The Cassava Source–Sink project: opportunities and challenges for crop improvement by metabolic engineering

Abstract

Summary Cassava (Manihot esculenta Crantz) is one of the important staple foods in Sub‐Saharan Africa. It produces starchy storage roots that provide food and income for several hundred million people, mainly in tropical agriculture zones. Increasing cassava storage root and starch yield is one of the major breeding targets with respect to securing the future food supply for the growing population of Sub‐Saharan Africa. The Cassava Source–Sink (CASS) project aims to increase cassava storage root and starch yield by strategically integrating approaches from different disciplines. We present our perspective and progress on cassava as an applied research organism and provide insight into the CASS strategy, which can serve as a blueprint for the improvement of other root and tuber crops. Extensive profiling of different field‐grown cassava genotypes generates information for leaf, phloem, and root metabolic and physiological processes that are relevant for biotechnological improvements. A multi‐national pipeline for genetic engineering of cassava plants covers all steps from gene discovery, cloning, transformation, molecular and biochemical characterization, confined field trials, and phenotyping of the seasonal dynamics of shoot traits under field conditions. Together, the CASS project generates comprehensive data to facilitate conventional breeding strategies for high‐yielding cassava genotypes. It also builds the foundation for genome‐scale metabolic modelling aiming to predict targets and bottlenecks in metabolic pathways. This information is used to engineer cassava genotypes with improved source–sink relations and increased yield potential.

Addressing Research Bottlenecks to Crop Productivity

“…This is consistent with the facts that (i) yield tends to be far better related to the number than to the average weight of seeds [219] and references quoted therein, (ii) seed weight tends to be insensitive or only marginally responsive to source strength per seed during seed filling [15,94,95], and (iii) photosynthesis and RUE are upregulated when sink strength is increased through breeding or treatments [85][86][87]96]. Sink limitation during storage organ development is not limited to grain crops but also applies to tuber and root crops such as potato or cassava [97,98].…”

“…An alternative way to improve sucrose supply towards the economic sinks is via an increase in sucrose phloem loading [112][113][114]. The loading mechanism is conserved across most crops [97,114]. Increased sucrose transport capacity improved productivity in model crops [115,116] as well as in oilseed rape and wheat (Dr Claus Frohberg, personal communication, 2020).…”

Flower Development in Cassava Is Feminized by Cytokinin, While Proliferation Is Stimulated by Anti-Ethylene and Pruning: Transcriptome Responses

“…Continual crop improvement is required to ensure efficient cassava production to meet the growing needs from an increasing population, new uses, and expanded markets, matched with changing environments globally. Cassava improvement has recently received renewed attention with major projects to investigate the potential use of genomic selection in breeding (Wolfe et al, 2017), its sourcesink relationships (Sonnewald et al, 2020) and its photosynthetic efficiency (De Souza and Long, 2018). These improvement efforts are geared toward smallholder farmers with an interest in translating cassava end-product quality traits into breeding outcomes (Iragaba et al, 2020).…”

A critical role of stable grain filling rate in maximizing rice yield revealed by whole plant carbon nitrogen interaction modeling

“…Despite the descriptive research of source sink interaction in single cells or organs, the plant level coordination of processes in different source and sink organs has been largely disregarded during crop improvement research; this has been due to difficulties in the interpretation of such a complex network 29 . Crop systems models have been used in designing agronomic practices 69,70 , testing hypotheses regarding the responses of crops to climate change 71,72 , guiding selection of traits for breeding 73 and assessing different strategies to adapt to future climate change 74 .…”

“…Ever since the proposal of the wheat ideotype by Donald 19 , improving crop morphology has been extensively studied in genetic research, modeling research, and in crop high-yield breeding, which is largely responsible for the dramatic increase in rice grain yield from around 3 t ha -1 to up to 15 t ha -1 in the past 60 years 17,20-26 . Although long recognized as the other equally important factor of crop ideotype 27-29 , crop physiological parameters, i.e., characteristics associated with photosynthesis, respiration, carbon and nitrogen metabolism in different organs, and also the transport of metabolites between organs, have so far generated little impact in crop breeding. This is due to there is still no consensus on the optimal physiological characteristics associated with a crop ideotype.…”