Unique histochemical gradients in a photosynthesis-deficient plant gall

• Gall metabolism varies both due to the insect taxa and gall structural features.
• Extralaminar galls of the sucking insect N. myrtoidis are photosynthesis-deficient.
• Carbohydrate metabolism generates a nutritional gradient in galls.
• Phenolics and proanthocyanidins secondarily modulate gall growth and physiology.
• Oxidative stress regulates tissue alterations during gall morphogenesis.

Galls usually present low chlorophyll content, and their metabolism may vary depending on the taxa of the inducer and on the complexity of the gall structure. Primary and secondary plant metabolites allocated in gall tissues are evidenced with histochemical tests and may indicate the physiological status of such tissues. The histochemical and biochemical profiles of the galls induced by Nothotrioza myrtoidis Burck. (Hemiptera: Psylloidea) on Psidium myrtoides (Myrtaceae) were compared to those reported for some other Neotropical galls. The extralaminar galls of N. myrtoidis have low chlorophyll and nitrogen contents, but they accumulate more polysaccharides than the non-galled leaves. The histochemical gradient of reducing sugars is guaranteed by the activity of acid phosphatase, and ensures the nourishment of the gall inducer, while the gradients of phenolics and proanthocyanidins are both related to protection of the gall inducer and modulation of plant cell growth. The accumulation of reactive oxygen species seems to play a major role on the determination of the extent of tissue alterations during gall morphogenesis. The lack of morphological continuum and physiological continuum between the extralaminar galls of N. myrtoidis and the leaves of P. myrtoides, together with the low impacting feeding activity of the sucking insect, determine the establishment of a photosynthesis-deficient structure with unique features among Neotropical galls.