?III-Gal is Involved in Galactan Reduction During Phloem Element Differentiation in Chickpea Stems

bIII-Gal, a member of the chickpea b-galactosidase family, is the enzyme responsible for the cell wall autolytic process. This enzyme, whose activity increases during epicotyl growth, displays significant hydrolytic activity against cell wall pectins, and its natural substrate has been determined as an arabinogalactan from the pectic fraction of the cell wall. In the present work, the localization of bIII-Gal in different seedling and plant organs was analyzed by using specific antibIII- Gal antibodies. Our results revealed that besides its possible role in cell wall loosening and in early events during primary xylem and phloem fiber differentiation bIII-Gal acts on the development of sieve elements. Localization of the enzyme in this tissue, both in epicotyls and radicles from seedlings and in the different stem internodes, is consistent with the reduction in galactan during the maturation of phloem elements, as can be observed with LM5 antibodies. Thus, bIII-Gal could act on its natural substrate, the neutral side chains of rhamnogalacturonan I, contributing to cell wall reinforcement allowing phloem elements to differentiate, and conferring the necessary strengthening of the cell wall to fulfill its function. This work completes the immunolocation studies of all known chickpea b-galactosidases. Taken together, our results reflect the broad range of developmental processes covered by different members of this protein family, and confirm their crucial role in cell wall remodeling during tissue differentiation.

The Ministerio de Ciencia e Innovacio?n, Spain [grant BFU2009- 08769].