The stage-specific regulation and role of root-knot nematode SWEET genes.

The root-knot nematode Meloidogyne incognita is a globally significant plant parasite that causes substantial crop losses. While pre-parasitic juveniles rely on innate energy reserves, later life stages acquire nutrients from host plants through specialized feeding structures. SWEET (Sugars Will Eventually be Exported Transporter) genes exhibit a conserved sugar transporting ability across all kingdoms of life, yet their function in plant-parasitic nematodes remains underexplored. Here, we functionally characterise the SWEET gene family in M. incognita, revealing their critical and stage-specific roles in nematode development and parasitism. We demonstrate that Mi-SWEETs segregate into two functional groups: those that facilitate mobility and invasion in motile juveniles (Mi-SWEET2, 4) and those support nutrient uptake during feeding (Mi-SWEET3, 5, 7). Although temporally distinct, all SWEET genes localise to the intestine, suggesting a conserved role in mediating sugar flux. Knockdown of Mi-SWEET2 and Mi-SWEET4 reduced root invasion, while silencing Mi-SWEET3, 5, and 7 impaired post-invasion growth, highlighting the varied roles of this large gene family across different life stages. Yeast complementation assays revealed distinct substrate preferences among Mi-SWEETs, aligning with the metabolic needs of different life stages. The transcription factor HBL1, a key regulator of nematode dietary responses, was found to control the expression of Mi-SWEET3 and is itself regulated through interaction with the post-transcriptional regulatory microRNA let-7. Our findings provide new insights into the metabolic adaptations and energy utilisation of plant-parasitic nematodes and outline a microRNA - transcription factor - target gene regulatory network. These findings have broader relevance given the fundamental importance of the regulation of resource transportation in plant-pathogen interactions.