Question

why does the curling of root hair take place when the factors are secreted by rhizobium in the symbiotic nitogen fixation by azotrophs?

Answer 1

to form a nodule so that the roots could create a environment where their is absence of oxygen for the enzymes to work.
leg haemoglobin is responsible for all this bcz it is an oxygen scavanger which create anaerobic condition for enzyme to work.
so this adaptation of nodule is only to ensure that the enzyme is protected from oxygen.