The yield of castor is influenced by the type of inflorescence and the proportion of female flowers. However, there are few studies on the genetic mechanism involved in the development and differentiation of castor inflorescences. In this study, we performed transcriptomic analyses of three different phenotypes of inflorescences at the five-leaf stage. In comparison to the MI (complete pistil without willow leaves), 290 and 89 differentially expressed genes (DEGs) were found in the SFI (complete pistil with willow leaves) and the BI (monoecious inflorescence), respectively. Among the DEGs, 104 and 88 were upregulated in the SFI and BI, respectively, compared to the MI. In addition, 186 DEGs and 1 DEG were downregulated in the SFI and BI compared to the MI. Moreover, we conducted GO and KEGG enrichment analyses of the DEGs. In comparison to the MI, the SFI and BI exhibited the enrichment of functional branches in DEGs, specifically in pollen wall assembly, pollen development, and cellular component assembly involved in morphogenesis. In our study, RADL5 showed low expression levels between SFI-vs.-MI types. In addition, we found that the expression of NAC in the SFI differed from that in MI and BI, and some genes related to hormonal signaling changed their expression levels during inflorescence differentiation. These results reveal the genetic mechanism of sex genotypes in castor, which will not only guide researchers in the breeding of castor but also provide a reference for genetic research on other flowering plants.

Castor is one of the top 10 oil crops in the world and has extremely valuable uses. Castor inflorescences directly affect yield, so the study of inflorescence development is very important in increasing castor yield. Our previous studies have shown that the PIP5K gene family (PIP5Ks) is associated with inflorescence development. In this study, to determine the function of each PIP5K gene in castor, a female Lm-type castor line, aLmAB2, was used to determine the relative expression levels of the PIP5Ks in castor inflorescences. Six PIP5K genes were heterologously overexpressed in Arabidopsis thaliana, the relative expression of each gene and the effect on plants was determined in A. thaliana, and the relationships among the PIP5Ks in castor were inferred. The expression levels of the PIP5Ks in the female Lm-type castor line aLmAB2 were analyzed. The relative expression levels of the PIP5K9 and PIP5K11 genes were high (p < 0.05) in isofemale inflorescences, and those of PIP5K1, PIP5K2, PIP5K6, and PIP5K8 were high (p < 0.05) in female inflorescences but low (p < 0.05) in bisexual inflorescences. The PIP5Ks were heterologously overexpressed in A. thaliana, and T3-generation plants with stable genetic resistance, i.e., AT-PIP5K⁺ plants (AT-PIP5K1⁺, AT-PIP5K2⁺, AT-PIP5K6⁺, AT-PIP5K8⁺, AT-PIP5K9⁺, and AT-PIP5K11⁺ plants), were obtained. Biological tests of the AT-PIP5K⁺ plants showed that the growth of the main stem was significantly delayed in AT-PIP5K⁺ plants compared with Columbia wild-type (WT) A. thaliana plants; the PIP5K1 and PIP5K2 genes promoted lateral stem growth and flower and silique development; and the PIP5K6, PIP5K8, PIP5K9 and PIP5K11 genes inhibited lateral stem growth and flower and silique development. The correlations among PIP5Ks in castor suggest that there may be a synergistic relationship among PIP5K1, PIP5K2, and PIP5K6 in castor inflorescences, and PIP5K8, PIP5K9, and PIP5K11 are complementary to the other three genes.