Life on the earth depends on plants. Microscopic aquatic plants and algae make most of the oxygen on the planet. The land plants are the primary producers of human and animal food. This is why it’s important to understand how they grow and reproduce.
In the last 450 million years, as plants slowly evolved from freshwater algae and moved from aquatic ecosystems to moist land to drier land, their life-cycles also changed significantly.
But something curious happened about 130 million years ago, soon after flowering plants first appeared. Fossils from that period suggest flowering plants diversified rapidly in terms of their anatomies and habitats. Evolution is understood to be a gradual process, and the rapid emergence of diverse flowering plants has thus been a puzzle. Charles Darwin called this an “abominable mystery”.
A recent paper by a team of researchers at the CSIR-Centre for Cellular and Molecular Biology (CCMB), Hyderabad, shed light on the molecular innovations in flowering plants that could help understand this mystery.
Life-cycles of land plants
A plant’s life-cycle has two distinct phases: when it’s a gametophyte (gamete-making plant) and when it’s a sporophyte (spore-making plant). The phases dictate their anatomies and functions.
Gametophyte cells contain one set of genes and make either sperm or egg. The fusion of a sperm and an egg gives rise to a sporophyte. Each sporophyte contains two sets of genes, one from each contributing gamete.
When it matures, the sporophyte cells divide to make new cells called spores. The spores have novel combinations of a single set of genes — and the diversity here is responsible for creating plants with diverse traits within a population.
Early land plants and those that evolved later are different in the duration of their phases. Mosses — which retain many features of the ancestors of the earliest land plants — spend most of their lives in the gametophyte stage. When moss makes sperm cells, it distributes them in its watery environment. The sperm cells swim to find egg cells. When a sperm fertilises an egg, a sporophyte is formed that remains attached to the gametophyte. It grows as a stalk, emerging with a capsule at the end. The capsule develops spores that disperse and grow into new gametophytes.
But flowering plants that evolved more recently spend most of their lives in the sporophyte phase. The flowers produce spores that give rise to the gametophytes. However, the number of gametophyte cells is small and they are completely enclosed in the sporophyte.
The gametophytes differentiate into male and female gametophytes. The male gametophytes develop as sperm-containing pollen that delivers sperm to the egg cells in female gametophytes through the wind, insects or other animals that come in contact with the flowers. The union of sperm cells and eggs gives rise to seeds, which germinate to make new sporophytes of flowering plants.
Scientists had previously found that genes controlling the early steps of sperm and egg development are conserved between mosses and flowering plants. That is, even as plants and mosses evolve and their genes change, those underlying the early steps of sperm and egg development don’t. Since moss gametophytes grow independently from the sporophyte, scientists assumed the mechanisms controlling flowering plant gametophyte development are also independent of the sporophyte.
But the recent report from CCMB challenges this assumption: the researchers have shown that the sporophyte controls gametophyte development in flowering plants.
Rapid evolution in flowering plants
The study, recently published in Nature Plants, described the role of a newfound gene called SHUKR (meaning ‘sperm’ in many Indian languages) in the plant Arabidopsis thaliana. This gene is expressed in the flower’s sporophyte cells and affects the development of pollen. When a functional SHUKR gene is absent, the flower fails to produce viable pollen.
(The researchers first found SHUKR in A. thaliana because it’s a model organism for plant biologists. They subsequently also found the gene in other eudicots through genome analyses, but restricted their detailed studies of the effects of SHUKR to A. thaliana alone.)
The SHUKR gene was also found to regulate a class of genes called F-box genes in pollen. These genes remove proteins that have served their functions and make room for new proteins to act in pollen development.
The team, led by emeritus scientist Imran Siddiqi, found the SHUKR gene in eudicots — a plant group that comprises 75% of all flowering plants. The gene first emerged in these plants about 125 million years ago. The team also found that the SHUKR gene and the pollen-specific F-box genes under SHUKR’s control are evolving rapidly.
Unlike mosses, where sperm cells always have enough water to swim through towards the egg cells, flowering plants operate in more variable conditions. Various families of flowering plants have to survive heat, cold, high humidity, and high aridity.
Siddiqi suggested that the fast-evolving nature of SHUKR and the F-box genes allowed the eudicot plants to explore, adapt to, and successfully reproduce in various environmental conditions through variations in pollen. According to him, this could provide key molecular insights to cracking Darwin’s “abominable mystery”.
Put another way, the sudden and drastic evolutionary changes among flowering plants about 125 million years ago could have happened because of the emergence of the SHUKR gene and its ability to control pollen quality, dictated by the conditions and needs of the larger sporophyte plant.
These adaptations add to others in many forms of land plants, including those that enabled them to grow on land and reproduce without constant access to abundant water. These abilities include a robust root system to draw water from the ground, a vasculature that allows water and minerals to move from the roots to different cells of the plant body, and the evolution of many forms of flower-pollinator strategies.
Focus on food security
If evolutionary success among plants was an exam, flowering plants would be the top scorers. Seeds in the form of cereals, pulses, and oilseeds are the biggest source of food for all animals on the earth. They also contribute to a billion-dollar global food industry.
Climate change poses a significant threat to these systems today and imperils food security. Higher temperature affects plant growth and reproduction by inducing metabolic changes in pollen and causing male sterility.
The new study and others like it can help researchers identify new mechanisms that ensure plants can survive increasingly harsh environments. Scientists are even today exploring genes responsible for plants’ physical sturdiness, immunity, and/or tolerance to salinity and drought. In this vein, SHUKR opens a new path to plant fitness.
Siddiqi and his team members have speculated that when exposed to a specific environmental condition, the sporophyte of a eudicot may create pollen fit for those conditions by modulating the protein composition in the pollen. Using the preconditioned pollen, it may be possible to naturally improve environmental resilience in plants.
Somdatta Karak heads science communication at CSIR-CCMB.
Published – June 03, 2025 05:30 am IST