laboratory mass micropropagation.

Attempts to regenerate shoots from internodes of the three drought tolerant cultivators on MS medium supplemented with 1.00 mg1-1 naphthalene acetic acid (NAA) plus KIN, BAP and ZEA each at 0.05, 0.1, 0.2 and 0.3mg1-1 concentration levels did not give rise to shoots but produced callus and roots.

The use of leaves and petioles for adventitious shoot production was also investigated. A two stage procedure was used. The leaf and petiole explants were incubated on MS medium supplemented with 0.2mg1-1, 4-dichlorophonexy acetic acid (2,4-D) for 2-3 days. They were then transferred to a second stage medium where 2,4-D was substituted with KIN, BAP and ZEA each at 0.1, 0.2, 0.3, and 0.4 mg1-1 for a period of 6-8 weeks. This investigation revealed that the leaf and petiole may not be suitable explants for in vitro plant regeneration in the three cultivars as only 15% of the leaf explants and 5% of the petiole exolants from KSP11 and KSP20 produced shoots. Both explants from KEMB10 did not give rise to shoots. These two explants are used in transformation work, they are first transformed then induced to produce shoots.

Of the twelve cultivars screened for their responsiveness to the somatic embryogenesis system of Cantliffe (1992) and Maingi (1996), none of them responded positively. However, callus from the apical domes of two cultivars 440012 and CPT560 on MS medium supplemented with 0.5, 1.0, 2.0 and 3.0 mg1-1 2,4-D produced shoots when transferred to hormone-free MS medium. Also calli from leaf pieces of these two cultivars on MS medium supplemented with 0.2 mg1-1 2,4-D and 0.5 mg1-1 KIN produced shoots when transferred to hormone-free MS medium. Calli from leaf pieces took a shorter period (12-16 weeks) to produce shoots while that from apical domes took up 21 weeks. Production of shoots from leaf piece calli within such a short time is good for in vitro plant production as it reduces chances of plants developing somaclonal variation.

Plants produced in this study appeared normal with no observed morphological or phenotypic abnormalities and were successesfully established in the green house.

It is recommended that other drought tolerant cultivars be screened for their responsiveness to in vitro regeneration and regeneration from internodes be tried using other media and since there is need to identify suitable Kenyan cultivars for transformation against the sweet potato weevil and viruses.