Leaf cells are specialized to optimize photosynthesis. Cells in the upper portion of the leaf are tightly packed and dense with chloroplasts to capture light energy. Cells in the lower portion are more irregularly shaped and loosely arranged to allow better exchange of oxygen and carbon dioxide. The difference between upper and lower parts of the leaf is also important for making new branches; new buds are made from the base of the upper, and not the lower, side of the leaf.
HD-ZIPIII proteins create the differences between upper and lower regions of the leaf by binding to and activating particular genes in the upper leaf domain. They bind to a palindromic DNA sequence as HD-ZIPIII pairs (dimers). However, like many other master regulators of development, keeping HD-ZIPIII proteins “OFF” in cells where their activity is inappropriate is vital to normal plant development. In previous work, Kathryn Barton’s lab identified a family of small proteins — LITTLE ZIPPER proteins — that inhibit the formation of HD-ZIPIII dimers, thereby preventing HD-ZIPIII proteins from binding DNA and activating genes inappropriately.
Now Enrico Magnani in the Barton lab has discovered a new layer of HD-ZIPIII inhibition. Magnani has shown that HD-ZIPIII proteins themselves include a region capable of inhibiting their partnering with other proteins. This region, called the MEKHLA domain, is at the carboxyterminal end of HD-ZIPIII proteins. When this domain is removed from HD-ZIPIII proteins, they form dimers in inappropriate cells. This causes alterations in leaf polarity and leaf curling due to improper coordination of growth between upper and lower sides of the leaf (see figure above). Magnani and Barton discovered that, unexpectedly, the addition of random sequences in place of the MEKHLA domain could inhibit dimerization. This suggests that the inhibition depends on a steric mechanism in which the MEKHLA domain spatially blocks HD-ZIPIII proteins from coming together.
The findings indicate there are at least two levels of inhibition of dimerization. The first, mediated by the MEKHLA domain, dictates whether the HD-ZIPIII proteins are competent to interact with other proteins. The second, mediated by the presence or absence of the LITTLE ZIPPER proteins, dictates whether they can form active HD-ZIPIII homodimers.
The presence of the inhibitory MEKHLA domain raises the question of how HD-ZIPIII proteins can be active in cells where they are needed. Efforts are ongoing to identify the signals that lift this inhibition. In the long run, understanding how HDZIPIII proteins shape the leaf in the myriad different leaf types that exist in nature will help us understand how leaf development and function are optimized for a wide range of environmental conditions. (Plant Cell 23:567-582, 2011)