Abstract
Cassava, a tropical storage-root crop, is a major source of food security for millions in the tropics. Cassava breeding, however, is hindered by the poor development of flowers and a low ratio of female flowers to male flowers. To advance the understanding of the mechanistic factors regulating cassava flowering, combinations of plant growth regulators (PGRs) and pruning treatments were examined for their effectiveness in improving flower production and fruit set in field conditions. Pruning the fork-type branches, which arise at the shoot apex immediately below newly formed inflorescences, stimulated inflorescence and floral development. The anti-ethylene PGR silver thiosulfate (STS) also increased flower abundance. Both pruning and STS increased flower numbers while having minimal influence on sex ratios. In contrast, the cytokinin benzyladenine (BA) feminized flowers without increasing flower abundance. Combining pruning and STS treatments led to an additive increase in flower abundance; with the addition of BA, over 80% of flowers were females. This three-way treatment combination of pruning+STS+BA also led to an increase in fruit number. Transcriptomic analysis of gene expression in tissues of the apical region and developing inflorescence revealed that the enhancement of flower development by STS+BA was accompanied by downregulation of several genes associated with repression of flowering, including homologs of TEMPRANILLO1 (TEM1), GA receptor GID1b, and ABA signaling genes ABI1 and PP2CA. We conclude that flower-enhancing treatments with pruning, STS, and BA create widespread changes in the network of hormone signaling and regulatory factors beyond ethylene and cytokinin.
Highlights
Cassava (Manihot esculenta) is a perennial tropical plant of the Euphorbiaceae family cultivated as an annual crop for its starchy storage roots (Alene et al, 2018)
From Experiment I, above, we obtained evidence that silver thiosulfate (STS) treatment increases flower and fruit numbers and that spraying BA to the shoot apex increased the proportion of flowers that were female
Using the DESeq2 package, we identified genes differentially expressed under the full model (Figure 7A)
Summary
Cassava (Manihot esculenta) is a perennial tropical plant of the Euphorbiaceae family cultivated as an annual crop for its starchy storage roots (Alene et al, 2018) It constitutes an important source of calories for over 800 million people (Jarvis et al, 2012). 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 source– sink relationships (Sonnewald et al, 2020) and its photosynthetic efficiency (De Souza and Long, 2018). Cassava can be clonally propagated by stem cuttings, crop improvement via breeding requires genetic recombination using sexual crosses between parents from diverse genetic populations that flower in a timely and synchronous manner
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