Density Stress has Minimal Impacts on the Barley or Maize Seedling Transcriptome

High planting density affects the morphology and productivity of many crop species. Our objectives were to examine the phenotypic and transcriptomic changes that occur during plant density stress in barley (Hordeum vulgare L.) and maize (Zea mays L.) seedlings. In maize and barley seedlings, density stress impacted several morphological traits. Gene expression profi les were examined in four barley and fi ve maize genotypes grown at low and high plant densities. Only 221 barley and 35 maize genes exhibited differential expression in response to plant density stress. The majority of the gene expression changes were observed in a subset of the genotypes and refl ected minor changes in the level of expression, indicating that the plant density stress imposed in this study did not result in major changes in gene expression. Also, little overlap was observed within barley or maize genotypes in gene expression during density stress, indicating that genotypic differences play a major role in the response to density stress. While it is clear that gene expression differences are involved in morphological changes induced by high plant densities, it is likely that many of these gene expression differences are subtle and restricted to particular tissues and developmental time. PLANT POPULATION DENSITY stress elicits a competitive response between neighboring plants for sunlight, water, and nutrients. Plants detect crowding by attempting to outgrow their neighbors by reallocating their resources into longer stems, longer leaves, and early fl owering (Ballare et al., 1991; Smith and Whitelam, 1997). Other vegetative and reproductive traits such as stem diameter, leaf width, leaf length, number of tillers, biomass per plant, and grain per plant are reduced (Casal, 1988; Hashemi-Dezfouli and Herbert, 1992; Smith and Whitelam, 1997). Even though biomass and grain yield are decreased on a per plant basis as density increases, yield can be increased on a per area unit basis (Duncan, 1958; Hashemi et al., 2005; Silva et al., 2007; Tollenaar, 1989). Plant breeders have understood this relationship and have been breeding for more density-tolerant plants to push the optimum plant density and further increase grain yield per unit area (Cox, 1996; Hashemi et al., 2005; Sarlangue et al., 2007; Tollenaar and Wu, 1999; Widdicombe and Th elen, 2002). In maize (Zea mays L.), plant density has increased from 39,520 plants ha–1 in the 1960s to 66,690 plants ha–1 in 2005 by both increasing planting rates and reducing row spacing, resulting in an increase of 21% in grain yield per hectare (Cardwell, 1982; Elmore and Abendroth, 2006). Th e ability of plant breeders to select for this trait indicates that there is genetic variation in the response to high plant densities. Several groups have assessed transcriptional changes that are induced by plant density stress and shade (a component of density stress). Several genes (2 out of 15) were found to exhibit allelic diff erences in transcript accumulation in Published in The Plant Genome 4:47–54. Published 16 Mar. 2011. doi: 10.3835/plantgenome2010.08.0020 © Crop Science Society of America 5585 Guilford Rd., Madison, WI 53711 USA An open-access publication All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher. S. St. Pierre and G.J. Muehlbauer, Dep. of Agronomy and Plant Genetics, Univ. of Minnesota, St. Paul, MN 55108. N.M. Springer, Dep. of Plant Biology, Univ. of Minnesota, St. Paul, MN 55108. Received 11 Aug. 2010. *Corresponding author ([email protected]). Abbreviations: cDNA, complementary DNA; FDR, false discovery rate; LTP, lipid transfer protein; qRT-PCR, quantitative reverse transcription-polymerase chain reaction; R:Fr, red to far red; RT-PCR, reverse transcription-polymerase chain reaction. Published March, 2011