The bacterial pathogen causing cucurbit yellow vine disease associates with the specialized extrafascicular phloem unique to cucurbits
Year: 2026
Type: preprint
Abstract: SUMMARY Cucurbit yellow vine disease (CYVD) is a phloem disease caused by a group of Serratia ureilytica bacteria within the S. marcescens complex. The CYVD pathogen is distinct from other phloem pathogens in two fundamental ways: i) unlike other bacterial phloem pathogens, the CYVD pathogen readily grows on standard laboratory media; and ii) whereas most bacterial phloem diseases are transmitted solely by hemipteran insects, CYVD can be transmitted by both hemipteran (squash bug) and non-hemipteran (cucumber beetle) vectors. The phloem of cucurbits is also distinct, with cucurbits having a unique secondary phloem system, the extrafascicular phloem (EFP), for which a role in CYVD has not been investigated. The distinctive features of this pathosystem prompted a re-evaluation of pathogen localization in planta using a green fluorescent protein-tagged CYVD strain during infection of squash ( Cucurbita pepo ). In epifluorescence and confocal microscopy images of transverse and longitudinal sections of stem and petiole tissues, the fluorescence patterns of the CYVD pathogen were consistent with pathogen colonization of all four known EFP tissues, namely the peripheral, entocyclic, ectocyclic, and commissural sieve tubes. The CYVD pathogen may enter the EFP during herbivory by pathogen-infected vectors that cause mechanical damage, as the EFP exhibits much slower blockage following injury than the conventional fascicular (bundle) phloem. Moreover, although EFP has been proposed to have an antimicrobial function, EFP exudate collected from the stem and petioles of C. pepo supported rapid pathogen growth. These results provide evidence for a unique tissue tropism of the newly emerged plant pathogen S. ureilytica . SIGNIFICANCE STATEMENT Bacterial phloem pathogens generally colonize the conventional fascicular (bundle) phloem of their host plant. We demonstrate that the bacterial pathogen causing cucurbit yellow vine disease is atypical in that it colonizes the extrafascicular phloem, a unique secondary phloem system of cucurbits that is thought to function beyond photosynthate transport. These findings reshape our understanding of phloem pathogenicity and indicate that the extrafascicular phloem is more physiologically accessible and biologically consequential than previously recognized.
Institution: Iowa State University
Language: English
Subfield: Plant Science
Domain: Life Sciences
SDG: Zero hunger
Open Access status: green
OpenAlex ID: W7214657505