Abstract
MADS-box genes have a wide range of functions in plant reproductive development and grain production. The ABCDE model of floral organ development shows that MADS-box genes are central players in these events in dicotyledonous plants but the applicability of this model remains largely unknown in many grass crops. Here, we show that transcript analysis of all MIKCc MADS-box genes through barley (Hordeum vulgare L.) inflorescence development reveals co-expression groups that can be linked to developmental events. Thirty-four MIKCc MADS-box genes were identified in the barley genome and single-nucleotide polymorphism (SNP) scanning of 22,626 barley varieties revealed that the natural variation in the coding regions of these genes is low and the sequences have been extremely conserved during barley domestication. More detailed transcript analysis showed that MADS-box genes are generally expressed at key inflorescence developmental phases and across various floral organs in barley, as predicted by the ABCDE model. However, expression patterns of some MADS genes, for example HvMADS58 (AGAMOUS subfamily) and HvMADS34 (SEPALLATA subfamily), clearly deviate from predicted patterns. This places them outside the scope of the classical ABCDE model of floral development and demonstrates that the central tenet of antagonism between A- and C-class gene expression in the ABC model of other plants does not occur in barley. Co-expression across three correlation sets showed that specifically grouped members of the barley MIKCc MADS-box genes are likely to be involved in developmental events driving inflorescence meristem initiation, floral meristem identity and floral organ determination. Based on these observations, we propose a potential floral ABCDE working model in barley, where the classic model is generally upheld, but that also provides new insights into the role of MIKCc MADS-box genes in the developing barley inflorescence.
Highlights
Flowers are often composed of four different floral organs organised in concentric whorls numbered from peripheral to central position
Our findings reveal that the ABCDE model can be mostly applied to barley, while deviations point to interesting adaptations that can reveal more about inflorescence development in grasses
The recent barley genome assembly contains 32 MIKCc MADS-box genes annotated as expressed sequences and a pseudogene strongly resembling HvMADS14 with 97% identity, but only covering the latter 67% (Mascher et al, 2017; Monat et al, 2019)
Summary
Flowers are often composed of four different floral organs organised in concentric whorls numbered from peripheral to central position. The ABCDE model, originally proposed for Arabidopsis and Antirrhinum majus, associates the developmental determination of specific flower organs with the combinatorial activity of several classes of homeotic selector genes, most of which encode MIKCc MADS domain developmental transcription factors. The putative 11 MIKC-type MADS-box genes from the last common ancestor of monocots and eudicots increased to at least 24 genes in the last common ancestor of rice, wheat and maize (Bremer, 2002; Ciaffi et al, 2011) During this time of duplication and diversification in the MIKC family, the complex grass inflorescence and floral structures of the Poaceae family evolved. Some MIKCc genes associated with the ABCDE model have adopted additional roles in grasses, like the AP1 clade gene HvMADS14 which is a vernalisation integrator in barley (Trevaskis et al, 2007a), expression of which is important for floral transition onset. Our findings reveal that the ABCDE model can be mostly applied to barley, while deviations point to interesting adaptations that can reveal more about inflorescence development in grasses
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