Question

What are the steps involved in formation of a root nodule?

Answer 1

Nodule formation involves a sequence of multiple interactions between Rhizobium and roots of the host plant. Main stages in the nodule formation are:

(i) Rhizobia multiply and colonise the surrounding of roots and get attached to epidermal and root hair cells (Figure a).

(ii)The root hair curl and the bacteria invade the root hair.

(iii)An infection thread is produced carrying the bacteria into the inner cortex of the root (Figure b and c)

(iv)The bacteria get modified into rod-shaped bacteroids and cause inner cortical and pericycle cells to divide. Division and growth of cortical and peri cycle cells lead to nodule formation.

(v) The nodule thus formed, establishes a direct vascular connection with the host for exchange of nutrients (Figure d).

(vi)The nodule contains all the necessary biochemical components, such as the enzyme nitrogenase and leghaemoglobin. The enzyme nitrogenase catalyses the conversion of atmospheric nitrogen to ammonia, the first stable product of nitrogen fixation.

                          (a)                                                                            (b)

 

                   (c)                                                                                       (d)

Figure Development of root nodules

Answer 2

Multiple interactions are involved in the formation of root nodules. The Rhizobium bacteria divide and form colonies. These get attached to the root hairs and epidermal cells. The root hairs get curled and are invaded by the bacteria. This invasion is followed by the formation of an infection thread that carries the bacteria into the cortex of the root. The bacteria get modified into rod-shaped bacteroides. As a result, the cells in the cortex and pericycle undergo division, leading to the formation of root nodules. The nodules finally get connected with the vascular tissues of the roots for nutrient exchange.