GUELPH, ONTARIO, CANADA
University of Guelph
Sucrose affects ID1 protein expression in developing leaves
— Wong, AYM; Colasanti, J
The indeterminate1 gene (id1) controls the transition to reproductive growth in maize. Mutant id1 plants remain in a prolonged vegetative state and are unable to undergo normal transition to flowering. The id1 gene encodes a zinc finger protein, suggesting that the ID1 protein is an important regulator of the transition to flowering in maize. Expression of id1 mRNA is detected only in leaf tissue, suggesting that id1 may control a long distance floral inductive signal (Colasanti et al., Cell 93: 593–603, 1998; Colasanti and Sundaresan, TIBS 25: 236–240, 2000). In particular, id1 mRNA accumulates in developing immature leaves and transcript levels decrease as the leaves emerge from the whorl and become active in photosynthesis. This reduction of id1 expression appears to be correlated with the sink-to-source transition in maize leaves; i.e., expression is reduced as leaves become green.
We used Western blotting with an anti-ID1 specific antibody to ask two specific questions:
To answer these questions we used a line of lemon white 1 (lw1) heterozygous seeds that segregate albino plants. Albino plants lack functional chloroplasts, and, therefore, all organs remain as carbon sinks because they do not carry out photosynthesis. The maize endosperm supplies enough nutrients to allow albino plants to form 3 to 4 visible leaves before dying. Detection of ID1 in mature leaves of albino plants would suggest that id1 expression is affected directly by the sink-to-source transition; i.e., albino plants form fully developed leaves, but they remain as sink tissue because they do not fix carbon.
Seeds that segregate albino and green seedlings were germinated in magenta boxes on MS media with 0%, 1%, 2%, and 5% sucrose. Nuclear proteins were extracted from immature leaf and mature leaf tissues from both albino mutants and normal green siblings at the 3 to 4 visible leaf stage. (Immature leaf samples included the stem section from 1 cm to 5 cm above the shoot apex and did not include outer sheath leaves; mature leaves included the green or albino leaf blades that had emerged from the whorl.) As shown in Figure 1, ID1 protein was detected only in immature leaves and not in leaf blades, in both albino and green plants. Therefore, in response to question 1, we found that ID1 protein is expressed only in particular tissues (i.e., developing leaves) and is not correlated with the sink-to-source transition. However, an interesting finding from this experiment was that immature leaves of albino plants grown in the absence of added sucrose had very low or undetectable levels of ID1 protein, whereas albino plants grown with as little as 1% sucrose in the media had similar amounts of ID1 protein as green siblings (Figure 1). Dissection of albino and green seedlings showed no differences in the number of immature leaves within the whorls of plants grown with or without sucrose, suggesting that differences in protein levels is not due to different amounts of leaf tissue. This suggests that ID1 expression in developing leaves may be controlled by the presence of sucrose. No differences in ID1 protein levels were observed in green plants grown with or without sucrose, suggesting that the endogenous sucrose produced by photosynthesis in green plants is sufficient to induce ID1 protein to its maximum expression level. Albino plants grown on more than 1% sucrose did not show correspondingly higher ID1 levels. Also, the externally supplied sugar did not induce ID1 expression in mature leaves, implying that altering the total amount of available carbon does not change the developmental expression pattern of control of ID1. These results are interesting in light of numerous physiological experiments that suggest a connection between carbon partitioning in plants and time to flowering. Future experiments are aimed at investigating the connection between carbon flow in maize and the transition to flowering.
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