At the request of Pope Francis, the Congregation for Divine Worship and the Discipline of the Sacraments is writing to Diocesan Bishops to remind them that it falls to them above all to duly provide for all that is required for the celebration of the Lord's Supper. [...]
"The bread used in the celebration of the Most Holy Eucharistic Sacrifice must be unleavened, purely of wheat, and recently made so that there is no danger of decomposition. It follows therefore that bread made from another substance, even if it is grain, or if it is mixed with another substance different from wheat to such an extent that it would not commonly be considered wheat bread, does not constitute valid matter for confecting the Sacrifice and the Eucharistic Sacrament. It is a grave abuse to introduce other substances, such as fruit or sugar or honey, into the bread for confecting the Eucharist. [...]
"Hosts that are completely gluten-free are invalid matter for the celebration of the Eucharist. Low-gluten hosts (partially gluten-free) are valid matter, provided they contain a sufficient amount of gluten to obtain the confection of bread without the addition of foreign materials and without the use of procedures that would alter the nature of bread" (A. 1-2). [...]
The same Congregation also decided that Eucharistic matter made with genetically modified organisms can be considered valid matter (cf. Letter to the Prefect of the Congregation for Divine Worship and the Discipline of the Sacraments, 9 December 2013, Prot. N. 89/78 -- 44897).
Letter to Bishops on the bread and wine for the Eucharist: