Heat Stress After Pollination Reduces Kernel Number in Maize by Insufficient Assimilates.

Shiduo Niu,Dahong Bian,Xiong Du,Zhen Gao,Yarong Zhang,Dejie Wei,Qian Tang,Yanhong Cui,Shanshan Liu

doi:10.3389/fgene.2021.728166

Abstract

Global warming has increased the occurrence of high temperature stress in plants, including maize, resulting in decreased the grain number and yield. Previous studies indicate that heat stress mainly damages the pollen grains and thus lowered maize grain number. Other field studies have shown that heat stress after pollination results in kernel abortion. However, the mechanism by which high temperature affect grain abortion following pollination remains unclear. Hence, this study investigated the field grown heat-resistant maize variety “Zhengdan 958” (ZD958) and heat-sensitive variety “Xianyu 335” (XY335) under a seven-day heat stress treatment (HT) after pollination. Under HT, the grain numbers of XY335 and ZD958 were reduced by 10.9% (p = 0.006) and 5.3% (p = 0.129), respectively. The RNA sequencing analysis showed a higher number of differentially expressed genes (DEGs) between HT and the control in XY335 compared to ZD958. Ribulose diphosphate carboxylase (RuBPCase) genes were downregulated by heat stress, and RuBPCase activity was significantly lowered by 14.1% (p = 0.020) in XY335 and 5.3% (p = 0.436) in ZD958 in comparison to CK. The soluble sugar and starch contents in the grains of XY335 were obviously reduced by 26.1 and 58.5%, respectively, with no distinct change observed in ZD958. Heat stress also inhibited the synthesis of grain starch, as shown by the low activities of metabolism-related enzymes. Under HT, the expression of trehalose metabolism genes in XY335 were upregulated, and these genes may be involved in kernel abortion at high temperature. In conclusion, this study revealed that post-pollination heat stress in maize mainly resulted in reduced carbohydrate availability for grain development, though the heat-resistant ZD958 was nevertheless able to maintain growth.

Highlights

The rising levels of carbon dioxide in the atmosphere causes a greenhouse effect that results in increased temperatures and climatic changes (Xuan et al, 2020)
The ear growth rate of XY335 and ZD958 under heat treatment (HT) was significantly reduced by 50.6 and 18.4% compared to CK, respectively (Figure 2A)
Under the same heat stress, the number of differentially expressed genes (DEGs) detected in ZD958 was considerably lower than in XY335, with 392 and 511 DGEs overlapping between the leaf and kernel, respectively (Figure 3B)

Summary

Introduction

The rising levels of carbon dioxide in the atmosphere causes a greenhouse effect that results in increased temperatures and climatic changes (Xuan et al, 2020). Simulation result indicated that a 10% reduction in maize yield was shown for each 1°C increase in global temperature (Zhang and Zhao, 2017; Dong et al, 2021). Maize tassels (male flowers) growing at the top of the plant were found to be vulnerable to low-level heat stress and this affected pollen viability (Dong et al, 2021; Wang et al, 2021). High temperatures inhibited anther dehiscence, pollen viability, and pollen germination, which caused kernel abortion and maize yield reduction (Carberry et al, 1989; Bakhtavar et al, 2015; Hatfield and Prueger, 2015; Li and Howell, 2021; Wang et al, 2021). Pollen sterility under heat stress has been intensely studied recently, but the manner in which high temperatures after pollination causes grain abortion remains unclear

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