Genetic Perturbation of the Starch Biosynthesis in Maize Endosperm Reveals Sugar-Responsive Gene Networks.

Christina Finegan,William F Tracy,Susan K Boehlein,Karen E Koch,L Curtis Hannah,Kristen A Leach,Gabriela Madrid,Marcio F R Resende

doi:10.3389/fpls.2021.800326

Christina Finegan, William F Tracy + Show 6 more

Open Access

https://doi.org/10.3389/fpls.2021.800326

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Abstract

In maize, starch mutants have facilitated characterization of key genes involved in endosperm starch biosynthesis such as large subunit of AGPase Shrunken2 (Sh2) and isoamylase type DBE Sugary1 (Su1). While many starch biosynthesis enzymes have been characterized, the mechanisms of certain genes (including Sugary enhancer1) are yet undefined, and very little is understood about the regulation of starch biosynthesis. As a model, we utilize commercially important sweet corn mutations, sh2 and su1, to genetically perturb starch production in the endosperm. To characterize the transcriptomic response to starch mutations and identify potential regulators of this pathway, differential expression and coexpression network analysis was performed on near-isogenic lines (NILs) (wildtype, sh2, and su1) in six genetic backgrounds. Lines were grown in field conditions and kernels were sampled in consecutive developmental stages (blister stage at 14 days after pollination (DAP), milk stage at 21 DAP, and dent stage at 28 DAP). Kernels were dissected to separate embryo and pericarp from the endosperm tissue and 3′ RNA-seq libraries were prepared. Mutation of the Su1 gene led to minimal changes in the endosperm transcriptome. Responses to loss of sh2 function include increased expression of sugar (SWEET) transporters and of genes for ABA signaling. Key regulators of starch biosynthesis and grain filling were identified. Notably, this includes Class II trehalose 6-phosphate synthases, Hexokinase1, and Apetala2 transcription factor-like (AP2/ERF) transcription factors. Additionally, our results provide insight into the mechanism of Sugary enhancer1, suggesting a potential role in regulating GA signaling via GRAS transcription factor Scarecrow-like1.

Highlights

Maize (Zea mays) is one of the most important cereal crops in terms of economic and nutritional value and, together with rice and wheat, provides over 60% of calories consumed by people throughout the world (Fao.org)
We utilized near isogenic lines (NILs) of starch biosynthetic mutants su1 and sh2 to perturb the maize endosperm transcriptome during progressive developmental stages
We identify ZmHXK1 as a key controller of a coexpression network containing transcription factors and genes related to grain filling and C:N balance in the endosperm, such as zeinencoding genes

Summary

Introduction

Maize (Zea mays) is one of the most important cereal crops in terms of economic and nutritional value and, together with rice and wheat, provides over 60% of calories consumed by people throughout the world (Fao.org). The major component of cereal seeds is the endosperm, which comprises approximately 85% of grain volume at maturity (Fao.org, 1998). Starch biosynthesis requires the coordination of multiple classes of enzymes, including adenosine 5 diphosphate-glucose (ADP-Glc) pyrophosphorylase (AGPase), starch synthases (SSs), starch branching enzymes (SBEs), and starch debranching enzymes (DBEs) (Smith et al, 1997; Hannah and Boehlein, 2017). Starch mutants have facilitated characterization of key genes involved in endosperm starch biosynthesis. The AGPases catalyze the first committed step of starch biosynthesis and require both large and small subunits for enzymatic function. The su allele is a mutation of Isoamylase I, which reduces starch content to approximately 50% of wildtype and shifts the ratio of amylose and amylopectin (Creech, 1968)

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