A class II KNOX gene, KNOX4, controls seed physical dormancy.
Physical dormancy of seed is an adaptive trait that widely exists in higher plants. This kind of dormancy is caused by a water-impermeable layer that blocks water and oxygen from the surrounding environment and keeps embryos in a viable status for a long time. Most of the work on hardseededness has focused on morphological structure and phenolic content of seed coat. The molecular mechanism underlying physical dormancy remains largely elusive. By screening a large number of Tnt1 retrotransposon-tagged Medicago truncatula lines, we identified nondormant seed mutants from this model legume species. Unlike wild-type hard seeds exhibiting physical dormancy, the mature mutant seeds imbibed water quickly and germinated easily, without the need for scarification. Microscopic observations of cross sections showed that the mutant phenotype was caused by a dysfunctional palisade cuticle layer in the seed coat. Chemical analysis found differences in lipid monomer composition between the wild-type and mutant seed coats. Genetic and molecular analyses revealed that a class II KNOTTED-like homeobox (KNOXII) gene, KNOX4, was responsible for the loss of physical dormancy in the seeds of the mutants. Microarray and chromatin immunoprecipitation analyses identified CYP86A, a gene associated with cutin biosynthesis, as one of the downstream target genes of KNOX4 This study elucidated a novel molecular mechanism of physical dormancy and revealed a new role of class II KNOX genes. Furthermore, KNOX4-like genes exist widely in seed plants but are lacking in nonseed species, indicating that KNOX4 may have diverged from the other KNOXII genes during the evolution of seed plants.
Forage Improvement Division, The Samuel Roberts Noble Foundation, Ardmore, OK 73401;
Maofeng
M
Chai
Forage Improvement Division, The Samuel Roberts Noble Foundation, Ardmore, OK 73401; School of Life Science, Shandong University, Jinan, Shandong 250100, China;
Chuanen
C
Zhou
Department of Biology, Algoma University, Sault Ste. Marie, Ontario, Canada;
Isabel
I
Molina
Forage Improvement Division, The Samuel Roberts Noble Foundation, Ardmore, OK 73401; Institute of BioEnergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao, Shandong 266101, China;
Chunxiang
C
Fu
Plant Biology Division, The Samuel Roberts Noble Foundation, Ardmore, OK 73401;
Jin
J
Nakashima
Plant Biology Division, The Samuel Roberts Noble Foundation, Ardmore, OK 73401;
Guifen
G
Li
Forage Improvement Division, The Samuel Roberts Noble Foundation, Ardmore, OK 73401;
Wenzheng
W
Zhang
Forage Improvement Division, The Samuel Roberts Noble Foundation, Ardmore, OK 73401; BioEnergy Science Center, Oak Ridge National Laboratory, Oak Ridge, TN 37831.
Jongjin
J
Park
Plant Biology Division, The Samuel Roberts Noble Foundation, Ardmore, OK 73401;
Yuhong
Y
Tang
Plant Biology Division, The Samuel Roberts Noble Foundation, Ardmore, OK 73401;
Qingzhen
Q
Jiang
Forage Improvement Division, The Samuel Roberts Noble Foundation, Ardmore, OK 73401; zywang@noble.org.
Zeng-Yu
0000-0002-6835-7917
ZY
Wang
D018506:Gene Expression Regulation, Plant; D005801:Genes, Homeobox; D017343:Genes, Plant; D029902:Medicago / Q000196:embryology / Q000235:genetics*; D009154:Mutation; D057445:Plant Dormancy / Q000235:genetics*; D012639:Seeds
Medicago truncatula; class II KNOX; hardseededness; seed coat; seed physical dormancy