Maize (Zea mays L.) root cap cells secrete a large variety of compounds including
proteins via an amorphous gel structure called mucilage into the rhizosphere. In the present study, mucilage secreted by primary roots of 3-4 day old maize seedlings was collected under axenic conditions, and the constitutively secreted
proteome was analyzed. A total of 2848 distinct extracellular
proteins were identified by nanoLC-MS/MS. Among those, metabolic
proteins (approximately 25%) represented the largest class of annotated
proteins. Comprehensive sets of
proteins involved in cell wall metabolism, scavenging of
reactive oxygen species, stress response, or nutrient acquisition provided detailed insights in functions required at the root-soil interface. For 85-94% of the mucilage
proteins previously identified in the relatively small data sets of the dicot species pea, Arabidopsis, and rapeseed, a close homologue was identified in the mucilage
proteome of the monocot model plant maize, suggesting a considerable degree of conservation between mono and dicot mucilage
proteomes. Homologues of a core set of 12 maize
proteins including three
superoxide dismutases and four
chitinases, which provide protection from
fungal infections, were present in all three mucilage
proteomes investigated thus far in the dicot species Arabidopsis, rapeseed, and pea and might therefore be of particular importance.