Low-Temperature X-ray Microanalysis Sheds New Light on Mineral Nutrition Aspects of Insect Leaf Galling.

Manipulation of host plant physiologies by leaf-galling insects is a complex, multifaceted process. Among fundamental knowledge gaps surrounding this scientifically intriguing phenomenon is the appropriation of plant mineral nutrients and moisture for galling advantage. Small, soluble mineral ions and watery cell contents in dense gall tissues are easily dislocated or lost to routine sample preparation. In this study, an X-ray microanalysis was applied to investigate gall mineral nutrition. Morphologically diverse leaf galls were sampled from three Australian rainforest tree species. Using cryo-analytical scanning electron microscopy, real-time X-ray analytical maps of localized cellular mineral nutrients and water were integrated with anatomical images of gall and leaf cross-sectional surfaces to capture mineral-nutrient distribution patterns in situ. A comparison of host-leaf and gall anatomies bore direct evidence of drastic changes to leaf cells through the galling process. Distinct "wet" and "dry" regions within galls were anatomically and/or chemically differentiated, suggesting specific functionality. Wet regions comprising hydrated cells including soft gall-cavity linings where larvae are known to feed contained soluble mineral nutrients, while C-rich dry tissues largely devoid of mineral nutrients likely contribute structural support. The findings here provided otherwise inaccessible insights into leaf-gall mineral nutrition.