What are the hydrocolloids that are secreted at a site of injury on a plant or surround theendosperm of some seeds and that are used as thickening agents?

it's called gums

Explanation:

Gum, in botany, adhesive substance of vegetable origin, mostly obtained as exudate from the bark of trees or shrubs belonging to the family Fabaceae (Leguminosae) of the pea order Fabales. Some plant gums are used in the form of water solutions in the manufacture of cosmetics, pharmaceuticals, and foods. When the water evaporates, a film having a considerable adhesive character is formed. Some plant gums, such as gum arabic, dissolve in water to give clear solutions. Other gums, such as gum tragacanth, form mucilages by the absorption of large amounts of water.