<p>Plant height is an important agronomic trait and a major
target for crop improvement. Owing to the ease of detection and measurement of
plant stature, as well as its high heritability, several height-related mutants
have been reported in maize. The genes underlying a few of those mutants have
also been identified, with a majority of them related to the biosynthesis or
signaling of two key phytohormones - gibberellins (GAs) and brassinosteroids
(BRs). However, most other maize dwarfing mutants, and especially those that
result from gain-of-function mutations, remain uncharacterized. The present
study was undertaken to characterize a novel dominant dwarfing mutant, named <i>D13</i>.
This mutant appeared in the M1 population of the inbred B73 that was generated
by mutagenesis with ethyl methanesulfonate (EMS). Like most other maize
dwarfing mutants, the reduction in <i>D13</i> height was largely due to the
compression of the internodes. However, unlike the GA or BR mutants, <i>D13</i>
had no defects in the female or male inflorescences. Further, in contrast to the
GA and BR mutants, the mesocotyl elongation during etiolation was not impacted
in <i>D13</i>. <i>D13</i> seedlings developed red coloration in two to three
lowermost leaves. In addition, <i>D13</i> also showed enhanced tillering when
the phenotype was very severe. The size of the shoot apical meristem of <i>D13</i>
was reduced slightly, and significant aberrations in the structure of vascular
bundles in the mutant were observed. All anatomical and phenotypic features of <i>D13</i>
were highly exaggerated in homozygous state, indicating the partially dominant
nature of the <i>D13</i> mutation. Interestingly, the heterozygous mutants
showed remarkable variation in their phenotype, which was maintained across
generations. Moreover, the <i>D13</i> phenotype was found to be sensitive to
the genetic background, being completely suppressed in Mo17, Oh7B, enhanced in CML322,
P39 and changed to different degrees in others. To identify the genetic defect
responsible for the <i>D13</i> mutant phenotype, a map-based cloning approach was
used, which identified a single base-pair
change from G to A (G2976A) in the coding region of a glutamate receptor gene (Zm00001d015007). The G2976A missense mutation resulted in the replacement of alanine with
threonine at the location 670. The replaced alanine is highly conserved in
glutamate receptors across all domains of life from cyanobacteria to plants to
mammals, suggesting a causal relationship between the G2976A substitution and the <i>D13</i>
phenotype. To validate this relationship, a targeted EMS-based mutagenesis
approach was used to knock-out (inactivate) the <i>D13</i> mutant allele. A
suppressor mutant was found in which the <i>D13</i> mutant phenotype reverted
to the normal tall phenotype. The sequence of the revertant allele, designated <i>D13</i>*,
revealed that the original <i>D13</i> mutant allele underwent a second G to A
mutation (G1520A) to change glycine into aspartic acid at position 473. This
intragenic second-site mutation in the <i>D13</i> allele suppressed the
function of the <i>D13</i> allele, thereby preventing it from interfering with
the function of the wild type allele. To further unveil the genes and underlying
mechanisms that enable the <i>D13</i> mutant to confer a dwarf phenotype,
transcriptomic and metabolomic analyses of <i>D13</i> mutants were conducted and
compared to the wild type sibs. While the omics analysis confirmed that stress
responses were upregulated and genes related to shoot system development were
downregulated in the mutant, the data did not allow us to pinpoint the
underlying mechanisms that connect the <i>D13</i> mutation with its dwarfing
phenotype. Furthermore, it remains unclear whether these stress and shoot
system-related changes result in the manifestation of <i>D13</i> phenotype, or the
dwarf phenotype due to <i>D13</i> mutation activates the stress-related
mechanisms. This is the first study that signifies the importance of a glutamate
receptor gene in controlling plant height.</p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/12736229 |
| Date | 30 July 2020 |
| Creators | Amanpreet Kaur (9183557) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/thesis/A_Novel_Maize_Dwarf_Resulting_From_a_Gain-of-Function_Mutation_In_a_Glutamate_Receptor_Gene/12736229 |
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