UC Berkeley

In plants, transport mediated by the PINFORMED (PIN) family of auxin efflux carriers helps create gradients on which many developmental processes depend. Current models suggest that Arabidopsis PIN1 has two concurrent functions during leaf initiation: 1) concentrating auxin to create local maxima in the meristem epidermis, and 2) transporting auxin away from the epidermal maxima and into the internal tissues. The resulting auxin gradient is required for leaf initiation and vein patterning. I identified an angiosperm PIN clade placed sister to PIN1, here termed Sister-of-PIN1 (SoPIN1), that has likely been lost within the Brassicaceae, including in Arabidopsis, but remains in all other angiosperms sampled. I also identify a conserved duplication of PIN1 to create PIN1a and PIN1b within the grasses. I used live-cell imaging and immuno-localization to characterize the expression and localization of SoPIN1, PIN1a and PIN1b members in both maize and Brachypodium. SoPIN1 expression is highest in the epidermis and is consistently oriented toward areas where the DR5 auxin reporter is highly expressed, suggesting that SoPIN1 functions in the creation of auxin maxima. PIN1a and PIN1b localization, largely absent from the epidermis and oriented rootward in the internal tissues, suggests that these PIN proteins transport auxin after maxima formation during the canalization of leaf and stem veins. These data support the functional division of PIN proteins into maxima creation and canalization modes. In addition, the loss of SoPIN1 within the Brassicaceae suggests that PIN1 in this group may be unique amongst the angiosperms in its ability to dynamically switch between these two functional modes. I then provide a model for how the PIN1a/PIN1b duplication in the lineage leading to the grasses may relate to the novel morphological and anatomical characteristics found in monocot plants. Finally, I summarize some preliminary PIN knockdown experiments that suggest a role for PIN mediated patterning in apical dominance, meristem maintenance and leaf proximal/distal patterning.