Does the sulfur assimilation pathway play a role in the response to Fe deficiency in maize (Zea mays L.) plants?

The finding that the methionine is the sole precursor of the mugineic acid family phytosiderophores induced us to evaluate whether sulfur assimilation pathway has a role in plant response to Fe deficiency. Maize plants were grown for 10 days in nutrient solution (NS) containing 80 PM Fe in the presence (+S) or absence (-S) of sulfate. After removing the root extra-plasmatic iron pool, half of the plants of each treatment (+S and -S) were transferred to a new Fe deficient NS (0.1 muM final Fe concentration) (-Fe). The remaining plants of each pre-culture condition (+S and -S) were transferred to a new NS containing 80 muM Fe (+Fe). Leaves were collected 4 and 24 hours from the beginning of Fe deprivation period and used for chemical analysis and enzyme assays. Results showed that iron content in the leaves was lower in plants grown in S-deficiency than in those grown in the presence of the macro-nutrient. Iron deprivation produced an increase in the level of SH compounds in both nutritive conditions (+S and -S). These observations are suggestive of some relationship between S nutrition and Fe uptake. For this reason, we next investigated the influence of Fe availability on S metabolism through the evaluation of changes in ATPs and OASs activity, the first and the last enzyme of S assimilation pathway respectively. Results showed that S-starvation increased the activity of both enzymes, but this effect disappeared in plants upon Fe deficiency suggesting that S metabolism is sensitive to Fe availability. Taken together these evidences suggest that S metabolism is sensitive to soil Fe-availability for plant nutrition and support the hypothesis of S involvement in plant response to Fe deprivation.