Spikelet abortion in six-rowed barley is mainly influenced by final spikelet number, with potential spikelet number acting as a suppressor trait

The Birth and Death of Floral Organs in Cereal Crops

Florets of cereal crops are the basic reproductive organs that produce grains for food or feed. The birth of a floret progresses through meristem initiation and floral organ identity specification and maintenance. During these processes, both endogenous and external cues can trigger a premature floral organ death, leading to reproductive failure. Recent advances in different cereal crops have identified both conserved and distinct regulators governing the birth of a floret. However, the molecular underpinnings of floral death are just beginning to be understood. In this review, we first provide a general overview of the current findings in the field of floral development in major cereals and outline different forms of floral deaths, particularly in the Triticeae crops. We then highlight the importance of vascular patterning and photosynthesis in floral development and reproductive success and argue for an expanded knowledge of floral birth-death balance in the context of agroecology.

Dynamic Phytomeric Growth Contributes to Local Adaptation in Barley

Vascular plants have segmented body axes with iterative nodes and internodes. Appropriate node initiation and internode elongation are fundamental to plant fitness and crop yield; however, how these events are spatiotemporally coordinated remains elusive. We show that in barley (Hordeum vulgare L.), selections during domestication have extended the apical meristematic phase to promote node initiation, but constrained subsequent internode elongation. In both vegetative and reproductive phases, internode elongation displays a dynamic proximal-distal gradient, and among subpopulations of domesticated barleys worldwide, node initiation and proximal internode elongation are associated with latitudinal and longitudinal gradients, respectively. Genetic and functional analyses suggest that, in addition to their converging roles in node initiation, flowering-time genes have been repurposed to specify the timing and duration of internode elongation. Our study provides an integrated view of barley node initiation and internode elongation and suggests that plant architecture should be recognized as a collection of dynamic phytomeric units in the context of crop adaptive evolution.

Multilayered regulation of developmentally programmed pre-anthesis tip degeneration of the barley inflorescence

Leaf and floral tissue degeneration is a common feature in plants. In cereal crops such as barley (Hordeum vulgare L.), pre-anthesis tip degeneration (PTD) starts with growth arrest of the inflorescence meristem dome, which is followed basipetally by the degeneration of floral primordia and the central axis. Due to its quantitative nature and environmental sensitivity, inflorescence PTD constitutes a complex, multilayered trait affecting final grain number. This trait appears to be highly predictable and heritable under standardized growth conditions, consistent with a developmentally programmed mechanism. To elucidate the molecular underpinnings of inflorescence PTD, we combined metabolomic, transcriptomic, and genetic approaches to show that barley inflorescence PTD is accompanied by sugar depletion, amino acid degradation, and abscisic acid responses involving transcriptional regulators of senescence, defense, and light signaling. Based on transcriptome analyses, we identified GRASSY TILLERS1 (HvGT1), encoding an HD-ZIP transcription factor, as an important modulator of inflorescence PTD. A gene-edited knockout mutant of HvGT1 delayed PTD and increased differentiated apical spikelets and final spikelet number, suggesting a possible strategy to increase grain number in cereals. We propose a molecular framework that leads to barley PTD, the manipulation of which may increase yield potential in barley and other related cereals.

A molecular framework for grain number determination in barley

Flowering plants with indeterminate inflorescences often produce more floral structures than they require. We found that floral primordia initiations in barley (Hordeum vulgare L.) are molecularly decoupled from their maturation into grains. While initiation is dominated by flowering-time genes, floral growth is specified by light signaling, chloroplast, and vascular developmental programs orchestrated by barley CCT MOTIF FAMILY 4 (HvCMF4), which is expressed in the inflorescence vasculature. Consequently, mutations in HvCMF4 increase primordia death and pollination failure, mainly through reducing rachis greening and limiting plastidial energy supply to developing heterotrophic floral tissues. We propose that HvCMF4 is a sensory factor for light that acts in connection with the vascular-localized circadian clock to coordinate floral initiation and survival. Notably, stacking beneficial alleles for both primordia number and survival provides positive implications on grain production. Our findings provide insights into the molecular underpinnings of grain number determination in cereal crops.

Genetic association of spikelet abortion with spike, grain, and shoot traits in highly-diverse six-rowed barley

Spikelet abortion is a phenomenon where apical spikelet primordia on an immature spike abort. Regardless of the row-type, both apical and basal spikelet abortion occurs, and their extent decides the number of grain-bearing spikelets retained on the spike-thus, affecting the yield potential of barley. Reducing spikelet abortion, therefore, represents an opportunity to increase barley yields. Here, we investigated the variation for apical spikelet abortion along with 16 major spike, shoot, and grain traits in a panel of 417 six-rowed spring barleys. Our analyses showed a significantly large genotypic variation resulting in high heritability estimates for all the traits. Spikelet abortion (SA) varies from 13 to 51% depending on the genotype and its geographical origin. Among the seven spike traits, SA was negatively correlated with final spikelet number, spike length and density, while positively with awn length. This positive correlation suggests a plausible role of the rapidly growing awns during the spikelet abortion process, especially after Waddington stage 5. In addition, SA also showed a moderate positive correlation with grain length, grain area and thousand-grain weight. Our hierarchical clustering revealed distinct genetic underpinning of grain traits from the spike and shoot traits. Trait associations showed a geographical bias whereby European accessions displayed higher SA and grain and shoot trait values, whereas the trend was opposite for the Asian accessions. To study the observed phenotypic variation of SA explained by 16 other individual traits, we applied linear, quadratic, and generalized additive regression models (GAM). Our analyses of SA revealed that the GAM generally performed superior in comparison to the other models. The genetic interactions among traits suggest novel breeding targets and easy-to-phenotype "proxy-traits" for high throughput on-field selection for grain yield, especially in early generations of barley breeding programs.