The seven mitzvot of the land
The Sages (Kiddushin 37a; Rambam, Terumot 1:6-10) counted seven mitzvot that apply in the Land of Israel specifically: terumot, ma'asrot, shemitah, kilayim, orlah, neta revai, and challah. Some add first fruits and the gifts to the priests.
The holiness of the land in action
The holiness of the land is expressed in that the Torah obligated special mitzvot in its produce. The Ramban (Vayikra 18:25) emphasizes that the holiness of the land is not just an idea but a real act - separating terumah and ma'aser, leaving the land fallow, guarding kilayim. And so it is stated (Devarim 11:8-9): "so that your days may be multiplied, and the days of your children, on the land" - length of days depends on fulfilling the land's mitzvot.
Partnership in the land's mitzvot
All these mitzvot depend on the agricultural act: on sowing, on reaping, on pressing the grapes, on baking the challah. One who is not in the land cannot perform them in his body. The emissaries in the field perform the agricultural act, separate terumot and ma'asrot, guard shemitah and kilayim, and the partner participates in the cost of the act. On the basis of "one's agent is as oneself" and partnership through money, all seven mitzvot are credited to him as if he performed them himself.
And so the Sages taught (Sotah 14a): "Whoever assists those who perform a mitzvah, Scripture considers it as if he performed it."