The role of general control non-derepressible 2 in maize growth, development and stress response.

Maize is among the most important food sources in the world. However, maize protein is not nutritionally balanced, limiting its value. As a result of climate change, frequent heat and drought stresses negatively affect maize yield. Therefore, it is critical to improve maize nutritional value, yield and performance under stress conditions to sustain global food security. General control non-derepressible-2 (GCN2) plays an important role in cellular responses to amino acid starvation. During amino acid starvation, GCN2 phosphorylates the α subunit of eukaryotic translation initiation factor-2 (eIF2), which enhances the translation of the transcription factor GCN4 by overcoming the inhibitory effect of upstream open reading frames (uORFs). This results in increased expression of many amino acid synthesis genes, and is known as general amino acid control. In this study, we first investigated the pleiotropic effects of maize opaque2 (o2) mutation. The o2 mutant was discovered to have enhanced essential amino acid lysine content, demonstrating nutritional superiority. We found elevated protein accumulations that could partially explain the high lysine content and altered gene expression associated with the increased insect and fungal susceptibility and the brittle endosperm texture of o2. Next, we showed that maize GCN2 kinase phosphorylated eIF2α in response to amino acid starvation in maize endosperm. It was associated with an increase of O2 protein accumulation but no alteration of O2 transcript was detected, indicating that the regulation of O2 was translational and that O2 could be a maize GCN4 ortholog. We then tested the role of GCN2 in other stress situations, such as drought. The gcn2 mutant showed increased tolerance to drought compared to wild type and had a higher level of steady state abscisic acid (ABA). RNA-Seq analysis indicated candidate genes responsible for the increased tolerance but further analysis was necessary for candidate genes involved in elevated ABA abundance. Finally, we screened for proteins involved in the localization of the 27 kDa γ- zein, which is important for the protein body formation in maize endosperm, and found that maize eIF5A was associated with γ-zein mRNA and an actin rich cytoskeleton, indicating the possible role of eIF5A in localizing γ-zein.