Is shattering in Aegilops longissima controlled by overlapping gene expression?

Seed dispersal in the tribe Triticeae, which holds three of the principal cereal crops – wheat, barley, and rye – is accomplished through a brittle rachis in about half of the wild members. This phenomenon, which leads to shattering and is bred out of domesticated crops, has three different presentations: brittleness above each node producing wedge-type dispersal units, brittleness above only the proximal node producing a whole-spike unit, and brittleness below the rachis node resulting in barrel-type units.

The genus Aegilops, which displays all three types of brittle rachis, also contributed two of the three sub-genomes found in hexaploid bread wheat and has many traits potentially of value to wheat improvement. Aegilops longissima is an unusual member of the genus in that although it normally forms whole-spike units, shattering sometimes fails to occur in some rachises, even when fertile spikelets have been formed.

In a new article published in a forthcoming issue of Annals of Botany, lead author Xiaoxue Zeng and colleagues investigated homologs of two genes implicated in brittle rachis in wild barley, Btr1 and Btr2, to understand what role they play in the unusual phenotype of Ae. longissima. The researchers used scanning electron microscopy to study the disarticulation surfaces (breaking points) formed by the grass, as well as characterizing gene expression in the inflorescence.

The authors found that disarticulation is accomplished via thinning of the cell walls in the rachis node, as in barley, rather than via an abscission zone as seen in rice. As for its molecular basis, “[a]t the transcriptional level, Btr1 homologs were found to be active in the proximal portion of the rachis, whereas Btr2 transcript was most abundant in the central portion of the rachis, tailing off towards the apex and effectively absent at the base,” write the authors. “The zone where the transcription of Btr1 and Btr2 overlapped coincided with the occurrence of rachis brittleness.”

This suggests that the two genes in concert produce the brittleness and that their suppression maintains the integrity of the other portions of the rachis.

However, a commentary by Elizabeth A. Kellogg, due to be published in the same issue of the journal, argues that few, if any, genes related to shattering are shared among grass species, which shatter at different points along the inflorescence and in anatomically and histologically different ways. Kellogg points to recent research (of which she is a co-author) that was unable to find conserved shattering genes across three grass genera, and the fact that “there is no evidence that Btr1-like and Btr2-like have anything to do with abscission.”

Still, Kellogg points out, if Btr gene involvement in shattering can be ascertained, the result may be a first step in identifying trends at the tribal level. “By showing that the Btr loci may control shattering in A. longissima, we can infer that the rachis may break up in the same way and under the same genetic control in many of the annual species of Triticeae, most of which have a shattering rachis that breaks just above the nodes,” she writes. “This result may appear simply confirmatory, but in fact it is important in establishing the level of generality.”

The next steps for this line of research, according to Kellogg, are the confirmation of Btr1/2-like functionality, and its testing across some species of the perennial Triticeae, which generally have a tough, non-breaking rachis. She underscores the possibility that there is no generality of gene function in shattering: “Possibly the disparate genes that regulate abscission in different crops are the very genes that make the crops morphologically distinct. For example, the Btr proteins could be involved in controlling rachis architecture and nodal anatomy. That their mutation leads to loss of shattering could be incidental.”