Gravity-Stimulated Changes in Auxin and Invertase Gene Expression in Maize Pulvinal Cells

Abstract

(A.M.R., G.K.M.) Maize (Zea mays) stem gravitropism involves differential elongation of cells within a highly specialized region, the stem internodal pulvinus. In the present study, we investigated factors that control gravitropic responses in this system. In the graviresponding pulvinus, hexose sugars (d-Glc and d-Fru) accumulated asymmetrically across the pulvinus. This correlated well with an asymmetric increase in acid invertase activity across the pulvinus. Northern analyses revealed asymmetric induction of one maize acid invertase gene, Ivr2, consistent with transcriptional regulation by gravistimulation. Several lines of evidence indicated that auxin redistribution, as a result of polar auxin transport, is necessary for gravity-stimulated Ivr2 transcript accumulation and differential cell elongation across the maize pulvinus. First, the auxin transport inhibitor, N-1-naphthylphthalamic acid, inhibited gravistimulated curvature and Ivr2 transcript accumulation. Second, a transient gradient of free indole-3-acetic acid (IAA) across the pulvinus was apparent shortly after initiation of gravistimulation. This temporarily free IAA gradient appears to be important for differential cell elongation and Ivr2 transcript accumulation. This is based on the observation that N-1-naphthylphthalamic acid will not inhibit gravitropic responses when applied to pulvinus tissue after the free IAA gradient peak has occurred. Third, IAA alone can stimulate Ivr2 transcript accumulation in non-gravistimulated pulvini. The gravity-and IAA-stimulated increase in Ivr2 transcripts was sensitive to the protein synthesis inhibitor, cycloheximide. Based on these results, a two-phase model describing possible relationships between gravitropic curvature, IAA redistribution, and Ivr2 expression is presented.Maintaining plant organs with respect to the gravity vector is crucial for proper plant development. Plants use the gravity vector as a cue to orient shoots and roots, positioning leaves for maximum light and roots for maximum water and nutrient uptake (Chen et al., 1999). Plants are able to perceive a change in the direction of the gravity vector. Signal transduction pathway(s) transmit this information, resulting in a differential growth response that returns the plant back to its normal position relative to the gravity vector. Using a genetic approach, some of the early key components of gravitropism have been elucidated (Chen et al., 1999). However, the pathways and mechanisms involved in gravity perception and signal transduction are still not very well understood.Grass shoots have recently emerged as an excellent model system for studying gravitropism signaling events and responses. In grass shoots, gravitropism occurs at a specific region called the pulvinus. In maize (Zea mays), the focus of this study, the pulvinus is located at the base of the internode (Collings et al., 1998). Surrounding the vascular bundles within the maize pulvinus are several layers of bundle sheath cells. These cells contain starch-filled, sedimenta...

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