Root systems of major tropical root and tuber crops: Root architecture, size, and growth and initiation of storage organs

Root and tuber crops (RTCs) produce a variety of edible belowground organs constituting the second most important source of carbohydrates to humans and the most important in sub-Saharan Africa. This review focuses on the development of adventitious roots (ARs), differentiation of storage roots and the growth and decay of non-storage roots. The root system of RTCs comprises ARs but the storage organs differ; confusingly, much of the literature refers to them all as tubers. Swelling of ARs to form storage roots (SRs) in cassava and sweet potato results from expansion of root cambium and proliferation of starch storage tissue with starch biosynthesis genes highly expressed, and lignin biosynthesis genes down regulated. Several genes play a role in SR development in sweet potato including two MADS-box transcription factors and in cassava, potato and sweet potato storage organ initiation and development is related to KNOX1 activity. The small number of studies makes generalization difficult, but several show that maximum length coincides with the start of rapid growth of storage roots/tubers. Different patterns of growth may reflect differences in soil water and nutrient availability. Many ARs appear to be short-lived. Typically the rooting depth of potato was < 1 m with a maximum root length of 7–12 km/m2 while for cassava, rooting depth was deeper (> 1 m), but root length was less (1–2.84 km/m2). We highlight the paucity of studies of RTCs, the inconsistent use of terms to describe roots and storage organs and the need to characterize the longevity and functionality of different root types especially SRs.