Ethylene and plant responses to nutritional stress

Although ethylene is known to be involved in plant response to a number of biotic and abiotic stresses, relatively little is known concerning its role in nutritional stress arising from nutrient deficiency or mineral toxicity. There is clear evidence for involvement of ethylene in the symbiosis between Rhizobium and legumes, and in the 'Strategy 1' response to Fe deficiency. Ethylene may also be generated during tissue necrosis induced by severe toxicities and deficiencies. Metal toxicity may generate ethylene through oxidative stress. Evidence for a more general role for ethylene in regulating plant responses to macronutrient deficiency is suggestive but incomplete. Few studies have addressed this interaction, and most published reports are difficult to interpret because of the unrealistic way that nutrient treatments were imposed. Deficiency of N and P appear to interact with ethylene production and sensitivity. A role for ethylene in mediating adaptive responses to P stress is suggested by the fact that P stress can induce a variety of morphological changes in root systems that are also affected by ethylene, such as gravitropism, aerenchyma formation, and root hair development. Other adaptive responses include senescence or abscission of plant parts which cannot be supported by the plant. Ethylene and other plant hormones may be involved in mediating the stress signal to generate these responses. Although existing literature is inconclusive, we speculate that ethylene may play an important role in mediating the morphological and physiological plasticity of plant responses to nutrient patches in time and space, and especially root responses to P stress.