Co-Orthologs of KATANIN1 Impact Plant Morphology and Show Differential Evolution in Maize
Understanding how the size and shape of crop plants and their specific organs are genetically controlled may allow for the development of cultivars with improved plant architecture. A microtubule-severing enzyme called katanin p60 is encoded by KATANIN1 (KTN1) in Arabidopsis or by an ortholog, dwarf and gladius leaf1 (dgl1), in rice. Katanin p60 has been implicated in the control of anisotropic cell growth, which is cell growth directed in a specific direction instead of equally in all directions. Anisotropic cell growth is crucial for proper plant shape and its disruption in ktn1/dgl1 mutants leads to morphological changes such as stunted plant height, shorter leaves and reduced inflorescence size. In this work, characterization of the maize mutant Clumped tassel1 (Clt1) led to the discovery of a putative dominant-negative allele of a KTN1 ortholog. Results indicate that the causative lesion is a missense mutation in the adenosine triphosphatase (ATPase) domain that, we hypothesize, disrupts the ability of the protein to hydrolyze ATP. In general, the phenotype of Clt1-1 was analogous to the phenotypes reported for ktn1/dgl1 mutants in Arabidopsis and rice, with reduced plant stature and more compact organs. Clt1-1 represents the first report of a dominant-negative allele of KTN1 or its orthologs among plant species, though similar mutants have been described in animal systems. By expressing it in specific tissues, the discovery of Clt1-1 can potentially be used to decouple the pleiotropic effects of KTN1 so that each effect can be studied without being confounded by other aspects of the phenotype, or to reduce the size of particular organs for agronomic purposes. In order to learn more about the mechanism of clt1, an enhancer/suppressor screen was conducted, using natural variation. Through this approach, multiple deleterious alleles of ktn1b, the other maize ortholog of KTN1, were identified. These alleles, whose effects are masked in the inbred parents presumably by redundant function from clt1, helped uncover functional differences between clt1 and ktn1b. First, clt1 appears to provide less function than ktn1b for the elongation of upper internodes. Conversely, clt1 seems to be more important than ktn1b for overall plant development because, in Clt1-1 heterozygotes, losing functionality in the other clt1 allele has a much more severe developmental impact than in one of the ktn1b alleles. Consistent with a more important biological role for clt1, sequencing data suggests that purifying selection is relaxed for ktn1b. Introns in ktn1b appear to have expanded dramatically compared to sorghum homologs, whereas clt1 retains homology with the sorghum homologs along most of the gene. Furthermore, examination of publicly available whole-genome sequencing data for approximately one thousand maize lines indicates a substantially greater ratio of nonsynonymous to synonymous nucleotide diversity (πa/πs) and more frequent occurrence of deleterious mutations in the coding sequence of ktn1b than in clt1. In summary, the results from this work indicate overlapping but not fully redundant functions between clt1 and ktn1b in maize, and suggest an evolutionary bias for the retention of clt1 over ktn1b.
Weil, Purdue University.
Agronomy|Genetics|Plant sciences|Developmental biology
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