Dissection of transcriptional events in graft incompatible reactions of “Bearss” lemon (Citrus limon) and “Valencia” sweet orange (C. sinensis) on a novel citrandarin (C. reticulata × Poncirus trifoliata) rootstock

Authors: FEBRES, VICENTE - University Of Florida; FADLI, ANAS - University Of Florida; MEYERING, BO - University Of Florida; YU, FAHONG - University Of Florida; Bowman, Kim; CHAPARRO, JOSE - University Of Florida; ALBRECHT, UTE - University Of Florida

Submitted to: Frontiers in Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/3/2024
Publication Date: 6/20/2024
Citation: Febres, V.J., Fadli, A., Meyering, B., Yu, F., Bowman, K.D., Chaparro, J.X., Albrecht, U. Dissection of transcriptional events in graft incompatible reactions of “Bearss” lemon (Citrus limon) and “Valencia” sweet orange (C. sinensis) on a novel citrandarin (C. reticulata × Poncirus trifoliata) rootstock. Frontiers in Plant Science. 15:1421734. 2024. https://doi.org/10.3389/fpls.2024.1421734.
DOI: https://doi.org/10.3389/fpls.2024.1421734

Interpretive Summary: Citrus crops are grown as grafted combinations of a fruiting/scion cultivar growing on a different rootstock cultivar that forms the lower trunk and roots of the tree. Normally, the scion and rootstock cultivars are grafted together in the citrus nursery, and in the field grow to produce a healthy and productive tree that survives and produces fruit for many years. Some graft combinations are known to be incompatible, and result in plants that fail to grow, do not produce healthy trees, and/or die prematurely. In the breeding of new scion and rootstock cultivars, evaluation of graft compatibility with a range of other common scion and rootstock cultivars is important to help define the range of commercial use. Generally, the causes of graft-incompatibility between scion and rootstock in citrus are not well understood. This study provides good new information about the incompatibility reaction that can improve our ability to breed for better graft compatibility in new citrus scion and rootstock cultivars.

Technical Abstract: Fruit crops, including citrus, are commercially propagated via grafting, which ensures scions have consistent traits for production, fruit quality, and disease resistance combined with favorable traits from the rootstock such as soil adaptability, vigor, and resistance to soil pathogens. Breeding new rootstocks requires careful agronomical evaluations, including graft compatibility tests with different scions. Graft incompatibility can occur when the scion and rootstock are not able to form a permanent, healthy union. Understanding and preventing graft incompatibility is therefore of paramount importance for breeders and growers. The rootstock US-1283 is a citrandarin generated from a cross of ‘Ninkat’ mandarin (Citrus reticulata) and ‘Gotha Road’ #6 trifoliate orange (Poncirus trifoliata). It was released in 2014 after years of field evaluation because of its superior productivity and good fruit quality on ‘Hamlin’ sweet orange (C. sinensis) under Florida’s growing conditions. Subsequently, it was discovered that US-1283 was graft-incompatible with several scions, including ‘Bearss’ lemon (C. limon) and ‘Valencia’ sweet orange (C. sinensis). The incompatibility manifested more severely in the rootstock as stem grooving and necrosis underneath the bark. A genetically similar citrandarin rootstock, US-812 (‘Sunki’ C. reticulata × ‘Benecke’ P. trifoliata), is fully graft compatible with the same scions. Transcriptome analysis was performed on the vascular tissues above and below the graft union of compatible US-812 and incompatible US-1283 graft combinations with ‘Bearss’ and ‘Valencia’ to identify expression networks associated with incompatibility and help understand the processes and potential causes of incompatibility in citrus. Transcriptional reprogramming was stronger in the incompatible rootstock than the grafted scions. Differentially expressed genes (DEGs) in US-1283, but not the scions, were associated with oxidative stress and plant defense, among others, similar to a pathogen-induced immune response localized to the rootstock. No pathogen infection was detected. It is hypothesized this response could have been triggered by signaling miscommunications between rootstock and scion either through 1) unknown molecules from the scion that were perceived as danger signals by the rootstock, 2) missing signals from the scion or missing receptors in the rootstock necessary for the formation of a healthy graft union, 3) the overall perception of the scion by the rootstock as non-self, or 4) a combination of the above.