The global genetic diversity of cassava and related Manihot species is revealed by sequencing of 53 cultivated and wild accessions and genotyping of 268 African cassavas, providing a vital resource for breeding.

Cassava is an important crop in Africa, Asia, Latin America, and the Caribbean. Cassava can be produced adequately in drought conditions making it the ideal food security crop in marginal environments. Although cassava can tolerate drought stress, it can be genetically improved to enhance productivity in such environments. Drought adaptation studies in over three decades in cassava have identified relevant mechanisms which have been explored in conventional breeding. Drought is a quantitative trait and its multigenic nature makes it very challenging to effectively manipulate and combine genes in breeding for rapid genetic gain and selection process. Cassava has a long growth cycle of 12–18 months which invariably contributes to a long breeding scheme for the crop. Modern breeding using advances in genomics and improved genotyping, is facilitating the dissection and genetic analysis of complex traits including drought tolerance, thus helping to better elucidate and understand the genetic basis of such traits. A beneficial goal of new innovative breeding strategies is to shorten the breeding cycle using minimized, efficient or fast phenotyping protocols. While high throughput genotyping have been achieved, this is rarely the case for phenotyping for drought adaptation. Some of the storage root phenotyping in cassava are often done very late in the evaluation cycle making selection process very slow. This paper highlights some modified traits suitable for early-growth phase phenotyping that may be used to reduce drought phenotyping cycle in cassava. Such modified traits can significantly complement the high throughput genotyping procedures to fast track breeding of improved drought tolerant varieties. The need for metabolite profiling, improved phenomics to take advantage of next generation sequencing technologies and high throughput phenotyping are basic steps for future direction to improve genetic gain and maximize speed for drought tolerance breeding.

The timing of developmental events and regulatory roles of auxin were examined in maize (Zea mays L.) endosperms. Zeatin, zeatin riboside, and indole-3-acetic acid (IAA) levels were determined by enzyme-linked immunosorbent (ELISA). Zeatin and zeatin riboside increased to maximal concentrations at an early stage (9 d after pollination [DAP]), corresponding to the stage when cell division rate was maximal. In contrast, IAA concentration was low at 9 DAP and abruptly increased from 9 to 11 DAP, thus creating a sharp decline in the cytokinin to auxin ratio. Coincident with the increase in IAA was an increase in DNA content per nucleus, attributed to postmitotic DNA replication via endoreduplication. Exogenous application of 2,4-dichlorophenoxyacetic acid (2,4-D) at 5 or 7 DAP hastened the time course of DNA accumulation per nucleus and increased the average nuclear diameter, whereas 2-(para-chlorophenoxy)isobutyric acid delayed such development. Exogenously applied 2,4-D hastened the accumulation of the zein polypeptides of apparent molecular masses of 12, 14, and 16 kD and the expression of mRNA hybridizing with a zein DNA probe. We conclude that an abrupt increase in auxin induces cellular differentiation events in endosperm, including endoredupliction and expression of particular zein storage proteins.

• Background and Aims Cassava (Manihot esculenta) is an important food crop in the tropics that has a high growth rate in optimal conditions, but also performs well in drought-prone climates. The objectives of this work were to determine the effects of water deficit and rewatering on the rate of expansion of leaves at different developmental stages and to evaluate the extent to which decreases in cell proliferation, expansion, and delay in development are responsible for reduced growth. • Methods Glasshouse-grown cassava plants were subjected to 8 d of water deficit followed by rewatering. Leaves at 15 developmental stages from nearly full size to meristematic were sampled, and epidermal cell size and number were measured on leaves at four developmental stages. • Key Results Leaf expansion and development were nearly halted during stress but resumed vigorously after rewatering. In advanced-stage leaves (Group 1) in which development was solely by cell expansion, expansion resumed after rewatering, but not sufficiently for cell size to equal that of controls at maturity. In Group 2 (cell proliferation), relative expansion rate and cell proliferation were delayed until rewatering, but then recovered partially, so that loss of leaf area was due to decreased cell numbers per leaf. In Group 3 (early meristematic development) final leaf area was not affected by stress, but development was delayed by 4–6 d. On a plant basis, the proportion of loss of leaf area over 26 d attributed to leaves at each developmental stage was 29, 50 and 21 % in Group 1, 2 and 3, respectively. • Conclusions Although cell growth processes were sensitive to mild water deficit, they recovered to a large extent, and much of the reduction in leaf area was caused by developmental delay and a reduction in cell division in the youngest, meristematic leaves.