Coordination of reproductive shoot architecture in cereals in response to resource availability

The successful development and maturation of seed is essential for determining the
reproductive success of almost all angiosperms and is therefore also an essential
determinant of yield in many of the world’s most consumed crops, including wheat
and barley. The development of structures such as tillers, ears and spikelets that
constrain final seed set is under tight developmental control in both space and time.
Evidence from various species indicates that this developmental control is heavily
influenced by resource availability (including nutrient availability and soil volume
availability), but is also subject to feedback control between different structures on
the shoot. The mechanisms underlying the resource-related coordination of shoot
architecture are generally poorly characterised in cereals but likely utilise long distance hormonal signalling. Here, I investigate how shoot architecture in wheat is
coordinated in response to environmental and endogenous stimuli and try to
understand the hormonal signalling mechanisms by which this occurs. I show that
actively developing spikelets repress tiller outgrowth in wheat and barley, in a
previously unreported mechanism of correlative inhibition. I further show that
cytokinin is likely to coordinate this feedback alongside the development of paired
spikelets. Cytokinin and strigolactone are shown to delay and accelerate SAM
development respectively, in a mechanism which could coordinate tiller and spikelet
number distinct from previously reported genetic networks. Additionally, I report how
wheat responds to environmental resource restriction by prioritizing its
developmental effort in a small number of the earliest emerging tillers and suggest
that strigolactone may be an essential hormone in coordinating this response. This
work develops species-specific understanding of the distribution of perceived
resource availability between different aspects of shoot architecture and supports
the growing academic consensus that manipulation of hormone levels in time and
space could result in improved crop yields without the need for increased resource
inputs.