Date Palm - Genomics of Cultivated Palms - page 51

palm [23] . Later, an S plasmid was found in some varieties of 1.45 kbp size [24] . R
plasmid evolved from S plasmid by a recombination event of 26 bp imperfect short
direct repeats. These two plasmids differed by a size of a 109 bp segment and were
amplified by [25] using Oli1 and Oil2 primers. Six Moroccan varieties resistant to
the disease had only R plasmid and lacked S plasmid. These varieties had poor fruit
quality. Hence, variation in the mitochondrial plasmid was found to be a potential
candidate marker for resistance breeding. However, later the association between the
marker and resistance [26] was verified and found only in 77% (seven out of nine)
of plants. The remaining two susceptible varieties had the R plasmid or both S and R
plasmids [26] . Various biochemical mechanisms, toxins, and the molecular biology
of the Bayoud disease resistance of date palm were reviewed recently [27] and pro-
vide additional details on the subject.
4.5 Probes for Brittle Leaf Disease
“Brittle leaf disease,” or the “Maladie des feuilles cassantes” (MFC), is a disor-
der affecting date palms in Tunisia. Leaflets of young fronds of the affected palms
become brittle and break when flexed and squeezed. Later, the other fronds also
develop similar symptoms and the tree dies. The disorder is prevalent in the Nafta
oasis of Tunisia [28] , Eastern Algeria, and Libya, and the leaves of the affected
palms are deficient in Mn [29] . Date palms with brittle leaf disease are associated
with the accumulation of two populations of small, chloroplast-encoded RNAs corre-
sponding to two different regions of the date palm chloroplast genome. The regions
are rrn5s/trnR and ATPE, coding for 5S rRNA, tRNA-Arg, and e subunit of ATP
synthase, respectively. Dot blot hybridization of these two RNA molecules gave a
positive hybridization signal in samples of affected tissues of any cultivar, with var-
ying severity of symptoms, sampled from different geographical locations. But the
signal was not seen in samples from the trees affected by other biotic and abiotic
stresses. Hence, the dot blot assay is simple and specific for the disorder and useful
for early diagnostic purposes [30] . Isolation and molecular characterization of these
two RNAs were analyzed for the properties of circular RNA viroids but were found
to be double-stranded RNAs of host origin [31] . Although the exact cause of the dis-
ease is still unknown, a rapid detection technique is available that uses the dot blot
technique, using an RNA probe specific to date palm.
4.6 Somaclonal Variations, DNA Methylation,
and Other Markers
Tissue culturing of date palm is done commercially to produce elite genotypes
quickly. It is helpful to overcome the few offshoots by a mother plant over a year.
Somaclonal variations do come up, which need to be detected to maintain genetic
stability. Tissue culture-derived date palm offshoots, analyzed using 37 RAPD