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A frame-shift mutation in MATRILINEAL, a pollen-specific phospholipase, triggers haploid induction in maize, which may be useful in developing improved haploid induction systems for crop breeding. Haploid induction in maize Haploid inbred lines are a valuable tool for genetic research and the hybrid breeding of crop plants. Timothy Kelliher et al . now examine the genetic basis of haploid induction in maize, and find that a frame-shift mutation in MATRILINEAL , a pollen-specific phospholipase, triggers haploid induction in these lines. This finding may be useful for the development of improved haploid induction systems for crop breeding. Sexual reproduction in flowering plants involves double fertilization, the union of two sperm from pollen with two sex cells in the female embryo sac. Modern plant breeders increasingly seek to circumvent this process to produce doubled haploid individuals, which derive from the chromosome-doubled cells of the haploid gametophyte. Doubled haploid production fixes recombinant haploid genomes in inbred lines, shaving years off the breeding process 1 . Costly, genotype-dependent tissue culture methods are used in many crops 2 , while seed-based in vivo doubled haploid systems are rare in nature 3 and difficult to manage in breeding programmes 4 . The multi-billion-dollar maize hybrid seed business, however, is supported by industrial doubled haploid pipelines using intraspecific crosses to in vivo haploid inducer males derived from Stock 6, first reported in 1959 (ref. 5 ), followed by colchicine treatment. Despite decades of use, the mode of action remains controversial 6 , 7 , 8 , 9 , 10 . Here we establish, through fine mapping, genome sequencing, genetic complementation, and gene editing, that haploid induction in maize ( Zea mays ) is triggered by a frame-shift mutation in MATRILINEAL ( MTL ), a pollen-specific phospholipase, and that novel edits in MTL lead to a 6.7% haploid induction rate (the percentage of haploid progeny versus total progeny). Wild-type MTL protein localizes exclusively to sperm cytoplasm, and pollen RNA-sequence profiling identifies a suite of pollen-specific genes overexpressed during haploid induction, some of which may mediate the formation of haploid seed 11 , 12 , 13 , 14 , 15 . These findings highlight the importance of male gamete cytoplasmic components to reproductive success and male genome transmittance. Given the conservation of MTL in the cereals, this discovery may enable development of in vivo haploid induction systems to accelerate breeding in crop plants.

The introduction of alleles into commercial crop breeding pipelines is both time consuming and costly. Two technologies that are disrupting traditional breeding processes are doubled haploid (DH) breeding and genome editing (GE). Recently, these techniques were combined into a GE trait delivery system called HI-Edit (Haploid Inducer-Edit) [1]. In HI-Edit, the pollen of a haploid inducer line is reprogrammed to deliver GE traits to any variety, obviating recurrent selection. For HI-Edit to operate at scale, an efficient transformable HI line is needed, but most maize varieties are recalcitrant to transformation, and haploid inducers are especially difficult to transform given their aberrant reproductive behaviors. Leveraging marker assisted selection and a three-tiered testing scheme, we report the development of new Iodent and Stiff Stalk maize germplasm that are transformable, have high haploid induction rates, and exhibit a robust, genetically-dominant anthocyanin native trait that may be used for rapid haploid identification. We show that transformation of these elite “HI-Edit” lines is enhanced using the BABYBOOM and WUSCHEL morphogenetic factors. Finally, we evaluate the HI-Edit performance of one of the lines against both Stiff Stalk and non-Stiff Stalk testers. The strategy and results of this study should facilitate the development of commercially scalable HI-Edit systems in diverse crops.

Increasing the grain protein concentration (GPC) of bread wheat is a concern of wheat breeders because this trait partially determines the bread-making properties of wheat flour and its nutritional value. Four cycles of recurrent selection for GPC were evaluated. Direct response to selection for GPC and correlated (indirect) responses of agronomic, bread-making quality, and nitrogen-use traits were determined. Ten hard red spring wheats selected for GPC and other traits were intermated to form the initial population. About 25 F$\\sb3$ lines were selected and recombined to initiate each successive cycle. Selection intensity of the parents ranged from 3.5 to 8%. In the first experiment, random F$\\sb3$-derived lines from cycles 0, 2, and 4 were evaluated at two locations for two years to provide an estimate of gain from selection. The average realized gain of GPC was 2.7% per cycle of selection. An indirect result of selection for GPC was a linear decrease in grain yield of 3.2% per cycle at one location, but no linear trend at another location. Flour protein concentration, mixogram water absorption, and kernel hardness linearly increased with cycles of selection for GPC. In a second experiment, five high and 5 low GPC lines from each of cycles 0, 2, and 4 were evaluated in three environments varying in soil nitrogen fertility to determine whether changes in nitrogen-use traits occurred in response to selection. Cycles of selection did not differ for nitrogen uptake or nitrogen remobilization. Recurrent selection effectively increased GPC linearly over cycles. However, gains in GPC were associated with loss in grain yield. Lines with the highest GPC and grain yield may be useful as germplasm sources for crossing to elite lines.