Investigating Contradictory Findings Related to Auxin and Thousand-Grain Weight 6 (TGW6) Gene and their Relationship with Grain Weight in Wheat

Abstract

<p>Grain size (GS) and grain weight (GW) are two key components of cereal yield that have been the subject of extensive research. Several candidate<i>GS</i> and <i>GW</i> genes are associated with quantitative trait loci (QTL) for grain weight. However, it is important to validate the precise roles of these genes. <i>THOUSAND-GRAIN WEIGHT 6 (TGW6)</i> is one such gene found in both rice (<i>Oryza sativa</i>) and wheat (<i>Triticum aestivum</i>). Inactive <i>TGW6</i> alleles were reported to result in lower levels of the plant hormone, indole-3-acetic acid (IAA) and higher grain weight. The active allele was proposed to encode an IAA-glucose (IAA-Glc) hydrolase. Conversely, IAA biosynthesis mutants of rice (<i>tsg1</i>) and maize (de18) with reduced IAA levels have small or defective grains. Furthermore, most IAA in cereals is produced from tryptophan via tryptophan aminotransferase (TAR) and indole-3-pyruvate monooxygenase (YUCCA). The <i>TGW6</i> work overlooked this source of IAA although <i>TAR</i> and <i>YUCCA</i> genes are also expressed in wheat grains. My study aimed to investigate whether <i>TGW6</i> is the main source of IAA in developing wheat grains and whether inhibiting IAA production can increase grain size. I examined the expression of all genes contributing to IAA production during grain fill and compared this with grain IAA content. I also investigated the availability of IAA-Glc in grains to act as a substrate for <i>TaTGW6</i>. Lastly, a pilot experiment investigated whether inhibition of IAA biosynthesis has a positive or negative effect on grain yield. Expression of <i>TaTAR2-B3, TaYUC9-1</i> and <i>TaYUC10</i> increased 7–52 fold from 5 to 15 days after anthesis (DAA) correlating with a 30-fold increase in grain IAA content over the same period. On the other hand, <i>TaTGW6</i> expression was not detected in grains. This was confirmed using published RNA-sequencing data, which showed <i>TaTGW6</i> and OsTGW6 are both expressed in the inflorescence. In addition, <i>TGW6</i> in cereals are part of a large protein family; <i>TaTGW6</i> has eight homologues with over 80% amino acid identity. Finally, inhibition of IAA biosynthesis and IAA action had a negative effect on spike yield, but this was primarily due to increased grain abortion rather than an effect on grain size. My results show that <i>TaTGW6</i> is unlikely to have any effect on the IAA content of grains or on grain size. On the other hand, I demonstrate that IAA is likely to have a positive effect on grain yield and primarily is produced from TAR/YUCCA pathway in developing wheat grains.</p>