WHEAT-GRAIN HARDNESS RESULTS FROM HIGHLY CONSERVED MUTATIONS IN THE FRIABILIN COMPONENTS PUROINDOLINE A AND B

''Soft'' and ''hard'' are the two main market classes of wheat (Tritic um aestivum L.) and are distinguished by expression of the Hardness ge ne. Friabilin, a marker protein for grain softness (Ha), consists of t wo proteins, puroindoline a and b (pinA and pinB, respectively), We pr eviously demonstrated that a glycine to serine mutation in pinB is lin ked inseparably to grain hardness, Here, we report that the pinB serin e mutation is present in 9 of 13 additional randomly selected hard whe ats and in none of 10 soft wheats. The four exceptional hard wheats no t containing the serine mutation in pinB express no pinA, the remainin g component of the marker protein friabilin. The absence of pinA prote in was linked inseparably to grain hardness among 44 near-isogenic lin es created between the soft variety Heron and the hard variety Falcon. Both pinA and pinB apparently are required for the expression of grai n softness. The absence of pinA protein and transcript and a glycine-t o-serine mutation in pinB are two highly conserved mutations associate d with grain hardness, and these friabilin genes are the suggested tig htly linked components of the Hardness gene. A previously described gr ain hardness related gene termed ''GSP-1'' (grain softness protein) is not controlled by chromosome 5D and is apparently not involved in gra in hardness, The association of grain hardness with mutations in both pinA or pinB indicates that these two proteins alone may function toge ther to effect grain softness. Elucidation of the molecular basis for grain hardness opens the way to understanding and eventually manipulat ing this wheat endosperm property.