Abstract:
Cassava (Manihot esculenta Crantz) ranks as the fourth most important staple food cultivated
and consumed mainly in the tropics and other developing countries. The crop yields well in areas
with uncertain rainfall patterns and prolonged drought. Leaf retention or longevity (staygreen)
has been associated with sustained cassava production under drought. This study analyzed and
screened for this characteristic in both transgenic and non transgenic cassava genotypes under
greenhouse conditions.
Significant (P<0.05) genotypic variations in intemode growth, leaf abscission, photosynthetic
rates, and stomatal conductance under drought stress treatments were observed between
transgenic and non-transgenic cassava genotypes. Based on these results, provisional levels of
staygreen or drought stress tolerance were assigned to the genotypes. Non-transgenic genotypes
98-0002, 98-2226 and transgenic line 529-48 showed high levels of staygreen, non transgenic
genotypes 91-02322, TME-3 and transgenic line 529-28 expressed moderate levels of staygreen
and non transgenic genotype 95-0306 and wild type TMS 60444 showed drought stress
susceptibility.
Expression of staygreen trait was clearly marked out under 30 and 60% levels of drought stress
as well as in fully irrigated or control treatment. Photosynthetic rates positively correlated with
stomatal conductance and negatively correlated with leaf abscission. Leaf abscission and
stomatal conductance correlated negatively.
The initial stages of characterization of cassava transcriptome upon drought treatment, (for the
identification of up- and down regulated genes), involves RNA purification. It is a pre-requisite
that for successful procedure of microarray-based gene expression profiling, the RNA extracted
should be of high integrity, quality and quantity.
In this study, the following five protocols were tested, optimized and used in the extraction of
RNA from the leaves of cassava genotype TMS 604444: modified CTAB based method, RNeasy
Plant Minikit, Total Nucleic acid and DNase treatment, protocol described by Reilly et al. (2001)
and Trizol Reagent-based method.
The modified CTAB method produced RNA of high concentration (more than 1 microgram),
high quality (A26o:A28o and A26o:A23o ratios more than 2.0) and high integrity (distinct and visible
28S and 18S rRNA bands) from young and old cassava leaves, compared to RNA (from the
same leaf tissues) generated by the other four methods.
