Cell length instead of cell number becomes the predominant factor contributing to hypocotyl length genotypic differences under abiotic stress in Medicago truncatula

Abstract

Hypocotyl elongation in the dark is a crucial process to ensure seedling emergence. It relies both on the cell number and cell length. The contribution of these two factors to the maximal hypocotyl length and the impact of environmental conditions on this contribution are not known. This is surprising considering the agronomic and economical importance of seedling emergence in crop establishment. Using 14 genotypes from a nested core collection representing Medicago truncatula (barrel medic) natural variation, we investigated how epidermal cell number and cell length contribute to hypocotyl length under optimal, low temperature (8°C) and water deficit (-0.50 MPa) conditions. Both cell number and length vary according to genotypes and contribute to maximal hypocotyl length differences between genotypes. This contribution, however, depends on growth conditions. Cell number is the major contributor under optimal conditions (60%) whereas cell length becomes the major determinant under stress. Maximal hypocotyl length is correlated with hypocotyl elongation rate under both stresses but not under optimal condition, revealing contrasted genotypes for cell elongation capacity under stress. To identify the genetic regulators determining cell number and cell length, quantitative trait loci (QTLs) were detected using a recombinant inbred lines population exhibiting segregation in maximal hypocotyl length. Two QTLs controlling cell number and three QTLs controlling cell length at low temperature were detected. One QTL for cell number and two for cell length were found to be associated with hypocotyl length under low temperature. This study provides new information to improve seedling emergence under abiotic stress.

Exploring natural diversity of Medicago truncatula reveals physiotypes and loci associated with the response of seedling performance to nitrate supply

“…Most of the loci identified in this study were new but some shared common regions with QTLs controlling SDW or seedling elongation under low temperature or water deficit previously identified using RIL populations (Dias et al 2011, Vandecasteele et al 2011, Youssef et al 2016). It was the case for loci associated with pHADW or HADW positioned within confidence intervals of QTLs controlling hypocotyl elongation related traits on chromosomes 2, 3 and 7, and for loci associated with radicle elongation related traits, on chromosomes 3, 4 and 5 (Fig.…”

“…1). Maximal hypocotyl length (HL) values (49–87 mm), estimating hypocotyl potential of elongation in darkness, were higher than those previously obtained on a smaller core collection of accessions (CC16) grown in water at the same temperature (42–57 mm, Youssef et al 2016). Values of seed dry weight use‐related traits were compared to those obtained by Brunel et al (2009) for two genotypes.…”

“…The co‐locations of the SNPs with previously identified QTLs for seed and seedling characteristics related to heterotrophic growth using recombinant inbred line populations are indicated by the projection of the support intervals of these QTLs on the right of the chromosomes (dark vertical bars). SM, seed biomass, ELR, early embryonic axis elongation rate (mm/h, 20°C, dark), HL, hypocotyl length (10°C, dark) from Dias et al (2011); CN, cell number in the epidermis of hypocotyl, CL, maximal length of epidermal cells of hypocotyl (10°C, dark) from Youssef et al (2016); HFwd, HGwd, hypocotyl growth rate under water deficit (−0.5 MPa, dark); RLL, radicle length (16 h light); RLLNO3, radicle length in 5 mm KNO3 (16 hr. light), RGLNO3, radicle growth rate in 5 mm KNO3 (16 hr.…”

“…Seedling growth parameters were determined at the end of heterotrophic growth in the presence (N+, 5 mM) and absence (N−) of nitrate at 17°C, previously determined as a control temperature for comparing genotypes during early seedling growth of M. truncatula (Brunel et al 2009, Youssef et al 2016). The seedling growth protocol and nitrate concentration were chosen based on previous results (Vandecasteele et al 2011, Pellizzaro et al 2014).…”

“…Most QTL (quantitative trait locus) studies on seedling heterotrophic performance were based on traditional linkage mapping using bi‐parental populations either in crop species (Bettey et al 2000, Lee et al 2001, Limami et al 2002, Finch‐Savage et al 2010, Rebetzke et al 2014), or in our previous studies in M. truncatula (Dias et al 2011, Vandecasteele et al 2011, Youssef et al 2016). Few were based on GWAS analyses (Li et al 2016, in wheat).…”