Leaf senescence is delayed in tobacco plants expressing the maize homeoboxgene knotted1 under the control of a senescence-activated promoter

Leaf senescence is an active process involving remobilization of nutrients from senescing leaves to other parts of the plant, Whereas senescence is ac companied by a decline in leaf cytokinin content, supplemental cytokinin de lays senescence. Plants that overexpress isopentenyl transferase (ipt), a c ytokinin-producing gene, or knotted1 (kn1), a homeobox gene, have many phen otypes in common. Many of these phenotypes are characteristic of altered cy tokinin physiology, The effect of kn1 on leaf senescence was tested by driv ing its expression using the promoter of the senescence-associated gene SAG 12, SAG:kn1 tobacco plants showed a marked delay in leaf senescence but oth erwise developed normally. The delay in senescence was revealed by an incre ase in chlorophyll content in SAG:kn1 leaves relative to leaves of the cont rol plants and by a decrease in the number of dead leaves. Senescence was a lso delayed in detached leaves of SAG:kn1 plants. Delayed senescence was ac companied by increased leaf cytokinin content in older leaves expressing kn 1. These experiments extend the current understanding of kn1 function and s uggest that in addition to mediating meristem maintenance, kn1 is capable o f regulating the onset of senescence in leaves.