Department of Plant Agriculture, University of Guelph, 50 Stone Road, Guelph, Ontario N1G 2W1, Canada.
BMC Genet. 2014 Feb 14;15:23. doi: 10.1186/1471-2156-15-23.
There was ancient human selection on the wild progenitor of modern maize, Balsas teosinte, for decreased shoot branching (tillering), in order to allow more nutrients to be diverted to grain. Mechanistically, the decline in shoot tillering has been associated with selection for increased expression of the major domestication gene Teosinte Branched 1 (Tb1) in shoot primordia. Therefore, TB1 has been defined as a repressor of shoot branching. It is known that plants respond to changes in shoot size by compensatory changes in root growth and architecture. However, it has not been reported whether altered TB1 expression affects any plant traits below ground. Previously, changes in dosage of a well-studied mutant allele of Tb1 in modern maize, called tb1-ref, from one to two copies, was shown to increase tillering. As a result, plants with two copies of the tb1-ref allele have a larger shoot biomass than heterozygotes. Here we used aeroponics to phenotype the effects of tb1-ref copy number on maize roots at macro-, meso- and micro scales of development.
An increase in the tb1-ref copy number from one to two copies resulted in: (1) an increase in crown root number due to the cumulative initiation of crown roots from successive tillers; (2) higher density of first and second order lateral roots; and (3) reduced average lateral root length. The resulting increase in root system biomass in homozygous tb1-ref mutants balanced the increase in shoot biomass caused by enhanced tillering. These changes caused homozygous tb1-ref mutants of modern maize to more closely resemble its ancestor Balsas teosinte below ground.
We conclude that a decrease in TB1 function in maize results in a larger root system, due to an increase in the number of crown roots and lateral roots. Given that decreased TB1 expression results in a more highly branched and larger shoot, the impact of TB1 below ground may be direct or indirect. We discuss the potential implications of these findings for whole plant coordination of biomass accumulation and maize domestication.
为了使更多的营养物质转移到谷物中，古代人类对现代玉米的野生祖本——巴尔萨玉米进行了选择性进化，使其减少了地上部分的分枝（分蘖）。从机制上讲，地上分枝的减少与主驯化基因 Teosinte Branched 1（Tb1）在芽原基中表达的增加有关。因此，TB1 被定义为分枝的抑制剂。人们知道，植物会通过根系生长和结构的代偿性变化来响应地上部分大小的变化。然而，目前还没有报道 TB1 表达的改变是否会影响地下任何植物性状。此前，在现代玉米中，对一个研究充分的 Tb1 突变等位基因 tb1-ref 的剂量变化进行了研究，从一个拷贝增加到两个拷贝，结果表明分蘖增加。因此，具有两个 tb1-ref 等位基因拷贝的植物比杂合体具有更大的地上生物量。在这里，我们使用气栽法表型分析了 tb1-ref 拷贝数对玉米根系在宏观、中观和微观发育尺度上的影响。
tb1-ref 拷贝数从一个增加到两个，导致：（1）由于从连续分蘖中累积发起冠根，冠根数量增加；（2）一级和二级侧根密度增加；（3）侧根平均长度降低。纯合 tb1-ref 突变体根系生物量的增加与增强分蘖引起的地上生物量增加相平衡。这些变化使现代玉米的纯合 tb1-ref 突变体在地下更类似于其祖先巴尔萨玉米。
我们得出的结论是，玉米中 TB1 功能的降低导致根系更大，这是由于冠根和侧根数量的增加。鉴于 TB1 表达的减少导致分枝更多、地上部分更大，TB1 对地下部分的影响可能是直接的或间接的。我们讨论了这些发现对整个植物生物量积累和玉米驯化的协调的潜在影响。
