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.