Figure 5.

Speculative model illustrating reactive oxygen species (ROS) mediated regulatory mechanisms that may account for observed patterns of inter- and intracellular formation of ingrowth walls (dark gray) in collection phloem companion cells (CC) and phloem parenchyma cells (PP) undergoing trans-differentiation to a transfer cell (TC) morphology. In all cases, developing sieve elements (SEs), undergoing partial programmed cell death, generate an intracellular ROS signal. (A) CC/TC trans-differentiation: SE-produced intracellular ROS (red dots) diffuses (thick red arrows) into CC, through interconnecting plasmodesmata, to induce formation of a non-polarized ingrowth wall covering the entire CC wall. (B) CC/TC and PP/TC trans-differentiation: an intracellular ROS signal induces ingrowth wall formation as described in (A). In addition, intracellular ROS, released from developing SEs through plasma membrane aquaporins abutting adjacent PP (thin red arrows) into the shared SE/PP cell wall space, elicits a polarized formation of ingrowth walls proximal to SEs. (C) PP/TC trans-differentiation: A transient and excessive intracellular generation of ROS by developing SEs gates plasmodesmata interconnecting CCs closed (filled in) thus preventing transmission of the ROS signal (curved red arrows) across the developmental period in which CCs are otherwise ROS-responsive. Ingrowth wall formation in PPs occurs as described in (B).