<p>Dhurrin is a cyanogenic glucoside (CG), an
important compound that can interplay with primary and secondary metabolism in sorghum.
Dhurrin metabolism contributes to insect
resistance, growth, nitrogen (N) metabolism, drought tolerance, and safety for
animal consumption when used as a forage. Through chemical mutagenesis with
ethyl methanesulfonate (EMS), a mutation in the gene encoding CYP79A1 (<i>cyp79a1</i>), the first enzyme in the
biosynthetic pathway of dhurrin, was discovered that inhibits the production of
dhurrin. The acyanogenic phenotype of this mutant could be a major benefit in
reducing the risk of hydrogen cyanide (HCN) toxicity within animals; however, understanding
the effects of inhibiting dhurrin biosynthesis is important in understanding
metabolic tradeoffs that could occur. This dissertation describes research to
assess impacts and tradeoffs of the dhurrin-free trait on susceptibility to
Fall Armyworm [<i>Spodoptera
frugiperda</i> (J.E. Smith)] (FAW) feeding, seedling growth, effects on
post-flowering drought tolerance, cold stress and utilization as a forage.
Insect susceptibility and seedling growth were examined using near-isogenic
lines (NILs) within the greenhouse utilizing non-destructive phenotyping
technologies for green plant area and in the field comparing total leaf area
and dry weight. Post-flowering drought stress was induced within a greenhouse,
growth chamber, and field environments. The <i>cyp79a1 </i>mutation was tested
in NILs, a near-isogenic backcross (NIBC) population, and near-isogenic hybrids
(NIH), to understand the impacts of the <i>cyp79a1 </i>mutation<i> </i>on the
stay-green trait. Palatability as forage was examined by comparing the feeding
preference of ruminant animals with multiple conventional hybrids and an
experimental hybrid carrying the <i>cyp79a1</i> mutation. This preference was
also examined using a set of NILs varying in the <i>cyp79a1 </i>mutation<i>. </i> Safety was assessed in preference trials by
testing for HCN release before grazing. To further our understanding of the
benefits of sorghum as a forage, the dhurrin-free experimental hybrid was
compared to seven conventional hybrids as a dry product. The dry sorghum
product was tested for the release of HCN and dhurrin content. Lastly, the
effects of low temperatures and frost were assessed for their effects on the
production of dhurrin in cyanogenic and dhurrin-free sorghum genotypes. </p>
<p>Overall, the
biosynthesis of dhurrin had a significant effect on the deterrence of FAW and
on the growth of sorghum seedlings. Dhurrin-free lines were more susceptible to
FAW feeding but also exhibited a significantly higher growth rate. Dhurrin-free
lines and hybrids only exhibited a slight increase in susceptibility to
post-flowering drought stresses with only one dhurrin-free hybrid discovered to
senesce faster than its wild-type NIH. Comparisons of the effects of dhurrin
biosynthesis on stay-green in a NIBC population in Tx642 (B35), one of the most
important sources of the stay-green trait, did not show any variation in
chlorophyll concentration (CC) and normalized difference vegetation index
(NDVI). Analyses of the impact of dhurrin on palatability as a forage showed
that ewes preferred grazing on the dhurrin-free hybrids and NILs, showing that
the ewes were able to detect the presence or absence of dhurrin while feeding.
Experiments to assess the safety and stability of dhurrin in dried plant
material demonstrated that dhurrin content did not change during drying and HCN
was released after rehydration. Furthermore, high levels of HCN were
immediately released when rumen fluid was added to dried plant materials
containing dhurrin; however, no detectable HCN was released from dhurrin-free
genotypes. Finally, sorghum plants exposed to freezing temperatures exhibited
an increase in dhurrin content in conventional sorghum hybrids while no
detectable dhurrin was noted within <i>cyp79a1 </i>mutants. </p>
<p>Taken together, these
studies demonstrate pleiotropic effects for the <i>cyp79a1 </i>mutation.
Dhurrin-free genotypes were more susceptible to insect herbivory and may be
slightly more susceptible to post-flowering drought within the hybrids;
however, these genotypes exhibited higher seedling growth rates, feeding
preference by ewes, no release of HCN in fresh or dry plant material, and frost
did not cause an increase in dhurrin content.</p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/15075048 |
| Date | 29 July 2021 |
| Creators | Shelby M Gruss (11204016) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/thesis/THE_EFFECTS_OF_INHIBITING_DHURRIN_BIOSYNTHESIS_IN_SORGHUM/15075048 |
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