Morphological characterization and identification of molecular markers for dwarfism genes in Sorghum bicolor L. [Moench]
Abstract
English: Sorghum is a grain crop with enormous potential in meeting the nutritional needs of
people in developing countries, as well as optimising current meat and poultry
production in the rest of the world. Sorghum has been overlooked in recent years and is
not utilised to its full potential, especially in African countries. The conversion of exotic
lines in terms of height and maturity holds great potential for the development of
improved varieties. The genes controlling height or dwarfism (dw) have also been found
to enhance drought tolerance making it an important trait to be incorporated for sorghum
improvement in semi-arid areas. However, little information is available on the genetic
and physiological basis of height.
In this study, near-isogenic lines developed for different height classes were
characterised (Chapter 2). Significant differences were observed in plant height,
internode length and flag leaf length between different height classes as previously
hypothesised. However, the current study also identified significant differences in flag
leaf width, panicle weight and number of panicle branches between the different height
classes, in contrast to previous reports comparing only two height classes. Furthermore,
this is the first report that plant height is correlated to flag leaf length, flag leaf width,
panicle weight, leaf sheath length and panicle length.
The exogenous application of gibberellic acid (GA) did not result in a final height
increase, as expected from previous reports (Chapter 3). However, it was observed that
GA application may affected the growth response in the different height classes. This
suggests that dwarf genes probably do not affect gibberellic acid biosynthesis but rather
may act as a negative regulator in response to GA and that other genes, possibly the
maturity genes determine the response to GA.
This is the first report of using SSRs on near-isogenic lines for different height classes to
identify specific dwarfism genes (Chapter 4). Potential markers were identified for the
dW1, dW2 and DW3 genes in sorghum. These markers also allowed the deduction of
genotypes for the different height classes. Linkage group A was confirmed to contain
loci affecting plant height. These markers will facilitate sorghum breeding and reduce
the time needed for line conversion.
The AFLP technique was effective in identifying a total of 164 possible markers for the
different dwarfism genes (Chapter 5). Markers were identified for recessive and
dominant dwarfism alleles at dW1, dW2 as well as dW3. Furthermore, the genetic distance
observed between the different near-isogenic height classes and the parental lines
indicate that a bias exists in the retention of an unexpectedly large donor segment,
possibly introduced through the selection of height and maturity characteristics.
Future research resulting from the present study includes further investigation of the
interaction between the dwarfism and maturity genes, as well as the verification of SSR
and AFLP markers in genetic material with confirmed genotypes in order to incorporate
these markers into international breeding programmes. The use of these markers in
conversion programmes will allow more cost effective and accelerated sorghum
improvement. Afrikaans: Sorghum is In graangewas met geweldige potensiaal om te voldoen aan voedings
behoeftes in ontwikkelende lande asook vee en pluimvee produksie in die wêreld. Die
volle potensiaal van sorghum in Afrika is nie die afgelope paar jaar verwesenlik nie.
Eksotiese sorghum lyne hou groot potensiaal in vir die ontwikkeling van verbeterde
sorghum lyne. Droogte-toleransie in sorghum kan verbeter word vir droër gebiede met
die inkorporering van dwerg-gene (dw) wat planthoogte beheer. Daar bestaan egter nie
voldoende inligting oor die fisiologiese beheer en interaksie van dwerg-gene nie.
In hierdie studie is naby-isogeniese-Iyne vir verskillende planthoogtes gekarakteriseer
(Hoofstuk 2). Betekenisvolle verskille is waargeneem tussen planthoogte en
internodale-Iengtesen vlagblaarlengtes, soos in vorige studies aangedui. In teenstelling
met vorige studies waar slegs twee planthoogte-Iyne gebruik is, is betekenisvolle
verskille tussen planthoogte en vlagblaarbreedtes, aargewig en die hoeveelheid
aarvertakkings waargeneem. In hierdie studie word planthoogte vir die eerste keer
gekorreleer met vlagblaarlengte, vlagblaarbreedte, blaarskede-lengte, aargewig en
aarlengte.
In teenstelling met wat verwag is, het die eksterne toedinging van gibberelliensuur (GA3)
nie die finale lengte van naby-isogeniese-dwerglyne vehoog nie (Hoofstuk 3). 'n
moontlikke reaksie op GA3 toedining is egter waargeneem in die groei van die
verskillende hoogte-lyne. Daar mag dus afgelei word dat die dwerggene nie die
produksie van die plantgroeihormoon GA3 beheer nie, maar eerder optree as 'n
negatiewe reguleerder tydens vrystelling van GA. Resultate in die studie dui ook op die
rol van ander gene, moontlik volwassenheidsgene (ma) in die regulering van GA3.
Hierdie is die eerste studie wat gebruik maak van SSRe (kort DNA basis opeenvolging
herhalings) op naby-isogeniese-Iyne vir verskillende hoogte-lyne (Hoofstuk 4).
Moontlikke merkers is geïdentifieseer vir die dW1, dW2 en DW2 gene in sorghum. Die
afleiding van genotipe vir die naby-isogeniese-Iyne is moontlik gemaak deur SSR
merkers. Koppelingsgroep A is wel geassosieer met 'n invloed op planthooghte in
sorghum. Met die gebruik van hierdie merkers kan sorghumteling bevoordeel word in
terme van tydvermindering en plantlynveranderinge.
Die AFLP (geamplifieseerde fragmentlengte polymorfisme) tegniek is effektief gebruik
om 164 moontlikke merkers vir die verskillende dwerggene asook vir dW3 te ientifiseer
(Hoofstuk 5). Genetiese afstande tussen die naby-isogeniese-Iyne en die ouerlyne dui
op 'n bo verwagte oordraging van chromosomale DNA deur die skenker ouer,
waarskynlik as gevolg van seleksie vir beide hoogte- en volwassenheidsgene.
Die interaksie tussen dwerggene (dw) en volwassenheids gene (ma) dien as uitvloeisel
van hierdie studie vir toekomstige navorsing. Bevestiging van die merkers met
bestaande genotipes wat in hierdie studie geïdentifiseer is, is nodig. Die gebuik van
hierdie merkers sal die verbetering van sorghum van groot waarde wees.