Crops have been domesticated by humans for thousands of years and have undergone significant variation in their inflorescences, spikelets, florets, and other floral organs. The most notable of these is the outermost floral organ of the spikelet—glume. In the natural evolution of cereals, seeds are protected by the glume or hull. However, in agricultural production, seeds have always been under selective pressure from humans to be easily threshed, which has led to the development of various hull-like floral organs.
In a review published in Trends in Plant Science, researchers led by Prof. XIE Qi from the Institute of Genetics and Developmental Biology of the Chinese Academy of Sciences comprehensively synthesized and discussed the evolutionary process of cereal floral organ and the genetic and molecular networks regulating cereal threshability.
By comparing the representative morphological structures of inflorescence, spikelet, floret, and hull-like floral organ among seven major crops (rice, wheat, barley, millet, maize, sorghum, and coix), similarities were observed in spikelet development within the same genus, while different traits exist in different genera. For example, in the "three-floret spikelet" hypothesis in rice, the original glumes are reduced, with two lateral putative florets developing into hard glumes, known as sterile lemmas. In sorghum and maize, sterile florets evolve into transparent or feathery lemma-like organs.
The researchers also found that crops have evolved diverse floral and hull-like organs, suggesting a balance struck between natural evolution and human selection. The key genes selected for the easy-threshing trait during domestication are different, but are located in similar regulatory pathways or within analogous gene families.
Future molecular design breeding of crop easy-threshing trait by using superior natural alleles and gene editing will fast achieve de novo domestication of wild species.