The immunity-associated protein ACIP1 counteracts MIDD1 to coordinate xylem cell wall patterning

Cell wall patterning plays a crucial role in determining the function of plant cells.¹ Protoxylem and metaxylem vessel cells form striped and pitted cell walls, respectively, to facilitate efficient water transport.^2,3,4 These patterned cell walls are deposited beneath the cortical microtubules.^5,6 The microtubule depletion domain 1 (MIDD1) pathway removes cortical microtubules to promote cell wall spacing in xylem vessels,^7,8,9 but overall, the regulatory network of cell wall patterning in xylem vessels and its links with external signals remain poorly understood. Here, we show that acetylated interacting protein 1 (ACIP1), the target of the bacterial type III effector, inhibits cell wall spacing in protoxylem vessels. An acip1 loss-of-function mutant displayed abnormally wide cell wall spacing, while ACIP1 overexpression resulted in narrower cell wall spacing. Additive phenotypes were observed when ACIP1 was knocked out or overexpressed in midd1 midd2 plants, suggesting that ACIP1 functions independently of the MIDD1 pathway. ACIP1 localized to microtubules in vitro. Overexpression of ACIP1 caused bundling and longitudinal rearrangement of cortical microtubules in the epidermis, suggesting that ACIP1 acted via microtubules as a negative regulator of cell wall spacing in protoxylem vessels, thereby counteracting the MIDD1 pathway. These findings reveal a novel mechanism in which two pathways, ACIP1 and MIDD1, act in opposition to coordinate cell wall patterning in protoxylem vessels, suggesting a potential link between xylem architecture and plant immunity.