CRISPR/Cas9-mediated mutagenesis of ClBG1 decreased seed size and promoted seed germination in watermelon

Yanping Wang,Jie Zhang,Haiying Zhang,Shouwei Tian,Guoyi Gong,Yong Xu,Shaogui Guo,Yi Ren,Jinfang Wang,Maoying Li

doi:10.1038/s41438-021-00506-1

Yanping Wang, Jie Zhang + Show 8 more

Open Access

https://doi.org/10.1038/s41438-021-00506-1

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Abstract

Abscisic acid (ABA) is a critical regulator of seed development and germination. β-glucosidases (BGs) have been suggested to be contributors to increased ABA content because they catalyze the hydrolysis of ABA-glucose ester to release free ABA. However, whether BGs are involved in seed development is unclear. In this study, a candidate gene, ClBG1, in watermelon was selected for targeted mutagenesis via the CRISPR/Cas9 system. Seed size and weight were significantly reduced in the Clbg1-mutant watermelon lines, which was mainly attributed to decreased cell number resulting from decreased ABA levels. A transcriptome analysis showed that the expression of 1015 and 1429 unique genes was changed 10 and 18 days after pollination (DAP), respectively. Cytoskeleton- and cell cycle-related genes were enriched in the differentially expressed genes of wild type and Clbg1-mutant lines during seed development. Moreover, the expression of genes in the major signaling pathways of seed size control was also changed. In addition, seed germination was promoted in the Clbg1-mutant lines due to decreased ABA content. These results indicate that ClBG1 may be critical for watermelon seed size regulation and germination mainly through the modulation of ABA content and thereby the transcriptional regulation of cytoskeleton-, cell cycle- and signaling-related genes. Our results lay a foundation for dissecting the molecular mechanisms of controlling watermelon seed size, a key agricultural trait of significant economic importance.

Highlights

The seed is a unique organ in a seed plant and is crucial for the plant life cycle
Among the six expressed ClBG genes in watermelon, the expression of ClBG1 (Cla97C08G153160) in cultivated watermelon 97103 increased with fruit development, while it was relatively less expressed and more stable in wild watermelon PI296341-FR (Figs. 1B and S1), which is in accordance with Abscisic acid (ABA) content variation during watermelon fruit development[35], making it the top candidate for involvement in releasing ABA from its conjugate form in the process of watermelon fruit ripening
ABA function depends on the variation of its level, which is mainly controlled by de novo biosynthesis, hydroxylation, and conjugation

Summary

Introduction

The seed is a unique organ in a seed plant and is crucial for the plant life cycle. Seed size and weight are important agronomic traits, and in addition to influencing plant fitness and adaption to environmental stresses, seeds can affect yield and quality, which is especially true for plants for which seeds are the main product organ[1,2,3]. Seed size regulation involves numerous genes that respond to developmental and environmental signals[5,6]. Recent studies have revealed some key genes and regulatory pathways that control seed size in plants[6,7], including G protein signaling, the ubiquitin-proteasome pathway, mitogen-activated protein kinase (MAPK) signaling, some transcriptional regulators, and phytohormone perception and homeostasis.

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