Plant organ size control: AINTEGUMENTA regulates growth and cell numbers during organogenesis

The control of cell proliferation during organogenesis plays an important r ole in initiation, growth, and acquisition of the intrinsic size of organs in higher plants. To understand the developmental mechanism that controls i ntrinsic organ size by regulating the number and extent of cell division du ring organogenesis, we examined the function of the Arabidopsis regulatory gene AINTEGUMENATA (ANT), Previous observations revealed that ANT regulates cell division in integuments during ovule development and is necessary for floral organ growth. Here we show that ANT controls plant organ cell numbe r and organ size throughout shoot development. Loss of ANT function reduces the size of all lateral shoot organs by decreasing cell number. Conversely , gain of ANT function, via ectopic expression of a 35S::ANT transgene, enl arges embryonic and all shoot organs without altering superficial morpholog y by increasing cell number in both Arabidopsis and tobacco plants. This hy perplasia results from an extended period of cell proliferation and organ g rowth. Furthermore, cells ectopically expressing ANT in fully differentiate d organs exhibit neoplastic activity by producing calli and adventitious ro ots and shoots. Based on these results, we propose that ANT regulates cell proliferation and organ growth by maintaining the meristematic competence o f cells during organogenesis.