The Lotus Sutra does not discuss a universal buddha-nature but often speaks of a universal buddha-seed. Chih-i re-uses the image of the seed to formulate a temporal succession in the process of enlightenment, which reflects also the way the Buddha acts: the seed is first sown, then left to sprout and grow, and finally the plant ripens. … [I]n Chih-i’s exegesis of the enlightenment of Śākyamuni the time between the sowing (the original enlightenment of Śākyamuni) and the ripening (the recent enlightenment of Śākyamuni) is an upāya, because the deeds Śākyamuni performs during this period are according to teachings other than the Lotus Sutra. The Lotus Sutra represents the world of the original enlightenment of Śākyamuni and that of his present enlightenment. The world in between is denoted by the other sutras Śākyamuni preached during his lifetime according to people’s capacity.

In Nichiren’s interpretation the upāya no longer has a function, the seed becomes equivalent to enlightenment, and the planting of the seed amounts to the attainment of buddhahood. The temporal interval between the primordial time and the present of Śākyamuni loses significance, and so does the difference between the original time and original land and the present of human beings. The Lotus Sutra is the buddha seed planted in people, the only means to realize the human potential for buddhahood. At any moment this scripture is read and diffused, the seed of buddhahood is again planted in everybody who chooses to listen and keep it, and the primordial relation with the Buddha is reestablished. If nobody “uses” the sutra, the seed disappears and no one is aware of their tie with the Buddha.

The seed is thus the necessary and sufficient cause of buddhahood. Yet, compared with the idea of buddha-nature, unchangeable by definition, the seed gives the idea of something belonging to the phenomenal world, subject to disappearance. “If people do not believe in this sutra and vilify it, then they cut off all the buddha-seeds in the world,” the sutra says. It is thus necessary to sow the seed again. If buddha-seeds occur “according to circumstances and conditioned cause,” as suggested in the Lotus Sutra itself, both the infinite action of the Buddha and one’s own activity are necessary. The image of the seed also conveys a more individual nuance than the universality of the buddha-nature: “Human beings defiled by evil encounter the bodhisattvas of the honmon, and the buddha-seeds are planted.”
A Buddhist Kaleidoscope; Lucia Dolce, Between Duration and Eternity: Hermeneutics of the ‘Ancient Buddha’ of the Lotus Sutra in Chih-i and Nichiren, Page 234-235