School of Environmental and Life Sciences, The University of Newcastle, Callaghan, NSW 2308, Australia
Transfer cells (TCs) trans-differentiate from existing cell types to facilitate enhanced membrane transport of nutrients at symplasmic/apoplasmic interfaces. The morphological specialization of TCs lies in their augmented surface area of plasma membrane resulting from intricate wall ingrowths. In Arabidopsis, phloem parenchyma (PP) TCs form from differentiated PP cells in vascular bundles of cotyledons, leaves and sepals. We report that PP TCs with extensive wall ingrowths are ubiquitous in juvenile leaves, but substantially less abundant in adult leaves, an observation consistent with PP TC development representing a novel trait of vegetative phase change (VPC) in Arabidopsis. Consistent with this conclusion, the abundance of PP TCs with extensive wall ingrowths varied across rosette development in three ecotypes that display different juvenile phase lengths, and extensive deposition of wall ingrowths was observed in rejuvenated leaves following defoliation. PP TC development across juvenile, transition and adult leaves correlated positively with levels of miR156, a major regulator of VPC in plants, and negatively with levels of miR156-targeted SQUAMOSA PROMOTER BINDING PROTEIN LIKE (SPL) genes. Corresponding changes in wall ingrowth deposition were observed when miR156 was overexpressed or its activity suppressed by target mimicry. Wall ingrowth deposition was reduced in plants carrying miR156-resistant forms of SPL9, SPL10 and SPL15, and was increased in the double mutant spl9/spl15. Importantly, no change in xylem abundance was observed in these lines, indicating a VPC-specific wall ingrowth response. Collectively, our results point to wall ingrowth deposition in PP TCs being under control of the genetic program regulating VPC via the miR156/SPL module.