Leaf color variation mechanism of the yellow-to-green mutant ‘<i>ytg-2</i>’ in <i>Phaseolus vulgaris</i> L.

Snap bean leaves are the primary site of photosynthesis. Mutant leaf color provides valuable tools for investigating leaf color changes, their role in photosynthesis, and pigment metabolic pathways. In this study, we found that the seedling leaves of '<i>ytg-2</i>' exhibited a yellow phenotype characterized by reduced chlorophyll content compared with 'A18'. Blockage of the uroporphyrinogen III (Urogen III) to the fecal porphyrinogen III (Coprogen III) synthesis pathway primarily affected the synthesis of chlorophyll intermediate metabolites. As the plants matured, their leaves transformed from yellow to light green, accompanied by an increase in the total chlorophyll content. Transcriptome analysis revealed that the significantly down-regulated expression of four genes in the HemE gene family (<i>Phvul.008G059400.1</i>, <i>Phvul.010G110900.1</i>, <i>Phvul.008G147200.1</i>, and <i>Phvul.010G069500.1</i>), which blocked the Urogen III to Coprogen III conversion, was the primary cause of the yellow phenotype in '<i>ytg-2</i>' seedlings. Additionally, the down-regulation of <i>POR</i> (<i>Phvul.004G113000.1</i>) and <i>NOL</i> (<i>Phvul.004G163900</i>) genes during the chlorophyll cycle explained the reduced total chlorophyll content in '<i>ytg-2</i>' and the gradual normalization of the chlorophyll a/b ratio in '<i>ytg-2</i>' yellow leaves. The expression of <i>PAO</i> (<i>Phvul.004G163900.1</i>), a key enzyme in chlorophyll degradation, further confirmed that the yellow-to-green transition in '<i>ytg-2</i>' leaves was linked to chlorophyll degradation processes.