COTYLEDON VASCULAR PATTERN2–Mediated Inositol (1,4,5) Triphosphate Signal Transduction Is Essential for Closed Venation Patterns of Arabidopsis Foliar Organs

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Abstract
Vein patterns in leaves and cotyledons form in a spatially regulated manner through the progressive recruitment of ground cells into vascular cell fate. To gain insight into venation patterning mechanisms, we have characterized the cotyledon vascular pattern2 (cvp2) mutants, which exhibit an increase in free vein endings and a resulting open vein network. We cloned CVP2 by a map-based cloning strategy and found that it encodes an inositol polyphosphate 5′ phosphatase (5PTase). 5PTases regulate inositol (1,4,5) triphosphate (IP3) signal transduction by hydrolyzing IP3 and thus terminate IP3 signaling. CVP2 gene expression is initially broad and then gradually restricted to incipient vascular cells in several developing organs. Consistent with the inferred enzymatic activity of CVP2, IP3 levels are elevated in cvp2 mutants. In addition, cvp2 mutants exhibit hypersensitivity to the plant hormone abscisic acid. We propose that elevated IP3 levels in cvp2 mutants reduce ground cell recruitment into vascular cell fate, resulting in premature vein termination and, thus, in an open reticulum.