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

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.

Flowering and Fruit-set in Cassava under Extended Red-Light Photoperiod Supplemented with Plant-Growth Regulators and Pruning

“…Regarding application of BA in this study, it may have stimulated pistillate ower formation and increased fruit set. This effect has also been reported by [14] in cassava and is corroborated by [68]. Thus, these results indicate that effective enhancement of owering and fruit set under RLE can best be maximized with PGR and/or pruning supplementation.…”

“…Feminization of staminate l owers due to BA (and STS) application with or without pruning was observed in this study and was also reported among the germplasms used by [14] and [31]. Normally, cassava owers exhibit unisexuality, being either pistillate or staminate owers [9,69].…”

“…Two PGRs, STS and BA, which came through as most effective candidates following screening of several PGRs for their effect on cassava owering [72,73)] were used. Efforts to optimize these PGRs were undertaken in a study by [14]. As such, in this study we chose to undertake (on small-scale) validation of the effectiveness of the STS and BA when used in combination on plants under RLE.…”

“…For the owering genotypes, time to owering and fruiting varies markedly [11] making synchronization of owering and crossing di cult [12]. Additionally, the ratio of female to male owers per in orescence is small [13,14]. Success of controlled pollination is usually one or two seeds per ower [15].…”

Environmental responsiveness of flowering time in cassava genotypes and associated transcriptome changes

“…In the current study, cassava flowering was repressed by warm temperatures (Fig 7), and TEM1 expression was elevated in warm temperatures corresponding to a postulated role as a flowering repressor. Consistent with the current work, enhancement of cassava flower development by the anti-ethylene silver thiosulfate (STS) and the cytokinin benzyladenine downregulated TEM1 in apical-region tissues [63]. Hence, the expression pattern of TEM1 identifies it as a key candidate for further investigation of flowering-time regulation in cassava under various environmental conditions.…”