杜邦先锋的研究人员通过EMS诱变的方法发现了一个玉米显性不育系ms44。经鉴定，造成不育的原因是编码Ms44蛋白的基因单碱基突变的结果。Ms44蛋白是一种在绒毡层中特异性表达的脂转运蛋白。单碱基的变异使得该蛋白中的一个丙氨酸变为了苏氨酸，这种变化阻碍了绒毡层中的蛋白转运过程，最终导致玉米表现为显性的雄性不育性状。更为惊奇的是，由于阻碍了雄穗中的运输通道，大量的N元素被运往玉米的雌穗，导致玉米籽粒数量增加了9.6%。在杂交授粉的情况下ms44不育系可以在正常栽培条件以及在氮缺乏和干旱的逆境条件下均表现增产。
此外，不同于隐性不育和核质不育，显性雄性不育的育性恢复和保持都是难题。而这一障碍也被杜邦公司完美地解决了。他们发现将ms44中的基因用人工小RNA干涉后，育性即可恢复，且正常的MS44基因被干涉后育性不受影响。而依据其自己研发的SPT系统即开发出与ms44不育系配套的保持系。该保持系中以Ms44基因的amiRNA作为育性恢复基因R，串联上花粉致死基因F和种子筛选标记基因S后，由于ami-F-S型雄配子含有F基因而不能存活，保持系只能产生一种可育的雄配子类型ms44，给不育性授粉后可直接生产不育系，且不含转基因成分。保持系自交后产生两种基因型ms44/ms44和ms44/ami-F-S，利用S导致的颜色差别可将保持系种子（ms44/ami-F-S）筛选出来。
由于ms44不育性是单碱基突变的结果，或许可以通过当下流行的基因编辑技术创制，而SPT创制的保持系又不会将转基因元件引入不育系，再加上杂交组合具有氮高效利用的优势，因此基于ms44的SPT杂交制种系统将在未来具有很好的应用前景。
Plant Biotechnology Journal, 5 January 2017
A single pointmutation in Ms44 results in dominant male sterility and improves nitrogen use efficiency in maize
Author
Tim Fox, Jason De Bruin, Kristin Haug Collet……Marc Albertsen, Dale Loussaert, Bo Shen
DuPont Pioneer, USA
Summary
Application of nitrogen fertilizer in the past 50 years has resulted in significant increases in crop yields. However, loss of nitrogen from crop fields has been associated with negative impacts on the environment. Developing maize hybrids with improved nitrogen use efficiency is a cost-effective strategy for increasing yield sustainably. We report that a dominant male sterile mutant Ms44 encodes a lipid transfer protein which is expressed specifically in the tapetum. A single amino acid change from alanine to threonine at the signal peptide cleavage site of the Ms44 protein abolished protein processing and impeded the secretion of protein from tapetal cells into the locule, resulting in dominant male sterility. While the total N content in plants was not changed, Ms44 male sterile plants reduced tassel growth and improved ear growth by partitioning more nitrogen to the ear, resulting in a 9.6% increase in kernel number. Hybrids carrying the Ms44 allele demonstrated a 4-8.5% yield advantage when N is limiting, 1.7% yield advantage under drought, and 0.9% yield advantage under optimal growth conditions relative to the yield of wildtype. Furthermore, we have developed an Ms44 maintainer line for fertility restoration, male-sterile inbred seed increase and hybrid seed production. This study reveals that protein secretion from the tapetum into the locule is critical for pollen development and demonstrates that a reduction in competition between tassel and ear by male sterility improves grain yield under low nitrogen conditions in maize.
