Drought adaptation and flowering time control in barley

The von Korff group uses natural genetic variation and quantitative genetics to unravel the genetic control of reproductive development in barley.

Our research aims are:

1)      To identify novel flowering time loci and alleles using natural variation in wild and cultivated barley

2)      To unravel the molecular and environmental control of the shoot apical and axillary meristem development in barley

3)      To characterize the pleiotropic effects of flowering time regulators on plant performance and adaptation to abiotic stresses.

Introduction

The control of flowering is central to reproductive success in plants, and has a major impact on grain yield in crop species. Variation in flowering time was crucial for the successful expansion of barley cultivation from the Fertile Crescent to temperate climates.  Wild barley H. vulgare ssp. spontaneum, the progenitor of cultivated barley originated in the Fertile Crescent and is still a widespread species found over the Eastern Mediterranean basin and Western Asiatic countries. Wild barley germinates in autumn and needs a period of cold (vernalisation) to flower in spring and mature in late spring. This vernalisation requirement prevents flowering during winter for the protection of the floral organs from cold. After exposure to cold and completed vernalisation, long days in spring accelerate reproductive development and thus ensure seed production before the onset of summer drought. For successful cultivation of barley in Northern latitudes with cold winters and long summers, barley cultivars with a spring growth habit were selected. These are sown in spring, do not need vernalisation to flower and are characterised by a reduced photoperiod response for late flowering and maturation in summer. Late flowering in temperate environments with a long growing season allows barley to exploit an extended vegetative period for resource storage. A further expansion of barley cultivation to Northern areas with cold winters and short summers required the selection of early flowering in spring grown barley. This led to the selection of early flowering genotypes which do not respond to photoperiod or vernalisation, and are characterised by the presence of the so called “earliness per se” (eps) or "early maturity” (eam) genes.  

Major genes controlling reproductive development in response to vernalisation (Vrn-H1, Vrn-H2) and photoperiod (Ppd-H1) have been described in barley and wheat. However, the flowering time network and the genetic control of the shoot apical meristem (SAM) development is not very well understood in temperate cereals. We use a combination of quantitative genetics, macroscopic and microscopic phenotyping, QTL mapping, and high-throughput sequencing of the barley transcriptome and genome to understand the genetic control of reproductive development and adaptation in barley.

Ongoing projects:

Identification and characterization of flowering time genes in barley

Despite the prominent role of Ppd-H1 in controlling photoperiodic flowering in barley, relatively little is known about the molecular mechanism by which this gene and other components of the barley circadian clock control flowering. We identified and cloned barley orthologs of known Arabidopsis clock genes which showed a high level of structural homology and conservation of diurnal and circadian expression patterns as compared to Arabidopsis (Campoli et al. 2012b). However, independent duplications/deletions of clock genes in barley as compared to Arabidopsis suggested that these evolved in a lineage specific manner.

We identified EAM8 and EAM10 as a barley orthologs of the Arabidopsis thaliana circadian clock regulator EARLY FLOWERING3 (ELF3) and LUX/ARRHYTHMO, respectively (Faure et al. 2012, Campoli et al. 2013). We show that EAM8 and EAM10 act as repressors of Ppd-H1 and thus control the transcription of the downstream floral activator HvFT1. Both eam8 and eam10 mutants show circadian defects. The selection of independent eam8 mutations suggested that this strategy facilitated short growth-season adaptation, despite the pronounced clock defect. We propose that HvELF3 and HvLUX1 together mediate the light input into the circadian clock by controlling expression of Ppd-H1. We are currently characterizing additional natural barley mutants with day-neutral flowering.

We functionally characterised HvCO1 as the closest barley ortholog of the Arabidopsis photoperiod response gene CONSTANS and its interaction with genetic variation at Ppd-H1 and Vrn-H1 (Campoli et al. 2012a). Over-expression of HvCO1 in the spring barley Golden Promise accelerated time to flowering in long and short day conditions and caused up-regulation of HvFT1. However, the transgenic plants retained a response to photoperiod, suggesting the presence of photoperiod response factors acting downstream of HvCO1 transcription. We are currently analyzing the functional role of HvCO2 and variation in HvCO1 protein for flowering time control.

Genetic dissection of meristem development

We are interested in understanding the effects of natural and induced genetic variation on meristem development in barley. Meristem development in grasses can be morphologically distinguished into 1) the vegetative phase, 2) spike initiation, and 3) spike growth during stem elongation. Over-expression of HvCO1 and natural genetic variation at Ppd-H1 and HvELF3 primarily affected spike development and stem elongation, suggesting that high levels of HvFT1 in these lines had a stronger effect on inflorescence development than on the transition to a reproductive meristem. We conduct global RNA expression analyses in leaf and shoot apical meristems (SAM) of barley lines under different environmental conditions to identify molecular changes during development. In addition, we analyse the genetic variation in axillary meristem development between cultivated and wild barley. Cultivated barley is characterized by low, synchronous tillering which is beneficial for harvesting. In contrary, the wild ancestor Hordeum vulgare spp. spontaneum exhibits more extensive and asynchronous tillering which improves adaptation to stress-prone environments.

3. Circadian clock, flowering time and stress adaptation

We use controlled experiments and field trials to study the effects of variation in flowering time and the circadian clock on adaptation to stress. We field trialed a recombinant inbred line population derived from the Syrian barley landrace Arta and the Australian feed cultivar, Keel in 13 environments in Syria (Rollins et al. 2013). The spring growth habit and early flowering determined by Vrn-H1 and Vrn-H2 and inherited from the Australian cultivar Keel increased plant height and biomass and improved yield stability in Syrian environments. We thus show that under changing climate conditions, spring barley might outperform the traditional vernalisation sensitive Syrian landraces. We are currently analyzing barley introgression lines varying at photoperiod and vernalisation response genes under rain-fed and irrigated conditions in the Middle East to further quantify pleiotropic effects of flowering time and circadian clock genes on agronomic performance.

In addition, we tested the effects of natural variation at Ppd-H1 and HvELF3 on gene expression and physiology under osmotic stress and control conditions (Habte et al. 2014). This study demonstrated that osmotic stress at the barley root altered clock gene expression in the shoot and acted as a spatial input signal into the clock. Unlike in Arabidopsis, barley primary assimilation was less controlled by the clock and more responsive to environmental perturbations, such as osmotic stress. We are currently analysing the effects of the clock on stress adaptation, carbon metabolism and growth.