Initiation of Lateral and Adventitious Roots

Initiation of Lateral and Adventitious Roots

Lateral roots are branches of the tap root. They are initiated in relatively mature tissues some distance from the apex, often in acropetal sequence; the most recently-formed lateral roots are usually those nearest to the root apical meristem. In angiosperms, lateral roots have a deep-seated (endogenous) origin. Root forma-tion is usually initiated in groups of ‘‘founder cells’’ in the peri-cycle, often adjacent to the xylem poles. The position of lateral root initiation in the pericycle is usually at a point adjacent to a protoxylem pole, unless the root is diarch, in which case initia-tion is sometimes opposite a phloem pole. However, in monocots lateral root initiation can be opposite either protoxylem or phloempoles, though in roots with a large number of vascular poles it is often difficult to determine the precise site of initiation. The founder cells undergo a series of periclinal and anticlinal divisions to form a lateral root primordium. In many species some subsequent cell divisions occur in the endodermis, so that ultimately both the pericycle and the endodermis contribute to the tissues of the lateral root. The growing lateral root pushes its way through the cortex and epidermis of the parent root, either by mechanical or enzymatic action.

Adventitious roots are formed in other parts of the plant, primarily stem tissue. They have various sites of origin, from deep-seated (endogenous) (Fig. 3.4), to (more rarely) exogenous, arising from superficial tissues such as the epidermis (e.g. in surface-rooting Begonia leaves). In most monocots adventitious roots arise from cell divisions in the pericyclic region of the stem; the primary thickening meristem contributes to adventitious root formation. Adventitious roots are often formed at nodes on the stem, which is why in horticulture cuttings are most commonly taken from just below a node. Adventitious roots may also form from callus tissue at the site of a wound.