Figure 11.

Model Representing the Possible Role of Pho1 in Starch Biosynthesis in the Rice Endosperm.

In this model, Pho1 is suggested to play a major role in one or more events of starch initiation. One possible role for Pho1 is to extend the chain length of the initial primer so that it is acted upon by SSs. This involvement of Pho1 in the starch initiation process would account for the shrunken phenotype and loss of starch in pho1 mutants grown at 20°C. A hypothetical X factor is also proposed that can partially complement the function of Pho1 in starch initiation at 30°C. The X factor is suggested to have low activity or be present at low amounts at 20°C but to be fully active at 30°C. Hence, at 20°C, the loss of Pho1 and low net activity of X factor results in a high incidence of shrunken seeds, while at higher temperature the X factor can partially complement Pho1 role in starch initiation leading to the production of a high percentage of seeds that accumulate near normal starch levels.