Compartmentation of photosynthesis gene expression in C4 maize depends on time of day.

Compared with the ancestral C3 state, C4 photosynthesis occurs at higher rates with improved water and nitrogen use efficiencies. In both C3 and C4 plants, rates of photosynthesis increase with light intensity and are maximal around midday. We determined that in the absence of light or temperature fluctuations, photosynthesis in maize (Zea mays) peaks in the middle of the subjective photoperiod. To investigate the molecular processes associated with these temporal changes, we performed RNA sequencing of maize mesophyll and bundle sheath strands over a 24-h time course. Preferential expression of C4 cycle genes in these cell types was strongest between 6 and 10 h after dawn when rates of photosynthesis were highest. For the bundle sheath, DNA motif enrichment and gene coexpression analyses suggested members of the DNA binding with one finger (DOF) and MADS (MINICHROMOSOME MAINTENANCE FACTOR 1/AGAMOUS/DEFICIENS/Serum Response Factor)-domain transcription factor families mediate diurnal fluctuations in C4 gene expression, while trans-activation assays in planta confirmed their ability to activate promoter fragments from bundle sheath expressed genes. The work thus identifies transcriptional regulators and peaks in cell-specific C4 gene expression coincident with maximum rates of photosynthesis in the maize leaf at midday.

Wheat EARLY FLOWERING 3 affects heading date without disrupting circadian oscillations.

Plant breeders have indirectly selected for variation at circadian-associated loci in many of the world's major crops, when breeding to increase yield and improve crop performance. Using an eight-parent Multiparent Advanced Generation Inter-Cross (MAGIC) population, we investigated how variation in circadian clock-associated genes contributes to the regulation of heading date in UK and European winter wheat (Triticum aestivum) varieties. We identified homoeologues of EARLY FLOWERING 3 (ELF3) as candidates for the Earliness per se (Eps) D1 and B1 loci under field conditions. We then confirmed a single-nucleotide polymorphism within the coding region of TaELF3-B1 as a candidate polymorphism underlying the Eps-B1 locus. We found that a reported deletion at the Eps-D1 locus encompassing TaELF3-D1 is, instead, an allele that lies within an introgression region containing an inversion relative to the Chinese Spring D genome. Using Triticum turgidum cv. Kronos carrying loss-of-function alleles of TtELF3, we showed that ELF3 regulates heading, with loss of a single ELF3 homoeologue sufficient to alter heading date. These studies demonstrated that ELF3 forms part of the circadian oscillator; however, the loss of all homoeologues was required to affect circadian rhythms. Similarly, loss of functional LUX ARRHYTHMO (LUX) in T. aestivum, an orthologue of a protein partner of Arabidopsis (Arabidopsis thaliana) ELF3, severely disrupted circadian rhythms. ELF3 and LUX transcripts are not co-expressed at dusk, suggesting that the structure of the wheat circadian oscillator might differ from that of Arabidopsis. Our demonstration that alterations to ELF3 homoeologues can affect heading date separately from effects on the circadian oscillator suggests a role for ELF3 in cereal photoperiodic responses that could be selected for without pleiotropic deleterious alterations to circadian rhythms.

Overexpression of the WAPO-A1 gene increases the number of spikelets per spike in bread wheat.

Two homoeologous QTLs for number of spikelets per spike (SPS) were mapped on chromosomes 7AL and 7BL using two wheat MAGIC populations. Sets of lines contrasting for the QTL on 7AL were developed which allowed for the validation and fine mapping of the 7AL QTL and for the identification of a previously described candidate gene, WHEAT ORTHOLOG OF APO1 (WAPO1). Using transgenic overexpression in both a low and a high SPS line, we provide a functional validation for the role of this gene in determining SPS also in hexaploid wheat. We show that the expression levels of this gene positively correlate with SPS in multiple MAGIC founder lines under field conditions as well as in transgenic lines grown in the greenhouse. This work highlights the potential use of WAPO1 in hexaploid wheat for further yield increases. The impact of WAPO1 and SPS on yield depends on other genetic and environmental factors, hence, will require a finely balanced expression level to avoid the development of detrimental pleiotropic phenotypes.

Circadian entrainment in Arabidopsis.

Circadian clocks coordinate physiology and development as an adaption to the oscillating day/night cycle caused by the rotation of Earth on its axis and the changing length of day and night away from the equator caused by orbiting the sun. Circadian clocks confer advantages by entraining to rhythmic environmental cycles to ensure that internal events within the plant occur at the correct time with respect to the cyclic external environment. Advances in determining the structure of circadian oscillators and the pathways that allow them to respond to light, temperature, and metabolic signals have begun to provide a mechanistic insight to the process of entrainment in Arabidopsis (Arabidopsis thaliana). We describe the concepts of entrainment and how it occurs. It is likely that a thorough mechanistic understanding of the genetic and physiological basis of circadian entrainment will provide opportunities for crop improvement.

Chronoculture, harnessing the circadian clock to improve crop yield and sustainability.

Human health is dependent on a plentiful and nutritious supply of food, primarily derived from crop plants. Rhythmic supply of light as a result of the day and night cycle led to the evolution of circadian clocks that modulate most plant physiology, photosynthesis, metabolism, and development. To regulate crop traits and adaptation, breeders have indirectly selected for variation at circadian genes. The pervasive impact of the circadian system on crops suggests that future food production might be improved by modifying circadian rhythms, engineering the timing of transgene expression, and applying agricultural treatments at the most effective time of day. We describe the applied research required to take advantage of circadian biology in agriculture to increase production and reduce inputs.