The barley mutant multiflorus2.b reveals quantitative genetic variation for new spikelet architecture

Ravi Koppolu,Martin Mascher,Quddoos H Muqaddasi,Twan Rutten,Thorsten Schnurbusch,Axel Himmelbach,Sara G Milner,Yu Guo,Nils Stein,Guojing Jiang

doi:10.1007/s00122-021-03986-w

Abstract

Key messageSpikelet indeterminacy and supernumerary spikelet phenotypes in barley multiflorus2.b mutant show polygenic inheritance. Genetic analysis of multiflorus2.b revealed major QTLs for spikelet determinacy and supernumerary spikelet phenotypes on 2H and 6H chromosomes.Understanding the genetic basis of yield forming factors in small grain cereals is of extreme importance, especially in the wake of stagnation of further yield gains in these crops. One such yield forming factor in these cereals is the number of grain-bearing florets produced per spikelet. Wild-type barley (Hordeum vulgare L.) spikelets are determinate structures, and the spikelet axis (rachilla) degenerates after producing single floret. In contrast, the rachilla of wheat (Triticum ssp.) spikelets, which are indeterminate, elongates to produce up to 12 florets. In our study, we characterized the barley spikelet determinacy mutant multiflorus2.b (mul2.b) that produced up to three fertile florets on elongated rachillae of lateral spikelets. Apart from the lateral spikelet indeterminacy (LS-IN), we also characterized the supernumerary spikelet phenotype in the central spikelets (CS-SS) of mul2.b. Through our phenotypic and genetic analyses, we identified two major QTLs on chromosomes 2H and 6H, and two minor QTLs on 3H for the LS-IN phenotype. For, the CS-SS phenotype, we identified one major QTL on 6H, and a minor QTL on 5H chromosomes. Notably, the 6H QTLs for CS-SS and LS-IN phenotypes co-located with each other, potentially indicating that a single genetic factor might regulate both phenotypes. Thus, our in-depth phenotyping combined with genetic analyses revealed the quantitative nature of the LS-IN and CS-SS phenotypes in mul2.b, paving the way for cloning the genes underlying these QTLs in the future.

Highlights

Grass inflorescences are constituted by the ordered arrangement of spikelets either directly on the inflorescence axis or on the branches differentiated from the inflorescence axis
The rachilla elongation suppression in LSs appears to be lost in mul2.b rendering the lateral spikelets indeterminate, a feature reminiscent of indeterminate wheat spikelets (Fig. 1e)
Similar to the QTLs identified from POP 2015, we identified two major QTLs for lateral spikelet indeterminacy (LS-IN) on 2H

Summary

Introduction

Grass inflorescences are constituted by the ordered arrangement of spikelets (specialized flower-bearing structures) either directly on the inflorescence axis (rachis) or on the branches differentiated from the inflorescence axis. Based on the architectural differences at the spikelet and spike levels, wheat and barley inflorescences can be classified into (i) determinate or (ii) indeterminate spikes (Bonnett 1966). In the determinate spikes of wheat, the inflorescence apex culminates with a terminal spikelet, producing a fixed number of spikelets per inflorescence. The spikelets in the wheat spike are indeterminate with multiple flowers (florets; up to 12), each produced at the articulation junction of the spikelet axis called rachilla (Fig. 1e). The inflorescence apex of barley remains indeterminate (no terminal spikelet), and the rachilla ceases to elongate upon initiation of the first floret and remains as a suppressed structure rendering determinacy to the spikelet (Fig. 1b). The spikelet determinacy or indeterminacy is dependent upon the extent of rachilla suppression or elongation

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