Arabidopsis Carboxylesterase 20 Binds Strigolactone and Increases Branches and Tillers When Ectopically Expressed in Arabidopsis and Maize.

Keith Roesler,Peter Vance,Cheng Lu,Lu Liu,Zhenglin Hou,Robert W Williams,Jill Thomas,Qingzhang Xu,Michaela A Owens,Jeffrey E Habben

doi:10.3389/fpls.2021.639401

Abstract

Severe drought stress can delay maize silk emergence relative to the pollen shedding period, resulting in poor fertilization and reduced grain yield. Methods to minimize the delay in silking could thus improve yield stability. An Arabidopsis enhancer-tagged carboxylesterase 20 (AtCXE20) line was identified in a drought tolerance screen. Ectopic expression of AtCXE20 in Arabidopsis and maize resulted in phenotypes characteristic of strigolactone (SL)-deficient mutants, including increased branching and tillering, decreased plant height, delayed senescence, hyposensitivity to ethylene, and reduced flavonols. Maize silk growth was increased by AtCXE20 overexpression, and this phenotype was partially complemented by exogenous SL treatments. In drought conditions, the transgenic maize plants silked earlier than controls and had decreased anthesis-silking intervals. The purified recombinant AtCXE20 protein bound SL in vitro, as indicated by SL inhibiting AtCXE20 esterase activity and altering AtCXE20 intrinsic fluorescence. Homology modeling of the AtCXE20 three-dimensional (3D) protein structure revealed a large hydrophobic binding pocket capable of accommodating, but not hydrolyzing SLs. The AtCXE20 protein concentration in transgenic maize tissues was determined by mass spectrometry to be in the micromolar range, well-above known endogenous SL concentrations. These results best support a mechanism where ectopic expression of AtCXE20 with a strong promoter effectively lowers the concentration of free SL by sequestration. This study revealed an agriculturally important role for SL in maize silk growth and provided a new approach for altering SL levels in plants.

Highlights

Maize production worldwide ranks first among the cereal crops, but drought stress can significantly limit maize yield (Boyer, 1982)
No phenotypes were observed with overexpression of the RNA helicase, but greater retention of leaf area during drought stress was evident with Arabidopsis enhancer-tagged carboxylesterase 20 (AtCXE20) overexpression (Figure 1C)
This increased branching from AtCXE20 overexpression was less pronounced than that observed with the Arabidopsis max1 and max2 mutants that are deficient in SL synthesis and signaling, respectively (Figure 1D)

Summary

Introduction

Maize production worldwide ranks first among the cereal crops, but drought stress can significantly limit maize yield (Boyer, 1982). If the anthesis-silking interval (ASI) from first pollen shed to first silk emergence becomes too great, poor fertilization, increased ovary abortion, AtCXE20 Ectopic Expression and decreased grain yield can result, as highlighted by multiple studies showing strong inverse correlations of ASI with kernel number and/or grain yield (Fischer et al, 1989; Bolanos and Edmeades, 1996; Campos et al, 2004). Breeding for decreased ASI has been effective in improving maize drought tolerance, and alternatively, breeding for increased yield in the presence of drought stress or stress associated with higher plant density has resulted in decreased ASI (Fischer et al, 1989; Bolanos and Edmeades, 1996; Campos et al, 2004). Silk emergence, and silking kinetics have all been proposed as important phenotypes to consider for the improvement of maize drought tolerance (Oury et al, 2016b; Habben and Schussler, 2017)

Methods

Results

Conclusion