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Article|28 Jan 2022|OPEN
Significant improvement of apple (Malus domestica Borkh.) transgenic plant production by pre-transformation with a Baby boom transcription factor. 
Jiajing Chen1,2 , Sumathi Tomes1 , Andrew P. Gleave1 , Wendy Hall1 , Zhiwei Luo1 and Juan Xu2 , Jia-Long Yao,1,3 ,
1The New Zealand Institute for Plant and Food Research Limited, Private Bag 92169, Auckland 1142, New Zealand
2Key Laboratory of Horticultural Plant Biology (Ministry of Education), College of Horticulture and Forestry, Huazhong Agricultural University, 1 Shizishan Street, Wuhan, 430070, China
3Zhengzhou Fruit Research Institute, Chinese Academy of Agricultural Sciences, 32 Gangwan Road Zhengzhou 450009, China
*Corresponding author. E-mail: Jia-Long.Yao@plantandfood.co

Horticulture Research 9,
Article number: uhab014 (2022)
doi: https://doi.org/10.1093/hr/uhab014
Views: 23

Received: 27 Jul 2021
Revised: 18 Jan 2022
Accepted: 16 Oct 2021
Published online: 28 Jan 2022

Abstract

BABY BOOM (BBM) is a member of the APETALA2/ETHYLENE RESPONSE FACTOR (AP2/ERF) family and its expression has been shown to improve herbaceous plant transformation and regeneration. However, this improvement has not been shown clearly for tree species. This study demonstrated that the efficiency of transgenic apple (Malus domestica Borkh.) plant production was dramatically increased by ectopic expression of the MdBBM1 gene. “Royal Gala” apple plants were first transformed with a CaMV35S-MdBBM1 construct (MBM) under kanamycin selection. These MBM transgenic plants exhibited enhanced shoot regeneration from leaf explants on tissue culture media, with most plants displaying a close-to-normal phenotype compared with CaMV35S-GUS transgenic plants when grown under greenhouse conditions, the exception being that some plants had slightly curly leaves. Thin leaf sections revealed the MBM plants produced more cells than the GUS plants, indicating that ectopic-expression of MdBBM1 enhanced cell division. Transcriptome analysis showed that mRNA levels for cell division activators and repressors linked to hormone (auxin, cytokinin and brassinosteroid) signalling pathways were enhanced and reduced, respectively, in the MBM plants compared with the GUS plants. Plants of eight independent MBM lines were compared with the GUS plants by re-transforming them with an herbicide-resistant gene construct. The number of transgenic plants produced per 100 leaf explants was 0–3% for the GUS plants, 3–8% for five MBM lines, and 20–30% for three MBM lines. Our results provided a solution for overcoming the barriers to transgenic plant production in apple, and possibly in other trees.