Hexitols in coconut milk: Their role in nurture of dividing cells

Abstract

Coconut milk, the liquid endosperm of the coconut (Cocos nucifera L.), has special interest because it will induce otherwise mature, non-growing cells to divide and to grow rapidly (3, 4). Similar properties reside in analogous morphological situations, such as the immature caryopsis of Zea mays and the liquid present in immature fruits of Juglans (18) or of Aesf alus (12, 24, & also see 25). A nutritive relationship also exists between the female gametophyte, sometimes called endosperm, and the archegonia and embryos of the gymnosperm Ginkgo; extracts of this gametophyte will also induce cells of mature tissue of carrot root to resume active growth (18). Therefore, the fluids that nourish immature embryos seem especially able to induce growth in the mature cells even of some other plants than the ones in which they were laid down. This raises the question whether the behavior of the zygote is due to its special nature or to its nurture by the special fluid contents of the embryo sac, by the substances which are contained in the endosperm, and by other special nutritive organs. Free cells obtained from mature carrot phloem may be cultured in media which contain coconut milk (21) and may grow and regenerate a complete and mature plant (20). In this7 respect the free cells imitate the zygote, and the coconut milk its normal nutritional supply. Moreover, as the cells grow and develop, they form structures which are strongly reminiscent of pro-embryonic development (15). Therefore, a full knowledge of the chemical constituents of coconut milk which cause these growth responses would have an important bearing upon many problems of cell growth and cell division. This knowledge would also have important implications for protein synthesis, which is stimulated in quantity and modified in kind during the induction of growth in carrot and potato cells (27). Following the observations of Blakeslee and van Overbeek (28, 29), work upon the chemical constituents of coconut milk was pursued sporadically in different laboratories (for references see 25). In this laboratory, investigation has been in progress for some years. Clearly, the growth induction which is pr,oduced by the coconut milk, over and above the effects due to common nutrients and vitamins, is not a simple effect due to a single substance. On the contrary, it has been emphasized that no substance singly and independently controls cell division (see 25 & references there cited). In part the effect of coconut milk is non-specific and is replaceable by casein hydrolysate, or by other sources of the reduced nitrogen compounds, from which the cells may synthesize protein more readily than they do from nitrate (13). Even whole coconut milk alone will not trigger the growth of some cells (e.g. potato tuber), for it needs to be supplemented by one of a large array of compounds which are now known to act synergistically with the coconut milk. The substance 2,4-dichlorophenoxyacetic acid (2,4-D), and many of its analogues with different ring configurations or different side chains, can also function in this manner (14,16, see also 25). Several of the halogen-substituted phenylacetic acids (26) and certain a-substituted propionic acids (14) can also work along with the coconut milk. This paper now designates certain hexitols to be responsible for part of the effect for which coconut milk (or its morphological equivalent) has hitherto been regarded as a specific source. Early work on the chemical fractionation of coconut milk and similar fluids recently has been reviewed (25). This work encountered the difficulty that, when purified, the isolated substances only expressed their activity in the presence of other sub-fractions from the coconut milk. While this statement still holds true, the work to be described permits the critical identification of at least three of the synergists which contribute to the total growth which is stimulated by whole coconut milk.