Grasshopper crop and midgut extract effects on plants: an example of reward feedback.

Abstract

Acid extracts and a resultant fraction from solid-phase extraction (SPE) of Romalea guttata crop and midgut tissues induce sorghum (Sorghum bicolor var. Rio) coleoptile growth in 24-h incubations an average of 49% above untreated controls. When combined with plant auxin, indole-3-acetic acid (IAA), the SPE fraction shows a synergistic reaction, yielding increases in coleoptile growth that average 295% above untreated controls and 8% above IAA standards.The interaction lowered the point of maximum sensitivity of IAA 3 orders of magnitude, resulting in a new IAA physiological set point at 10-7 g/ml. This synergism suggests that contents in animal regurgitants making their way into plant tissue during feeding may produce a positive feedback in plant growth and development following herbivory. Such a process, also known as reward feedback, may exert major controls on ecosystem-level relationships in nature.As herbivores feed on their host plants they induce a variety of changes in plant responses. These include production of defense mechanisms (1, 2) and changes in internal physiology (3, 4) and productivity (5, 6), providing overall a series of positive and negative feedback signals to the plant that can reorient its C and N distribution as well as its growth and development potential (7,8). The induction signal itself may emanate from endogenous biochemical pathways set in motion by physical damage to plant leaves (9-11) or, perhaps as likely, exogenously from biochemical messengers found in salivary and digestive systems of herbivores (12)(13)(14)(15)(16). Epidermal growth factor (EGF), a highly conserved mitogenic peptide found in salivary glands of many animal species that acts as an intracellular messenger in animal tissues (17), also affects plant cell growth, seedling development, and plant production processes (12)(13)(14)(15) peptides that induce plant responses, which in turn control production processes similar to those actions found in applications of vertebrate EGF. MATERIALS AND METHODSDuring the summers of 1992 and 1993 we collected >1000 late-instar and adult male and female Romalea grasshoppers from a park in Athens, Georgia. We kept the grasshoppers in 0.23-m3 laboratory cages in a greenhouse, fed them washed lettuce and high-protein chick starter meal for several days, and then starved them for at least 2 days to clear out their digestive tracts. We dissected the crop and midgut and froze the tissues immediately on dry ice for storage at -80°C.We extracted the tissues by a method developed for purification of bioactive polypeptides with known molecular masses of <13 kDa (20). We homogenized the frozen crops and midguts (5-25 g) in 100 ml of ice-cold 0.2 M acetic acid solution containing protease inhibitors. After centrifugation (20 min; 17,200 x g) and reextraction of the pellet twice more, we pooled the supernatant solutions ('300 ml) and lyophilized them. We rehydrated the lyophilized solutions with HPLC grade water and centrifuged them (as before) to obtain a supernatant solution or ...

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