Autoimmune response and repression of mitotic cell division occur in inter‐specific crosses between tetraploid wheat and Aegilops tauschii Coss. that show low temperature‐induced hybrid necrosis

Abstract

SUMMARYCommon wheat is an allohexaploid species originating from a naturally occurring inter-specific cross between tetraploid wheat and the diploid wild wheat Aegilops tauschii Coss. Artificial allopolyploidization can produce synthetic hexaploid wheat. However, synthetic triploid hybrids show four types of hybrid growth abnormalities: type II and III hybrid necrosis, hybrid chlorosis, and severe growth abortion. Of these hybrid abnormalities, type II necrosis is induced by low temperature. Under low temperature, elongation of stems and expansion of new leaves is repressed in type II necrosis lines, which later exhibit necrotic symptoms. Here, we characterize type II necrosis in detail. Comparative transcriptome analysis showed that a number of defense-related genes were highly up-regulated in seedling leaves that showed type II necrosis. Transmission electron microscopy revealed extensive cell death in the leaves under low-temperature conditions, accompanied by abundant generation of reactive oxygen species. In addition, down-regulation of cell cycle-related genes was observed in shoot apices of type II necrosis lines under low-temperature conditions. Quantitative RT-PCR and in situ hybridization showed repression of accumulation of histone H4 transcripts in the shoot apical meristem of type II necrosis lines. These results strongly suggest that an autoimmune response-like reaction and repression of cell division in the shoot apical meristem are associated with the abnormal growth phenotype in type II necrosis lines.

Low temperature-induced necrosis shows phenotypic plasticity in wheat triploid hybrids

“…In addition, type II necrosis lines exhibit distinct phenotypes from those showing type III necrosis, and is a marked growth repression induced by low temperature. 18 This repression was supported by observation …”

“…In addition, type II necrosis lines exhibit distinct phenotypes from those showing type III necrosis, and is a marked growth repression induced by low temperature. 18 This repression was supported by observation of a significant decrease in cell cycle-and division-related gene expression at the crown tissues including shoot apical meristems. Interestingly, tillering number is dramatically increased at normal temperatures in type II necrosis, although plant height is significantly shorter.…”

“…9,18 Epistatic interaction between the AB and D genomes induces upregulation of a set of defense-related genes in the necrosis-exhibiting triploids, which implies that hybrid necrosis in wheat triploid hybrids shares similar responses with those observed in Arabidopsis and lettuce. In addition, type II necrosis lines exhibit distinct phenotypes from those showing type III necrosis, and is a marked growth repression induced by low temperature.…”

“…Interestingly, tillering number is dramatically increased at normal temperatures in type II necrosis, although plant height is significantly shorter. 18 This temperature-dependent developmental plasticity might be due to an abnormality in meristematic activity at the crown tissues. Therefore, epistatic interaction between the AB and D genomes greatly influences plant architecture in type II necrosis plants, probably through the alteration of meristematic activity.…”

Early Disruption of Maternal–Zygotic Interaction and Activation of Defense-Like Responses inArabidopsisInterspecific Crosses

“…The molecular basis of hybrid incompatibility has been variously attributed to cisor trans-regulatory changes, copy number changes, and amino acid changes (Krüger et al, 2002;Bomblies et al, 2007;Dilkes et al, 2008;, and, thus far, there is little overlap between genes detected in one genus to another (reviewed in Rieseberg and Blackman, 2010). These interactions can affect different targets including pathogen responses (Bomblies et al, 2007;Jeuken et al, 2009;Yamamoto et al, 2010;Mizuno et al, 2011), suppression of transposable elements (TEs) (McClintock, 1984;Shaked et al, 2001;Kashkush et al, 2003;Madlung et al, 2005;Josefsson et al, 2006;Ungerer et al, 2006;Martienssen, 2010), small RNA pathways (Ha et al, 2009;Ng et al, 2012;Shivaprasad et al, 2012;Zhang et al, 2012), and developmental regulatory pathways, such as genomic imprinting in the seed (Josefsson et al, 2006). Importantly, although all these molecular mechanisms have been documented as affected by hybridization in at least one system, their relative contribution to hybrid incompatibility is unclear.…”