Abstract
Hybrid chlorosis, a type of hybrid incompatibility, has frequently been reported in inter- and intraspecific crosses of allopolyploid wheat. In a previous study, we reported some types of growth abnormalities such as hybrid necrosis and observed hybrid chlorosis with mild or severe abnormalities in wheat triploids obtained in crosses between tetraploid wheat cultivar Langdon and four Ae. tauschii accessions and in their derived synthetic hexaploids. However, the molecular mechanisms underlying hybrid chlorosis are not well understood. Here, we compared cytology and gene expression in leaves to characterize the abnormal growth in wheat synthetics showing mild and severe chlorosis. In addition, we compared disease resistance to wheat blast fungus. In total 55 and 105 genes related to carbohydrate metabolism and 53 and 89 genes for defense responses were markedly up-regulated in the mild and severe chlorosis lines, respectively. Abnormal chloroplasts formed in the mesophyll cells before the leaves yellowed in the hybrid chlorosis lines. The plants with mild chlorosis showed increased resistance to wheat blast and powdery mildew fungi, although significant differences only in two, third internode length and maturation time, out of the examined agricultural traits were found between the wild type and plants showing mild chlorosis. These observations suggest that senescence might be accelerated in hybrid chlorosis lines of wheat synthetics. Moreover, in wheat synthetics showing mild chlorosis, the negative effects on biomass can be minimized, and they may show substantial fitness under pathogen-polluted conditions.
Highlights
Allopolyploid speciation, which is one of the major evolutionary processes in higher plants [1,2], is achieved through various processes including interspecific hybrid formation and endoreduplication
The severe chlorosis line Ldn/KU-2111 was crossed with the mild chlorosis line Ldn/IG47202 for allelism test, and 156 F2 plants obtained from the cross were grown in the 2011–2012 season
In type III necrosis, necrotic cell death gradually begins from older tissues, whereas the lesion color is clearly different from that in hybrid chlorosis
Summary
Allopolyploid speciation, which is one of the major evolutionary processes in higher plants [1,2], is achieved through various processes including interspecific hybrid formation and endoreduplication. Hexaploid wheat plants with the AABBDD genome can be obtained through artificial hybrids (2n = 3x = 21, ABD genome) between tetraploid wheat and Ae. tauschii, called synthetic hexaploid wheat [4]. The tetraploid wheat cultivar Langdon (Ldn) was found to be an efficient AB genome parent for the production of synthetic hexaploid wheat [8]. Numerous synthetic wheat hexaploids have been produced from ABD hybrids between Ldn and various Ae. tauschii accessions [9], whereas several types of hybrid growth abnormalities were observed in many cross combinations [10,11]. The incompatibilities between the wheat AB and D genomes include hybrid chlorosis, severe growth abortion, and two types of hybrid necrosis (type II and type III necrosis), which function as postzygotic reproductive barriers preventing the production of synthetic hexaploid wheat [11]. Cell death occurs gradually beginning with older tissues in hybrid lines showing type III necrosis, whereas type II necrosis lines show a necrotic phenotype under low temperature conditions [11]
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