Rootstock of distant hybrid activates transcription factors of scion cultivar to promote growth and yield in walnut

Walnut (Juglans regia L.) is a significant commercial tree, valued both for its timber and nut production and it is extensively cultivated across the globe. We exploit heterografting by using cultivar-'Shangsong14' (Juglans regia; 'S14') as scion and hybrid elite-'Zhongningqi' (Juglans hindsii × Juglans regia; 'Q') as rootstock and self-grafting each other as both scion and rootstock to investigate the genetic effects of the rootstock. Heterografting induces vigour and yield in walnut scions through 15 years of continuous observation. Grafting experiment indicated that 'Q' as a rootstock caused a reduction of miR396a in scions (S14/Q) compared to self-grafting (S14/S14), while the transcript levels of JrGRF4b and JrGIF1a were upregulated in S14/Q compared with S14/S14. Moreover, we indicated that JrGRF4b might directly target and enhance JrGATA4 expression, further upregulating JrCycD4;1 and JrCycJ18 transcript abundance and affecting the proliferation of cambium cells. To examine the role of miR396a-JrGRF4b/JrGIF1a in growth vigour, we produced white birch transgenic lines that overexpressed miR396a, rJrGRF4b and JrGIF1a. Overexpression of rJrGRF4b and JrGIF1a enhanced plant vigour and cambium cell proliferation, while overexpression of miR396a had the opposite effect in white birch. Our findings suggest that Jr-miR396a was decreased in S14/Q compared to S14/S14, where mRNAs of JrGRF4b were released to enhance the JrGATA4 transcript level, strengthening cell proliferation in scions. The research revealed a novel molecular mechanism about rootstocks enhancing vigour in scions, which have the potential to accelerate breeding rootstocks for perennial woody species.