Cloning and identification of highly expressed genes in barley lemma and palea

The lemma and palea (lemma/palea) of cereals are photosynthetic organs that supply the developing kernel with carbon and nitrogen. Because of their rigid structure, the lemma/palea can also protect the kernel from pathogens and herbivory. However, very little is known about specific gene expression that enabled the lemma/palea carry out their functions. We have constructed three subtracted cDNA libraries from lemma/palea of barley (Hordeum vulgare L. cv. Morex) at the elongation (between pollination and milky stages) through dough stages of kernel development. Differential screening and northern hybridization showed that the cloned genes were highly expressed in the lemma/palea, compared with the nag leaf. Thus, they contained unique sequences not found in the flag leaf or were expressed in the lemma/palea at much higher levels, appearing as if they were induced. Sequence analysis of 226 clones identified a high proportion of genes for defense, structure, amino acid biosynthesis, and photosynthesis. High expression levels of defense-related genes strongly suggest that lemma/palea constitutively accumulate defensive molecules to inhibit invasion of florets and kernels by pathogens. Increased expression of genes involved in cell wall synthesis and structural repair can improve physical barriers to herbivores and pathogens. High expression of genes for amino acid biosynthesis and photosynthesis indicates that the lemma/palea are major sources of nitrogen and carbon for the growing kernel.