ERECTA Family Genes Regulate the Shoot Apical Meristem and Organ Formation

Plants are sessile and must adjust their organ growth to their environments. A reservoir of stem cells in the shoot apical meristem (SAM) supplies cells for differentiation into organs. The SAM must balance organ production with stem cell maintenance. The ERECTA family (ERfs) encodes the leucine-rich repeat receptor-like kinases ERECTA (ER), ERECTA-LIKE 1 (ERL1), and ERL2. ERf signaling regulates organ initiation and stem cell maintenance. Results presented in this work include the following:

1) WUSCHEL (WUS) and CLAVATA3 (CLV3) make up a negative feedback loop to maintain SAM size. WUS and CLV3 expression localization is critical for their function. Using genetic analysis, we show that ERfs and EPIDERMAL PATTERNING FACTOR ligands (EPFL) synergistically regulate SAM size with CLV3, and the wus mutation is epistatic to ERf signaling. The erf triple mutant has enlarged CLV3 and WUS expression domains, and ERf activation by EPFL causes a decrease in WUS and CLV3 transcript levels independently of protein translation. These findings show that ERf-EPFL signaling is required to restrict lateral stem cell overproliferation in the SAM.

2) Leaf initiation is regulated by the hormone auxin. Auxin peaks precede organ initiation. The auxin transport protein, PIN1, concentrates auxin in the L1 layer of the SAM during polar auxin transport. Mutants deficient in ERfs have decreased leaf formation and altered auxin signaling. The auxin responsive reporter, DR5rev::GFP, shows that er erl1 erl2 has elevated auxin signaling in the L1 layer, but cannot initiate leaves. Exogenous auxin or auxin efflux inhibition elevates auxin accumulation in the L1 layer of the er erl1 erl2 SAM and rescues leaf initiation, indicating the threshold for leaf initiation is elevated in the mutant. ERfs regulate organ formation synergistically with PID, a PIN polarity regulator. Analysis of DR5rev::GFP in pid er erl1 erl2 indicates leaves do not form due to decreased auxin response. Taken together, we show that ERfs have a dual role in organ initiation by promoting auxin responses and inhibiting auxin accumulation.

This work shows ERECTA family signaling functions in the SAM to prevent lateral overproliferation of stem cells and promotes auxin response to drive leaf initiation.