Screening for water deficit tolerance, relative growth analysis and agrobacterium-infectivity among tropical maize (zea mays l.) Inbred lines in Nairobi, Kenya

ABSTRACT

Maize is ranked the third most important cereal under global cultivation after wheat and rice. It is valued for food, fiber, feed and for bio fuel production. However the gap between maize demand and regional supply is increasing as small-holder farmers grapple with many challenges, key among them drought. Thus, research in identifying maize lines that are tolerant to water deficit and that are amenable to A. tumefaciens-mediated transformation is a step towards enhancing food security in the region. The objectives of this study were, to assess the physiological response of tropical maize inbred lines to water deficit, to determine whether A. tumefaciens elicits host resistance when in contact with zygotic embryos and determine if YEP is an effective infection medium compared to MS medium in immature zygotic embryo infection. Tropical maize lines CML 395, CML 216, CML 144, TL 21, A 04, E 04 and T 04 and Agrobacterium strain EHA101 habouring pTF102 vector containing GUS reporter gene were used in this research. Physiological response of tropical maize genotypes to drought stress was evaluated by measuring plant height, leaf length, leaf width and fresh weight at the vegetative stage of growth. In addition, competence of immature zygotic embryos (IZE) of all the selected genotypes to Agrobacterium infectivity was assayed through transient GUS assay. All data collected were analyzed using ANOVA at 95 % confidence interval with MINITAB statistical computer software (version 23.22). Mean separation was carried out using Turkey’s pairwise comparison test at 5 % probability level. Transformation frequencies were calculated as the number of embryos showing GUS expression over the total number of embryos infected x 100. In this study, T 04 and CML 216 seedlings exhibited the fastest growth rates of 4.33 cm and 4.28 cm respectively between the 7 th and 8 th day post leaf four emergence while TL 21 and CML 395 seedlings had the lowest rates of 2.93 cm and 3.59 cm respectively under normal growth conditions. A 04 seedlings exhibited the highest differences in fresh weight between stressed and unstressed plants (56.13 gm) while CML 216 seedlings had the lowest (24.57 gm). Upon salt stress, leaf discs of CML 216 seedlings, exhibited low chlorophyll a: b ratios of 0.84, 0.78, 0.74, 0.75 and 0.70 in 100 mM, 125 mM, 150 mM, 175 mM and 200 mM NaCl concentrations respectively, while leaf discs of CML 144 seedlings exhibited higher chlorophyll a: b ratios of 1.51, 1.19, 1.24, 1.26, 1.45 and 1.39 under the same concentrations. There was however no significant differences noted in chlorophyll a: b ratios among the selected genotypes upon mannitol stress, and no evidence of effect of mannitol concentration on carotenoids among all genotypes. Upon infection of immature embryos with Agrobacterium on YEP medium, the immature embryos elicited host resistance as observed by lack of growth of Agrobacterium within the vicinity of the immature embryos. The use of YEP medium in contrast to MS medium led to an improvement in transient GUS expression observed in immature embryos and significant increase in transformation frequency. The transformation protocol using YEP infection media as used in this study should be optimized and used in transformation of tropical maize inbreds.