Nuclear Genes Controlling Male Fertility.

In higher plants, the developmental programs of the male and female organs are substantially independent of each other and of the vegetative plant. This developmental independence permits the genetic dissection of fertility processes without impairing vegetative growth. Male fertility requires a number of coordinated developmental events. These include the formation, as part of the flower, of stamens and anthers from the vegetative meristem; the development of pollen grains inside the anther locules; the timely release and deposition of pollen on the stigma; and finally, in self-compatible plants, the interaction of the male and female gametes to attain self-fertilization. Mutations can be obtained that perturb any of the above processes, giving rise to a plant with impaired pollen function but normal female fertility. Such mutations in self-compatible plants have been described as male steriles (ms). Genetic studies in a wide variety of plants indicate that most of the developmental steps leading to male fertility are controlled by nuclear genes. ms mutants are self-sterile but female fertile and, hence, retain their ability to respond to outcrossing. Whereas some of the male-sterile mutants lack male organs altogether (Hill and Lord, 1989; Coen and Meyerowitz, 1991), others have normal male organs but lack pollen (Kaul, 1988); in a third category, visible pollen is formed that is nonfunctional. There is also a category of mutants with defective anthers (Hafen and Stevenson 1958; Chaudhury et al., 1992). Other ms mutants are defective in the temporal control of pollen formation (Kaul, 1988). In this last type of mutant, pollen is formed either to0 early or to0 late for efficient self-fertilization. However, when pollinated by functional pollen from a malefertile plant, each of these mutants produces as many seeds as normal self-pollinating plants. Pollen from this type of mutant is also able to fertilize other plants. The goal of this review is to describe male-sterile mutants that define nuclear genes controlling male fertility. I shall review work done in severa1 plants so as to cover adequately various landmark developmental events. For a more comprehensive review of male sterility, the reader is referred to Kaul (1 988).