Regulation of sex expression in pistillate lines and its implications for hybrid breeding in castor (Ricinus communis L.)

Castor is an economically important non-edible oilseed crop cultivated globally for its oil and diverse industrial applications. The crop displays multiple sex forms including pistillate, monoecious, interspersed staminate flowers (ISF) and sex revertants, which complicate hybrid seed production. Hybrid development in castor depends on the pistillate lines, particularly stigma-emerged type (S-type) and non-elongating stigma (NES)-type systems, which are highly responsive to environmental factors such as temperature, photoperiod, nutrition and plant age. Plant growth regulators, such as Ethrel, decrease ISF expression and increase the number of capsules per spike, resulting in higher seed output. Genetic research indicates that 4 pairs of cumulatively acting recessive nuclear genes regulate pistillateness. Advances in genetic and molecular studies have explained the involvement of genes such as SDR2a, WUSCHEL, RcACS5, RcHK1 and glycosyltransferase gene 29916.m000523 in controlling sex differentiation during the floral developmental stage. Despite major advancements in pistillate-line breeding and the release of stable, high-yielding hybrids, constraints such as sex reversion, environmental influence and high roguing costs persist. To overcome this problem, the development of cytoplasmic male sterility (CMS) is vital for castor improvement. Approaches such as spontaneous mutant screening, intraspecific crosses, wide hybridisation and mutation breeding can be used to identify CMS sources in castor. Identification of CMS would significantly enhance hybrid seed purity, thus increasing production efficiency. This review synthesises current knowledge on genetic, molecular, hormonal and environmental regulation of sex expression in castor and discusses their implications for hybrid breeding strategies.