AUTHOR=Kaulfürst-Soboll Heidi , Mertens-Beer Melanie , Brehler Randolf , Albert Markus , von Schaewen Antje 

TITLE=Complex N-Glycans Are Important for Normal Fruit Ripening and Seed Development in Tomato

JOURNAL=Frontiers in Plant Science

VOLUME=Volume 12 - 2021

YEAR=2021

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2021.635962

DOI=10.3389/fpls.2021.635962

ISSN=1664-462X

ABSTRACT=Complex N-glycan modification of secretory glycoproteins in plants is still not well understood. Essential in animals, where a lack of complex N-glycans is embryo-lethal, their presence in plants seemed less relevant for a long time. Mostly, because Arabidopsis thaliana cgl1 mutants lacking N-acetyl-glucosaminyltransferase I (GNTI, the enzyme initiating complex N-glycan maturation in the Golgi apparatus), are viable and showed only minor impairments regarding stress tolerance or development. A different picture emerged when a rice (Oryza sativa) gntI T-DNA mutant was found to be unable to reach the reproductive stage. Here we report on tomato (Solanum lycopersicum) lines that showed severe impairments upon two RNAi approaches. Originally created to shed light on the role of core α1,3-fucose and β1,2-xylose residues in food allergy, plants with strongly reduced GNTI activity developed necrotic fruit-attached stalks and early fruit drop, combined with patchy incomplete ripening. Correspondingly, semi-quantitative RT-PCR of the abscission zone revealed an increase of abscission markers. Also, GNTI-RNAi plants were more susceptible to sporadic infection. To obtain vital tomatoes with comparable low allergenic potential, Golgi α-mannosidase II (MANII) was chosen as second target. The resulting phenotypes were oppositional: MANII-reduced plants carried normal-looking fruits that remained attached for extended time without signs of necrosis. Fruits contained no or only few, but enlarged seeds. Furthermore, leaves developed rolled-up rims simultaneously during the reproductive stage. Trials to cross MANII-reduced plants failed, while GNTI-reduced plants could be (back-)crossed, retaining their characteristic phenotype. This phenotype could not be overcome by Ethephon or IAA application, but the latter was able to mimic patchy fruit ripening in wild-type. Phytohormones measured in leaves and ACC contents in fruits showed no significant differences. Together, the findings hint at altered liberation/perception of protein-bound N-glycans, known to trigger auxin-like effects. Concomitantly, semi-quantitative RT-PCR analysis revealed differences in auxin-responsive genes, indicating importance of complex N-glycan modification for hormone signaling/crosstalk. Another possible role of altered glycoprotein life-span seems subordinate, as concluded from transient expression of KOR1-GFP fusion proteins in RNAi plants of Nicotiana benthamiana. In summary, our analyses stress the importance of complex N-glycan maturation for normal plant responses, especially in fruit-bearing crops like tomato.